9. Separation of the Embryo

The embryo increases rapidly in size, but the circumference of the embryonic disk, or line of meeting of the embryonic and amniotic parts of the ectoderm, is of relatively slow growth and gradually comes to form a constriction between the embryo and the greater part of the yolk-sac. By means of this constriction, which corresponds to the future umbilicus, a small part of the yolk-sac is enclosed within the embryo and constitutes the primitive digestive tube.

FIG. 21– Section through the embryo which is represented in Fig. 17. (After Graf Spee.)

The embryo increases more rapidly in length than in width, and its cephalic and caudal ends soon extend beyond the corresponding parts of the circumference of the embryonic disk and are bent in a ventral direction to form the cephalic and caudal folds respectively (Figs. 26 and 27).

The cephalic fold is first formed, and as the proamniotic area (page 47) lying immediately in front of the pericardial area (page 47) forms the anterior limit of the circumference of the embryonic disk, the forward growth of the head necessarily carries with it the posterior end of the pericardial area, so that this area and the buccopharyngeal membrane are folded back under the head of the embryo which now encloses a diverticulum of the yolk-sac named the fore-gut.

The caudal end of the embryo is at first connected to the chorion by a band of mesoderm called the body-stalk, but with the formation of the caudal fold the body-stalk assumes a ventral position; a diverticulum of the yolk-sac extends into the tail fold and is termed the hind-gut.

Between the fore-gut and the hind-gut there exists for a time a wide opening into the yolk-sac, but the latter is gradually reduced to a small pear-shaped sac (sometimes termed the umbilical vesicle), and the channel of communication is at the same time narrowed and elongated to form a tube called the vitelline duct.

10. The Yolk-sac

The yolk-sac (Figs. 22 and 23) is situated on the ventral aspect of the embryo; it is lined by entoderm, outside of which is a layer of mesoderm. It is filled with fluid, the vitelline fluid, which possibly may be utilized for the nourishment of the embryo during the earlier stages of its existence.

Blood is conveyed to the wall of the sac by the primitive aortæ, and after circulating through a wide-meshed capillary plexus, is returned by the vitelline veins to the tubular heart of the embryo. This constitutes the vitelline circulation, and by means of it nutritive material is absorbed from the yolk-sac and conveyed to the embryo.

At the end of the fourth week the yolk-sac presents the appearance of a small pear-shaped vesicle (umbilical vesicle) opening into the digestive tube by a long narrow tube, the vitelline duct. The vesicle can be seen in the after-birth as a small, somewhat oval-shaped body whose diameter varies from 1 mm. to 5 mm.; it is situated between the amnion and the chorion and may lie on or at a varying distance from the placenta.

As a rule the duct undergoes complete obliteration during the seventh week, but in about three per cent. of cases its proximal part persists as a diverticulum from the small intestine, Meckel’s diverticulum, which is situated about three or four feet above the ileocolic junction, and may be attached by a fibrous cord to the abdominal wall at the umbilicus. Sometimes a narrowing of the lumen of the ileum is seen opposite the site of attachment of the duct.

FIG. 22– Human embryo of 2.6 mm. (His.)

FIG. 23– Human embryo from thirty-one to thirty-four days. (His.)

11. Development of the Fetal Membranes and Placenta

The Allantois (Figs. 25 to 28).—The allantois arises as a tubular diverticulum of the posterior part of the yolk-sac; when the hind-gut is developed the allantois is carried backward with it and then opens into the cloaca or terminal part of the hind-gut: it grows out into the body-stalk, a mass of mesoderm which lies below and around the tail end of the embryo. The diverticulum is lined by entoderm and covered by mesoderm, and in the latter are carried the allantoic or umbilical vessels. 1
In reptiles, birds, and many mammals the allantois becomes expanded into a vesicle which projects into the extra-embryonic celom. If its further development be traced in the bird, it is seen to project to the right side of the embryo, and, gradually expanding, it spreads over its dorsal surface as a flattened sac between the amnion and the serosa, and extending in all directions, ultimately surrounds the yolk. Its outer wall becomes applied to and fuses with the serosa, which lies immediately inside the shell membrane. Blood is carried to the allantoic sac by the two allantoic or umbilical arteries, which are continuous with the primitive aortæ, and after circulating through the allantoic capillaries, is returned to the primitive heart by the two umbilical veins. In this way the allantoic circulation, which is of the utmost importance in connection with the respiration and nutrition of the chick, is established. Oxygen is taken from, and carbonic acid is given up to the atmosphere through the egg-shell, while nutritive materials are at the same time absorbed by the blood from the yolk. 2

FIG. 24– Diagram showing earliest observed stage of human ovum. (See enlarged image)

FIG. 25– Diagram illustrating early formation of allantois and differentiation of body-stalk. (See enlarged image)

FIG. 26– Diagram showing later stage of allantoic development with commencing constriction of the yolk-sac. (See enlarged image)

FIG. 27– Diagram showing the expansion of amnion and delimitation of the umbilicus.

In man and other primates the nature of the allantois is entirely different from that just described. Here it exists merely as a narrow, tubular diverticulum of the hind-gut, and never assumes the form of a vesicle outside the embryo. With the formation of the amnion the embryo is, in most animals, entirely separated from the chorion, and is only again united to it when the allantoic mesoderm spreads over and becomes applied to its inner surface. The human embryo, on the other hand, as was pointed out by His, is never wholly separated from the chorion, its tail end being from the first connected with the chorion by means of a thick band of mesoderm, named the body-stalk (Bauchstiel); into this stalk the tube of the allantois extends (Fig. 21).

The Amnion.—The amnion is a membranous sac which surrounds and protects the embryo. It is developed in reptiles, birds, and mammals, which are hence called “Amniota;” but not in amphibia and fishes, which are consequently termed “Anamnia.”

In the human embryo the earliest stages of the formation of the amnion have not been observed; in the youngest embryo which has been studied the amnion was already present as a closed sac (Figs. 24 and 32), and, as indicated on page 46, appears in the inner cell-mass as a cavity. This cavity is roofed in by a single stratum of flattened, ectodermal cells, the amniotic ectoderm, and its floor consists of the prismatic ectoderm of the embryonic disk—the continuity between the roof and floor being established at the margin of the embryonic disk. Outside the amniotic ectoderm is a thin layer of mesoderm, which is continuous with that of the somatopleure and is connected by the body-stalk with the mesodermal lining of the chorion.

FIG. 28– Diagram illustrating a later stage in the development of the umbilical cord. (See enlarged image)

When first formed the amnion is in contact with the body of the embryo, but about the fourth or fifth week fluid (liquor amnii) begins to accumulate within it. This fluid increases in quantity and causes the amnion to expand and ultimately to adhere to the inner surface of the chorion, so that the extra-embryonic part of the celom is obliterated. The liquor amnii increases in quantity up to the sixth or seventh month of pregnancy, after which it diminishes somewhat; at the end of pregnancy it amounts to about 1 liter. It allows of the free movements of the fetus during the later stages of pregnancy, and also protects it by diminishing the risk of injury from without. It contains less than 2 per cent. of solids, consisting of urea and other extractives, inorganic salts, a small amount of protein, and frequently a trace of sugar. That some of the liquor amnii is swallowed by the fetus is proved by the fact that epidermal debris and hairs have been found among the contents of the fetal alimentary canal.

In reptiles, birds, and many mammals the amnion is developed in the following manner: At the point of constriction where the primitive digestive tube of the embryo joins the yolk-sac a reflection or folding upward of the somatopleure takes place. This, the amniotic fold (Fig. 29), first makes its appearance at the cephalic extremity, and subsequently at the caudal end and sides of the embryo, and gradually rising more and more, its different parts meet and fuse over the dorsal aspect of the embryo, and enclose a cavity, the amniotic cavity. After the fusion of the edges of the amniotic fold, the two layers of the fold become completely separated, the inner forming the amnion, the outer the false amnion or serosa. The space between the amnion and the serosa constitutes the extra-embryonic celom, and for a time communicates with the embryonic celom.

FIG. 29– Diagram of a transverse section, showing the mode of formation of the amnion in the chick. The amniotic folds have nearly united in the middle line. (From Quain’s Anatomy.) Ectoderm, blue; mesoderm, red; entoderm and notochord, black.

FIG. 30– Fetus of about eight weeks, enclosed in the amnion. Magnified a little over two diameters. (Drawn from stereoscopic photographs lent by Prof. A. Thomson, Oxford.)

The Umbilical Cord and Body-stalk.—The umbilical cord (Fig. 28) attaches the fetus to the placenta; its length at full time, as a rule, is about equal to the length of the fetus, i.e., about 50 cm., but it may be greatly diminished or increased. The rudiment of the umbilical cord is represented by the tissue which connects the rapidly growing embryo with the extra-embryonic area of the ovum. Included in this tissue are the body-stalk and the vitelline duct—the former containing the allantoic diverticulum and the umbilical vessels, the latter forming the communication between the digestive tube and the yolk-sac. The body-stalk is the posterior segment of the embryonic area, and is attached to the chorion. It consists of a plate of mesoderm covered by thickened ectoderm on which a trace of the neural groove can be seen, indicating its continuity with the embryo. Running through its mesoderm are the two umbilical arteries and the two umbilical veins, together with the canal of the allantois—the last being lined by entoderm (Fig. 31). Its dorsal surface is covered by the amnion, while its ventral surface is bounded by the extra-embryonic celom, and is in contact with the vitelline duct and yolk-sac. With the rapid elongation of the embryo and the formation of the tail fold, the body stalk comes to lie on the ventral surface of the embryo (Figs. 27 and 28), where its mesoderm blends with that of the yolk-sac and the vitelline duct. The lateral leaves of somatopleure then grow round on each side, and, meeting on the ventral aspect of the allantois, enclose the vitelline duct and vessels, together with a part of the extra-embryonic celom; the latter is ultimately obliterated. The cord is covered by a layer of ectoderm which is continuous with that of the amnion, and its various constitutents are enveloped by embryonic gelatinous tissue, jelly of Wharton. The vitelline vessels and duct, together with the right umbilical vein, undergo atrophy and disappear; and thus the cord, at birth, contains a pair of umbilical arteries and one (the left) umbilical vein.

FIG. 31– Model of human embryo 1.3 mm. long. (After Eternod.)

Implantation or Imbedding of the Ovum.—As described (page 44), fertilization of the ovum occurs in the lateral or ampullary end of the uterine tube and is immediately followed by segmentation. On reaching the cavity of the uterus the segmented ovum adheres like a parasite to the uterine mucous membrane, destroys the epithelium over the area of contact, and excavates for itself a cavity in the mucous membrane in which it becomes imbedded. In the ovum described by Bryce and Teacher 7 the point of entrance was visible as a small gap closed by a mass of fibrin and leucocytes; in the ovum described by Peters, 8 the opening was covered by a mushroom-shaped mass of fibrin and blood-clot (Fig. 32), the narrow stalk of which plugged the aperture in the mucous membrane. Soon, however, all trace of the opening is lost and the ovum is then completely surrounded by the uterine mucous membrane. 9
The structure actively concerned in the process of excavation is the trophoblast of the ovum, which possesses the power of dissolving and absorbing the uterine tissues. The trophoblast proliferates rapidly and forms a network of branching processes which cover the entire ovum and invade and destroy the maternal tissues and open into the maternal bloodvessels, with the result that the spaces in the trophoblastic network are filled with maternal blood; these spaces communicate freely with one another and become greatly distended and form the intervillous space. 10

FIG. 32– Section through ovum imbedded in the uterine decidua. Semidiagrammatic. (After Peters.) am. Amniotic cavity. b.c. Blood-clot. b.s. Body-stalk. ect. Embryonic ectoderm. ent. Entoderm. mes. Mesoderm. m.v. Maternal vessels. tr. Trophoblast. u.e. Uterine epithelium. u.g. Uterine glands. y.s. Yolk-sac. (See enlarged image)

The Decidua.—Before the fertilized ovum reaches the uterus, the mucous membrane of the body of the uterus undergoes important changes and is then known as the decidua. The thickness and vascularity of the mucous membrane are greatly increased; its glands are elongated and open on its free surface by funnel-shaped orifices, while their deeper portions are tortuous and dilated into irregular spaces. The interglandular tissue is also increased in quantity, and is crowded with large round, oval, or polygonal cells, termed decidual cells. These changes are well advanced by the second month of pregnancy, when the mucous membrane consists of the following strata (Fig. 33): (1) stratum compactum, next the free surface; in this the uterine glands are only slightly expanded, and are lined by columnar cells; (2) stratum spongiosum, in which the gland tubes are greatly dilated and very tortuous, and are ultimately separated from one another by only a small amount of interglandular tissue, while their lining cells are flattened or cubical; (3) a thin unaltered or boundary layer, next the uterine muscular fibers, containing the deepest parts of the uterine glands, which are not dilated, and are lined with columnar epithelium; it is from this epithelium that the epithelial lining of the uterus is regenerated after pregnancy. Distinctive names are applied to different portions of the decidua. The part which covers in the ovum is named the decidua capsularis; the portion which intervenes between the ovum and the uterine wall is named the decidua basalis or decidua placentalis; it is here that the placenta is subsequently developed. The part of the decidua which lines the remainder of the body of the uterus is known as the decidua vera or decidua parietalis. 11
Coincidently with the growth of the embryo, the decidua capsularis is thinned and extended (Fig. 34) and the space between it and the decidua vera is gradually obliterated, so that by the third month of pregnancy the two are in contact. By the fifth month of pregnancy the decidua capsularis has practically disappeared, while during the succeeding months the decidua vera also undergoes atrophy, owing to the increased pressure. The glands of the stratum compactum are obliterated, and their epithelium is lost. In the stratum spongiosum the glands are compressed and appear as slit-like fissures, while their epithelium undergoes degeneration. In the unaltered or boundary layer, however, the glandular epithelium retains a columnar or cubical form. 12

FIG. 33– Diagrammatic sections of the uterine mucous membrane: A. The non-pregnant uterus. B. The pregnant uterus, showing the thickened mucous membrane and the altered condition of the uterine glands. (Kundrat and Engelmann.) (See enlarged image)

FIG. 34– Sectional plan of the gravid uterus in the third and fourth month. (Modified from Wagner.) (See enlarged image)

The Chorion (Figs. 23 to28).—The chorion consists of two layers: an outer formed by the primitive ectoderm or trophoblast, and an inner by the somatic mesoderm; with this latter the amnion is in contact. The trophoblast is made up of an internal layer of cubical or prismatic cells, the cytotrophoblast or layer of Langhans, and an external layer of richly nucleated protoplasm devoid of cell boundaries, the syncytiotrophoblast. It undergoes rapid proliferation and forms numerous processes, the chorionic villi, which invade and destroy the uterine decidua and at the same time absorb from it nutritive materials for the growth of the embryo. The chorionic villi are at first small and non-vascular, and consist of trophoblast only, but they increase in size and ramify, while the mesoderm, carrying branches of the umbilical vessels, grows into them, and in this way they are vascularized. Blood is carried to the villi by the branches of the umbilical arteries, and after circulating through the capillaries of the villi, is returned to the embryo by the umbilical veins. Until about the end of the second month of pregnancy the villi cover the entire chorion, and are almost uniform in size (Fig. 25), but after this they develop unequally. The greater part of the chorion is in contact with the decidua capsularis (Fig. 34), and over this portion the villi, with their contained vessels, undergo atrophy, so that by the fourth month scarcely a trace of them is left, and hence this part of the chorion becomes smooth, and is named the chorion læve; as it takes no share in the formation of the placenta, it is also named the non-placental part of the chorion. On the other hand, the villi on that part of the chorion which is in contact with the decidua placentalis increase greatly in size and complexity, and hence this part is named the chorion frondosum (Fig. 28). 13

FIG. 35– Transverse section of a chorionic villus. (See enlarged image)

FIG. 36– Primary chorionic villi. Diagrammatic. (Modified from Bryce.) (See enlarged image)

The Placenta.—The placenta connects the fetus to the uterine wall, and is the organ by means of which the nutritive, respiratory, and excretory functions of the fetus are carried on. It is composed of fetal and maternal portions. 14

FIG. 37– Secondary chorionic villi. Diagrammatic. (Modified from Bryce.) (See enlarged image)

Fetal Portion.—The fetal portion of the placenta consists of the villi of the chorion frondosum; these branch repeatedly, and increase enormously in size. These greatly ramified villi are suspended in the intervillous space, and are bathed in maternal blood, which is conveyed to the space by the uterine arteries and carried away by the uterine veins. A branch of an umbilical artery enters each villus and ends in a capillary plexus from which the blood is drained by a tributary of the umbilical vein. The vessels of the villus are surrounded by a thin layer of mesoderm consisting of gelatinous connective tissue, which is covered by two strata of ectodermal cells derived from the trophoblast: the deeper stratum, next the mesodermic tissue, represents the cytotrophoblast or layer of Langhans; the superficial, in contact with the maternal blood, the syncytiotrophoblast (Figs. 36 and 37). After the fifth month the two strata of cells are replaced by a single layer of somewhat flattened cells. 15

Maternal Portion.—The maternal portion of the placenta is formed by the decidua placentalis containing the intervillous space. As already explained, this space is produced by the enlargement and intercommunication of the spaces in the trophoblastic network. The changes involve the disappearance of the greater portion of the stratum compactum, but the deeper part of this layer persists and is condensed to form what is known as the basal plate. Between this plate and the uterine muscular fibres are the stratum spongiosum and the boundary layer; through these and the basal plate the uterine arteries and veins pass to and from the intervillous space. The endothelial lining of the uterine vessels ceases at the point where they terminate in the intervillous space which is lined by the syncytiotrophoblast. Portions of the stratum compactum persist and are condensed to form a series of septa, which extend from the basal plate through the thickness of the placenta and subdivide it into the lobules or cotyledons seen on the uterine surface of the detached placenta. 16

FIG. 38– Fetus in utero, between fifth and sixth months. (See enlarged image)

The fetal and maternal blood currents traverse the placenta, the former passing through the bloodvessels of the placental villi and the latter through the intervillous space (Fig. 39). The two currents do not intermingle, being separated from each other by the delicate walls of the villi. Nevertheless, the fetal blood is able to absorb, through the walls of the villi, oxygen and nutritive materials from the maternal blood, and give up to the latter its waste products. The blood, so purified, is carried back to the fetus by the umbilical vein. It will thus be seen that the placenta not only establishes a mechanical connection between the mother and the fetus, but subserves for the latter the purposes of nutrition, respiration, and excretion. In favor of the view that the placenta possesses certain selective powers may be mentioned the fact that glucose is more plentiful in the maternal than in the fetal blood. It is interesting to note also that the proportion of iron, and of lime and potash, in the fetus is increased during the last months of pregnancy. Further, there is evidence that the maternal leucocytes may migrate into the fetal blood, since leucocytes are much more numerous in the blood of the umbilical vein than in that of the umbilical arteries. 17
The placenta is usually attached near the fundus uteri, and more frequently on the posterior than on the anterior wall of the uterus. It may, however, occupy a lower position and, in rare cases, its site is close to the orificium internum uteri, which it may occlude, thus giving rise to the condition known as placenta previa. 18

FIG. 39– Scheme of placental circulation. (See enlarged image)

Separation of the Placenta.—After the child is born, the placenta and membranes are expelled from the uterus as the after-birth. The separation of the placenta from the uterine wall takes place through the stratum spongiosum, and necessarily causes rupture of the uterine vessels. The orifices of the torn vessels are, however, closed by the firm contraction of the uterine muscular fibers, and thus postpartum hemorrhage is controlled. The epithelial lining of the uterus is regenerated by the proliferation and extension of the epithelium which lines the persistent portions of the uterine glands in the unaltered layer of the decidua. 19
The expelled placenta appears as a discoid mass which weighs about 450 gm. and has a diameter of from 15 to 20 cm. Its average thickness is about 3 cm., but this diminishes rapidly toward the circumference of the disk, which is continuous with the membranes. Its uterine surface is divided by a series of fissures into Iobules or cotyledons, the fissures containing the remains of the septa which extended between the maternal and fetal portions. Most of these septa end in irregular or pointed processes; others, especially those near the edge of the placenta, pass through its thickness and are attached to the chorion. In the early months these septa convey branches of the uterine arteries which open into the intervillous space on the surfaces of the septa. The fetal surface of the placenta is smooth, being closely invested by the amnion. Seen through the latter, the chorion presents a mottled appearance, consisting of gray, purple, or yellowish areas. The umbilical cord is usually attached near the center of the placenta, but may be inserted anywhere between the center and the margin; in some cases it is inserted into the membranes, i. e., the velamentous insertion. From the attachment of the cord the larger branches of the umbilical vessels radiate under the amnion, the veins being deeper and larger than the arteries. The remains of the vitelline duct and yolk-sac may be sometimes observed beneath the amnion, close to the cord, the former as an attenuated thread, the latter as a minute sac. 20
On section, the placenta presents a soft, spongy appearance, caused by the greatly branched villi; surrounding them is a varying amount of maternal blood giving the dark red color to the placenta. Many of the larger villi extend from the chorionic to the decidual surface, while others are attached to the septa which separate the cotyledons; but the great majority of the villi hang free in the intervillous space.

FIG. 40– Embryo between eighteen and twenty-one days. (His.)

FIG. 41– Head end of human embryo, about the end of the fourth week. (From model by Peter.)

Note 7. Contribution to the study of the early development and imbedding of the human ovum, 1908. [back]
Note 8. Die Einbettung des menschlichen Eies, 1899. [back]

12. The Branchial Region

The Branchial or Visceral Arches and Pharyngeal Pouches.—In the lateral walls of the anterior part of the fore-gut five pharyngeal pouches appear (Fig. 42); each of the upper four pouches is prolonged into a dorsal and a ventral diverticulum.

Over these pouches corresponding indentations of the ectoderm occur, forming what are known as the branchial or outer pharyngeal grooves. The intervening mesoderm is pressed aside and the ectoderm comes for a time into contact with the entodermal lining of the fore-gut, and the two layers unite along the floors of the grooves to form thin closing membranes between the fore-gut and the exterior.

Later the mesoderm again penetrates between the entoderm and the ectoderm. In gill-bearing animals the closing membranes disappear, and the grooves become complete clefts, the gill-clefts, opening from the pharynx on to the exterior; perforation, however, does not occur in birds or mammals.

The grooves separate a series of rounded bars or arches, the branchial or visceral arches, in which thickening of the mesoderm takes place (Figs. 40 and 41). The dorsal ends of these arches are attached to the sides of the head, while the ventral extremities ultimately meet in the middle line of the neck.

In all, six arches make their appearance, but of these only the first four are visible externally. The first arch is named the mandibular, and the second the hyoid; the others have no distinctive names.

In each arch a cartilaginous bar, consisting of right and left halves, is developed, and with each of these there is one of the primitive aortic arches.

FIG. 42– Floor of pharynx of embryo shown in Fig. 40.

The mandibular arch lies between the first branchial groove and the stomodeum; from it are developed the lower lip, the mandible, the muscles of mastication, and the anterior part of the tongue.

Its cartilaginous bar is formed by what are known as Meckel’s cartilages (right and left) (Fig. 43); above this the incus is developed.

The dorsal end of each cartilage is connected with the ear-capsule and is ossified to form the malleus; the ventral ends meet each other in the region of the symphysis menti, and are usually regarded as undergoing ossification to form that portion of the mandible which contains the incisor teeth.

The intervening part of the cartilage disappears; the portion immediately adjacent to the malleus is replaced by fibrous membrane, which constitutes the spheno-mandibular ligament, while from the connective tissue covering the remainder of the cartilage the greater part of the mandible is ossified.

From the dorsal ends of the mandibular arch a triangular process, the maxillary process, grows forward on either side and forms the cheek and lateral part of the upper lip.

The second or hyoid arch assists in forming the side and front of the neck. From its cartilage are developed the styloid process, stylohyoid ligament, and lesser cornu of the hyoid bone. The stages probably arises in the upper part of this arch.

The cartilage of the third arch gives origin to the greater cornu of the hyoid bone. The ventral ends of the second and third arches unite with those of the opposite side, and form a transverse band, from which the body of the hyoid bone and the posterior part of the tongue are developed.

The ventral portions of the cartilages of the fourth and fifth arches unite to form the thyroid cartilage; from the cartilages of the sixth arch the cricoid and arytenoid cartilages and the cartilages of the trachea are developed.

The mandibular and hyoid arches grow more rapidly than those behind them, with the result that the latter become, to a certain extent, telescoped within the former, and a deep depression, the sinus cervicalis, is formed on either side of the neck.

This sinus is bounded in front by the hyoid arch, and behind by the thoracic wall; it is ultimately obliterated by the fusion of its walls.

FIG. 43– Head and neck of a human embryo eighteen weeks old, with Meckel’s cartilage and hyoid bar exposed. (After Kölliker.) (See enlarged image)

FIG. 44– Under surface of the head of a human embryo about twenty-nine days old. (After His.) (See enlarged image)

From the first branchial groove the concha auriculæ and external acoustic meatus are developed, while around the groove there appear, on the mandibular and hyoid arches, a number of swellings from which the auricula or pinna is formed.

The first pharyngeal pouch is prolonged dorsally to form the auditory tube and the tympanic cavity; the closing membrane between the mandibular and hyoid arches is invaded by mesoderm, and forms the tympanic membrane.

No traces of the second, third, and fourth branchial grooves persist. The inner part of the second pharyngeal pouch is named the sinus tonsillaris; in it the tonsil is developed, above which a trace of the sinus persists as the supratonsillar fossa.

The fossa of Rosenmüller or lateral recess of the pharynx is by some regarded as a persistent part of the second pharyngeal pouch, but it is probably developed as a secondary formation.

From the third pharyngeal pouch the thymus arises as an entodermal diverticulum on either side, and from the fourth pouches small diverticula project and become incorporated with the thymus, but in man these diverticula probably never form true thymus tissue.

The parathyroids also arise as diverticula from the third and fourth pouches. From the fifth pouches the ultimobranchial bodies originate and are enveloped by the lateral prolongations of the median thyroid rudiment; they do not, however, form true thyroid tissue, nor are any traces of them found in the human adult.

The Nose and Face.—During the third week two areas of thickened ectoderm, the olfactory areas, appear immediately under the fore-brain in the anterior wall of the stomodeum, one on either side of a region termed the fronto-nasal process (Fig. 44).

By the upgrowth of the surrounding parts these areas are converted into pits, the olfactory pits, which indent the fronto-nasal process and divide it into a medial and two lateral nasal processes (Fig. 45).

The rounded lateral angles of the medial process constitute the globular processes of His.

The olfactory pits form the rudiments of the nasal cavities, and from their ectodermal lining the epithelium of the nasal cavities, with the exception of that of the inferior meatuses, is derived.

The globular processes are prolonged backward as plates, termed the nasal laminæ: these laminæ are at first some distance apart, but, gradually approaching, they ultimately fuse and form the nasal septum; the processes themselves meet in the middle line, and form the premaxillæ and the philtrum or central part of the upper lip (Fig. 48).

The depressed part of the medial nasal process between the globular processes forms the lower part of the nasal septum or columella; while above this is seen a prominent angle, which becomes the future apex (Figs. 45, 46), and still higher a flat area, the future bridge, of the nose. The lateral nasal processes form the alæ of the nose.

FIG. 45– Head end of human embryo of about thirty to thirty-one days. (From model by Peters.)

FIG. 46– Same embryo as shown in Fig. 45, with front wall of pharynx removed.

FIG. 47– Head of a human embryo of about eight weeks, in which the nose and mouth are formed. (His.)

FIG. 48– Diagram showing the regions of the adult face and neck related to the fronto-nasal process and the branchial arches. (See enlarged image)

FIG. 49– Primitive palate of a human embryo of thirty-seven to thirty-eight days. (From model by Peters.) On the left side the lateral wall of the nasal cavity has been removed.

FIG. 50– The roof of the mouth of a human embryo, aged about two and a half months, showing the mode of formation of the palate. (His.) (See enlarged image)

Continuous with the dorsal end of the mandibular arch, and growing forward from its cephalic border, is a triangular process, the maxillary process, the ventral extremity of which is separated from the mandibular arch by a > shaped notch (Fig. 44). The maxillary process forms the lateral wall and floor of the orbit, and in it are ossified the zygomatic bone and the greater part of the maxilla; it meets with the lateral nasal process, from which, however, it is separated for a time by a groove, the naso-optic furrow, that extends from the furrow encircling the eyeball to the olfactory pit. The maxillary processes ultimately fuse with the lateral nasal and globular processes, and form the lateral parts of the upper lip and the posterior boundaries of the nares (Figs. 47, 48). From the third to the fifth month the nares are filled by masses of epithelium, on the breaking down and disappearance of which the permanent openings are produced. The maxillary process also gives rise to the lower portion of the lateral wall of the nasal cavity. The roof of the nose and the remaining parts of the lateral wall, viz., the ethmoidal labyrinth, the inferior nasal concha, the lateral cartilage, and the lateral crus of the alar cartilage, are developed in the lateral nasal process. By the fusion of the maxillary and nasal processes in the roof of the stomodeum the primitive palate (Fig. 49) is formed, and the olfactory pits extend backward above it. The posterior end of each pit is closed by an epithelial membrane, the bucco-nasal membrane, formed by the apposition of the nasal and stomodeal epithelium. By the rupture of these membranes the primitive choanæ or openings between the olfactory pits and the stomodeum are established. The floor of the nasal cavity is completed by the development of a pair of shelf-like palatine processes which extend medial-ward from the maxillary processes (Figs. 50 and 51); these coalesce with each other in the middle line, and constitute the entire palate, except a small part in front which is formed by the premaxillary bones. Two apertures persist for a time between the palatine processes and the premaxillæ and represent the permanent channels which in the lower animals connect the nose and mouth. The union of the parts which form the palate commences in front, the premaxillary and palatine processes joining in the eighth week, while the region of the future hard palate is completed by the ninth, and that of the soft palate by the eleventh week. By the completion of the palate the permanent choanæ are formed and are situated a considerable distance behind the primitive choanæ. The deformity known as cleft palate results from a non-union of the palatine processes, and that of harelip through a non-union of the maxillary and globular processes (see page 199). The nasal cavity becomes divided by a vertical septum, which extends downward and backward from the medial nasal process and nasal laminæ, and unites below with the palatine processes. Into this septum a plate of cartilage extends from the under aspect of the ethmoid plate of the chodrocranium. The anterior part of this cartilaginous plate persists as the septal cartilage of the nose and the medial crus of the alar cartilage, but the posterior and upper parts are replaced by the vomer and perpendicular plate of the ethmoid. On either side of the nasal septum, at its lower and anterior part, the ectoderm is invaginated to form a blind pouch or diverticulum, which extends backward and upward into the nasal septum and is supported by a curved plate of cartilage. These pouches form the rudiments of the vomero-nasal organs of Jacobson, which open below, close to the junction of the premaxillary and maxillary bones. 5

FIG. 51– Frontal section of nasal cavities of a human embryo 28 mm. long. (Kollmann.) (See enlarged image)

The Limbs.—The limbs begin to make their appearance in the third week as small elevations or buds at the side of the trunk (Fig. 52). Prolongations from the muscle- and cutis-plates of several primitive segments extend into each bud, and carry with them the anterior divisions of the corresponding spinal nerves. The nerves supplying the limbs indicate the number of primitive segments which contribute to their formation—the upper limb being derived from seven, viz., fourth cervical to second thoracic inclusive, and the lower limb from ten, viz., twelfth thoracic to fourth sacral inclusive. The axial part of the mesoderm of the limb-bud becomes condensed and converted into its cartilaginous skeleton, and by the ossification of this the bones of the limbs are formed. By the sixth week the three chief divisions of the limbs are marked off by furrows—the upper into arm, forearm, and hand; the lower into thigh, leg, and foot (Fig. 53). The limbs are at first directed backward nearly parallel to the long axis of the trunk, and each presents two surfaces and two borders. Of the surfaces, one—the future flexor surface of the limb—is directed ventrally; the other, the extensor surface, dorsally; one border, the preaxial, looks forward toward the cephalic end of the embryo, and the other, the postaxial, backward toward the caudal end. The lateral epicondyle of the humerus, the radius, and the thumb lie along the preaxial border of the upper limb; and the medial epicondyle of the femur, the tibia, and the great toe along the corresponding border of the lower limb. The preaxial part is derived from the anterior segments, the postaxial from the posterior segments of the limb-bud; and this explains, to a large extent, the innervation of the adult limb, the nerves of the more anterior segments being distributed along the preaxial (radial or tibial), and those of the more posterior along the postaxial (ulnar or fibular) border of the limb. The limbs next undergo a rotation or torsion through an angle of 90° around their long axes the rotation being effected almost entirely at the limb girdles. In the upper limb the rotation is outward and forward; in the lower limb, inward and backward. As a consequence of this rotation the preaxial (radial) border of the fore-limb is directed lateralward, and the preaxial (tibial) border of the hind-limb is directed medialward; thus the flexor surface of the fore-limb is turned forward, and that of the hind-limb backward. 6

FIG. 52– Human embryo from thirty-one to thirty-four days. (His.) (See enlarged image)

FIG. 53– Embryo of about six weeks. (His.) (See enlarged image)

13. Development of the Body Cavities

In the human embryo described by Peters the mesoderm outside the embryonic disk is split into two layers enclosing an extra-embryonic cœlom; there is no trace of an intra-embryonic cœlom. At a later stage four cavities are formed within the embryo, viz., one on either side within the mesoderm of the pericardial area, and one in either lateral mass of the general mesoderm. All these are at first independent of each other and of the extra-embryonic celom, but later they become continuous. The two cavities in the general mesoderm unite on the ventral aspect of the gut and form the pleuro-peritoneal cavity, which becomes continuous with the remains of the extra-embryonic celom around the umbilicus; the two cavities in the pericardial area rapidly join to form a single pericardial cavity, and this from two lateral diverticula extend caudalward to open into the pleuro-peritoneal cavity (Fig. 54). 1

FIG. 54– Figure obtained by combining several successive sections of a human embryo of about the fourth week (From Kollmann.) The upper arrow is in the pleuroperitoneal opening, the lower in the pleuropericardial. (See enlarged image)

Between the two latter diverticula is a mass of mesoderm containing the ducts of Cuvier, and this is continuous ventrally with the mesoderm in which the umbilical veins are passing to the sinus venosus. A septum of mesoderm thus extends across the body of the embryo. It is attached in front to the body-wall between the pericardium and umbilicus; behind to the body-wall at the level of the second cervical segment; laterally it is deficient where the pericardial and pleuro-peritoneal cavities communicate, while it is perforated in the middle line by the foregut. This partition is termed the septum transversum, and is at first a bulky plate of tissue. As development proceeds the dorsal end of the septum is carried gradually caudalward, and when it reaches the fifth cervical segment muscular tissue with the phrenic nerve grows into it. It continues to recede, however, until it reaches the position of the adult diaphragm on the bodies of the upper lumbar vertebræ. The liver buds grow into the septum transversum and undergo development there. 2
The lung buds meantime have grown out from the fore-gut, and project laterally into the forepart of the pleuro-peritoneal cavity; the developing stomach and liver are imbedded in the septum transversum; caudal to this the intestines project into the back part of the pleuro-peritoneal cavity (Fig. 55). Owing to the descent of the dorsal end of the septum transversum the lung buds come to lie above the septum and thus pleural and peritoneal portions of the pleuro-peritoneal cavity (still, however, in free communication with one another) may be recognized; the pericardial cavity opens into the pleural part. 3

FIG. 55– Upper part of celom of human embryo of 6.8 mm., seen from behind. (From model by Piper.) (See enlarged image)

The ultimate separation of the permanent cavities from one another is effected by the growth of a ridge of tissue on either side from the mesoderm surrounding the duct of Cuvier (Figs. 54, 55). The front part of this ridge grows across and obliterates the pleuro-pericardial opening; the hinder part grows across the pleuro-peritoneal opening. 4

FIG. 56– Diagram of transverse section through rabbit embryo. (After Keith.) (See enlarged image)

With the continued growth of the lungs the pleural cavities are pushed forward in the body-wall toward the ventral median line, thus separating the pericardium from the lateral thoracic walls (Fig. 53). The further development of the peritoneal cavity has been described with the development of the digestive tube (page 168 et seq.).

FIG. 57– The thoracic aspect of the diaphragm of a newly born child in which the communication between the peritoneum and pleura has not been closed on the left side; the position of the opening is marked on the right side by the spinocostal hiatus. (After Keith.) (See enlarged image)

II. Osteology

THE GENERAL framework of the body is built up mainly of a series of bones, supplemented, however, in certain regions by pieces of cartilage; the bony part of the framework constitutes the skeleton. 1
In the skeleton of the adult there are 206 distinct bones, as follows:— 2
Axial
Skeleton Vertebral column 26

Skull 22
Hyoid bone 1
Ribs and sternum 25

— 74
Appendicular
Skeleton Upper extremities 64

Lower extremities 62

— 126
Auditory ossicles

6

—

Total
206

The patellæ are included in this enumeration, but the smaller sesamoid bones are not reckoned. 3
Bones are divisible into four classes: Long, Short, Flat, and Irregular. 4

Long Bones.—The long bones are found in the limbs, and each consists of a body or shaft and two extremities. The body, or diaphysis is cylindrical, with a central cavity termed the medullary canal; the wall consists of dense, compact tissue of considerable thickness in the middle part of the body, but becoming thinner toward the extremities; within the medullary canal is some cancellous tissue, scanty in the middle of the body but greater in amount toward the ends. The extremities are generally expanded, for the purposes of articulation and to afford broad surfaces for muscular attachment. They are usually developed from separate centers of ossification termed epiphyses, and consist of cancellous tissue surrounded by thin compact bone. The medullary canal and the spaces in the cancellous tissue are filled with marrow. The long bones are not straight, but curved, the curve generally taking place in two planes, thus affording greater strength to the bone. The bones belonging to this class are: the clavicle, humerus, radius, ulna, femur, tibia, fibula, metacarpals, metatarsals, and phalanges. 5

Short Bones.—Where a part of the skeleton is intended for strength and compactness combined with limited movement, it is constructed of a number of short bones, as in the carpus and tarsus. These consist of cancellous tissue covered by a thin crust of compact substance. The patellæ, together with the other sesamoid bones, are by some regarded as short bones. 6

Flat Bones.—Where the principal requirement is either extensive protection or the provision of broad surfaces for muscular attachment, the bones are expanded into broad, flat plates, as in the skull and the scapula. These bones are composed of two thin layers of compact tissue enclosing between them a variable quantity of cancellous tissue. In the cranial bones, the layers of compact tissue are familiarly known as the tables of the skull; the outer one is thick and tough; the inner is thin, dense, and brittle, and hence is termed the vitreous table. The intervening cancellous tissue is called the diploë, and this, in certain regions of the skull, becomes absorbed so as to leave spaces filled with air (air-sinuses) between the two tables. The flat bones are: the occipital, parietal, frontal, nasal, lacrimal, vomer, scapula, os coxæ (hip bone), sternum, ribs, and, according to some, the patella. 7

Irregular Bones.—The irregular bones are such as, from their peculiar form, cannot be grouped under the preceding heads. They consist of cancellous tissue enclosed within a thin layer of compact bone. The irregular bones are: the vertebræ, sacrum, coccyx, temporal, sphenoid, ethmoid, zygomatic, maxilla, mandible, palatine, inferior nasal concha, and hyoid. 8

Surfaces of Bones.—If the surface of a bone be examined, certain eminences and depressions are seen. These eminences and depressions are of two kinds: articular and non-articular. Well-marked examples of articular eminences are found in the heads of the humerus and femur; and of articular depressions in the glenoid cavity of the scapula, and the acetabulum of the hip bone. Non-articular eminences are designated according to their form. Thus, a broad, rough, uneven elevation is called a tuberosity, protuberance, or process, a small, rough prominence, a tubercle; a sharp, slender pointed eminence, a spine; a narrow, rough elevation, running some way along the surface, a ridge, crest, or line. Non-articular depressions are also of variable form, and are described as fossæ, pits, depressions, grooves, furrows, fissures, notches, etc. These non-articular eminences and depressions serve to increase the extent of surface for the attachment of ligaments and muscles, and are usually well-marked in proportion to the muscularity of the subject. A short perforation is called a foramen, a longer passage a canal. 9

1. Development of the Skeleton

The Skeleton.—The skeleton is of mesodermal origin, and may be divided into (a) that of the trunk (axial skeleton), comprising the vertebral column, skull, ribs, and sternum, and (b) that of the limbs (appendicular skeleton). 10

The Vertebral Column.—The notochord (Fig. 19) is a temporary structure and forms a central axis, around which the segments of the vertebral column are developed. 11 It is derived from the entoderm, and consists of a rod of cells, which lies on the ventral aspect of the neural tube and reaches from the anterior end of the mid-brain to the extremity of the tail. On either side of it is a column of paraxial mesoderm which becomes subdivided into a number of more or less cubical segments, the primitive segments (Figs. 19 and 20). These are separated from one another by intersegmental septa and are arranged symmetrically on either side of the neural tube and notochord: to every segment a spinal nerve is distributed. At first each segment contains a central cavity, the myocœl, but this is soon filled with a core of angular and spindle-shaped cells. The cells of the segment become differentiated into three groups, which form respectively the cutis-plate or dermatome, the muscle-plate or myotome, and the sclerotome (Fig. 64). The cutis-plate is placed on the lateral and dorsal aspect of the myocœl, and from it the true skin of the corresponding segment is derived; the muscle-plate is situated on the medial side of the cutis-plate and furnishes the muscles of the segment. The cells of the sclerotome are largely derived from those forming the core of the myocœl, and lie next the notochord. Fusion of the individual sclerotomes in an antero-posterior direction soon takes place, and thus a continuous strand of cells, the sclerotogenous layer, is formed along the ventro-lateral aspects of the neural tube. The cells of this layer proliferate rapidly, and extending medialward surround the notochord; at the same time they grow backward on the lateral aspects of the neural tube and eventually surround it, and thus the notochord and neural tube are enveloped by a continuous sheath of mesoderm, which is termed the membranous vertebral column. In this mesoderm the original segments are still distinguishable, but each is now differentiated into two portions, an anterior, consisting of loosely arranged cells, and a posterior, of more condensed tissue (Fig. 65, A and B). Between the two portions the rudiment of the intervertebral fibrocartilage is laid down (Fig. 65, C). Cells from the posterior mass grow into the intervals between the myotomes (Fig. 65, B and C) of the corresponding and succeeding segments, and extend both dorsally and ventrally; the dorsal extensions surround the neural tube and represent the future vertebral arch, while the ventral extend into the body-wall as the costal processes. The hinder part of the posterior mass joins the anterior mass of the succeeding segment to form the vertebral body. Each vertebral body is therefore a composite of two segments, being formed from the posterior portion of one segment and the anterior part of that immediately behind it. The vertebral and costal arches are derivatives of the posterior part of the segment in front of the intersegmental septum with which they are associated. 11

FIG. 64– Transverse section of a human embryo of the third week to show the differentiation of the primitive segment. (Kollmann.) ao. Aorta. m.p. Muscle-plate. n.c. Neural canal. sc. Sclerotome. s.p. cutis-plate. (See enlarged image)

FIG. 65– Scheme showing the manner in which each vertebral centrum is developed from portions of two adjacent segments. (See enlarged image)

This stage is succeeded by that of the cartilaginous vertebral column. In the fourth week two cartilaginous centers make their appearance, one on either side of the notochord; these extend around the notochord and form the body of the cartilaginous vertebra. A second pair of cartilaginous foci appear in the lateral parts of the vertebral bow, and grow backward on either side of the neural tube to form the cartilaginous vertebral arch, and a separate cartilaginous center appears for each costal process. By the eighth week the cartilaginous arch has fused with the body, and in the fourth month the two halves of the arch are joined on the dorsal aspect of the neural tube. The spinous process is developed from the junction of the two halves of the vertebral arch. The transverse process grows out from the vertebral arch behind the costal process. 12
In the upper cervical vertebræ a band of mesodermal tissue connects the ends of the vertebral arches across the ventral surfaces of the intervertebral fibrocartilages. This is termed the hypochordal bar or brace; in all except the first it is transitory and disappears by fusing with the fibrocartilages. In the atlas, however, the entire bow persists and undergoes chondrification; it develops into the anterior arch of the bone, while the cartilage representing the body of the atlas forms the dens or odontoid process which fuses with the body of the second cervical vertebra. 13

FIG. 66– Sagittal section through an intervertebral fibrocartilage and adjacent parts of two vertebræ of an advanced sheep’s embryo. (Kölliker.) (See enlarged image)

The portions of the notochord which are surrounded by the bodies of the vertebræ atrophy, and ultimately disappear, while those which lie in the centers of the intervertebral fibrocartilages undergo enlargement, and persist throughout life as the central nucleus pulposus of the fibrocartilages (Fig. 66). 14

The Ribs.—The ribs are formed from the ventral or costal processes of the primitive vertebral bows, the processes extending between the muscle-plates. In the thoracic region of the vertebral column the costal processes grow lateralward to form a series of arches, the primitive costal arches. As already described, the transverse process grows out behind the vertebral end of each arch. It is at first connected to the costal process by continuous mesoderm, but this becomes differentiated later to form the costotransverse ligament; between the costal process and the tip of the transverse process the costotransverse joint is formed by absorption. The costal process becomes separated from the vertebral bow by the development of the costocentral joint. In the cervical vertebrœ (Fig. 67) the transverse process forms the posterior boundary of the foramen transversarium, while the costal process corresponding to the head and neck of the rib fuses with the body of the vertebra, and forms the antero-lateral boundary of the foramen. The distal portions of the primitive costal arches remain undeveloped; occasionally the arch of the seventh cervical vertebra undergoes greater development, and by the formation of costovertebral joints is separated off as a rib. In the lumbar region the distal portions of the primitive costal arches fail; the proximal portions fuse with the transverse processes to form the transverse processes of descriptive anatomy. Occasionally a movable rib is developed in connection with the first lumbar vertebra. In the sacral region costal processes are developed only in connection with the upper three, or it may be four, vertebræ the processes of adjacent segments fuse with one another to form the lateral parts of the sacrum. The coccygeal vertebrœ are devoid of costal processes. 15

FIG. 67– Diagrams showing the portions of the adult vertebræ derived respectively from the bodies, vertebral arches, and costal processes of the embryonic vertebræ. The bodies are represented in yellow, the vertebral arches in red, and the costal processes in blue. (See enlarged image)

The Sternum.—The ventral ends of the ribs become united to one another by a longitudinal bar termed the sternal plate, and opposite the first seven pairs of ribs these sternal plates fuse in the middle line to form the manubrium and body of the sternum. The xiphoid process is formed by a backward extension of the sternal plates. 16

The Skull.—Up to a certain stage the development of the skull corresponds with that of the vertebral column; but it is modified later in association with the expansion of the brain-vesicles, the formation of the organs of smell, sight, and hearing, and the development of the mouth and pharynx. 17
The notochord extends as far forward as the anterior end of the mid-brain, and becomes partly surrounded by mesoderm (Fig. 68). The posterior part of this mesodermal investment corresponds with the basilar part of the occipital bone, and shows a subdivision into four segments, which are separated by the roots of the hypoglossal nerve. The mesoderm then extends over the brain-vesicles, and thus the entire brain is enclosed by a mesodermal investment, which is termed the membranous cranium. From the inner layer of this the bones of the skull and the membranes of the brain are developed; from the outer layer the muscles, bloodvessels, true skin, and subcutaneous tissues of the scalp. In the shark and dog-fish this membranous cranium undergoes complete chondrification, and forms the cartilaginous skull or chondrocranium of these animals. In mammals, on the other hand, the process of chondrification is limited to the base of the skull—the roof and sides being covered in by membrane. Thus the bones of the base of the skull are preceded by cartilage, those of the roof and sides by membrane. The posterior part of the base of the skull is developed around the notochord, and exhibits a segmented condition analogous to that of the vertebral column, while the anterior part arises in front of the notochord and shows no regular segmentation. The base of the skull may therefore be divided into (a) a chordal or vertebral, and (b) a prechordal or prevertebral portion. 18

FIG. 68– Sagittal section of cephalic end of notochord. (Keibel.) (See enlarged image)

FIG. 69– Diagrams of the cartilaginous cranium. (Wiedersheim.) (See enlarged image)

In the lower vertebrates two pairs of cartilages are developed, viz., a pair of parachordal cartilages, one on either side of the notochord; and a pair of prechordal cartilages, the trabeculæ cranii, in front of the notochord (Fig. 66). The parachordal cartilages (Fig. 69) unite to form a basilar plate, from which the cartilaginous part of the occipital bone and the basi-sphenoid are developed. On the lateral aspects of the parachordal cartilages the auditory vesicles are situated, and the mesoderm enclosing them is soon converted into cartilage, forming the cartilaginous ear-capsules. These cartilaginous ear-capsules, which are of an oval shape, fuse with the sides of the basilar plate, and from them arise the petrous and mastoid portions of the temporal bones. The trabeculæ cranii (Fig. 69) are two curved bars of cartilage which embrace the hypophysis cerebri; their posterior ends soon unite with the basilar plate, while their anterior ends join to form the ethmoidal plate, which extends forward between the fore-brain and the olfactory pits. Later the trabeculæ meet and fuse below the hypophysis, forming the floor of the fossa hypophyseos and so cutting off the anterior lobe of the hypophysis from the stomodeum. The median part of the ethmoidal plate forms the bony and cartilaginous parts of the nasal septum. From the lateral margins of the trabeculæ cranii three processes grow out on either side. The anterior forms the ethmoidal labyrinth and the lateral and alar cartilages of the nose; the middle gives rise to the small wing of the sphenoid, while from the posterior the great wing and lateral pterygoid plate of the sphenoid are developed (Figs. 70, 71). The bones of the vault are of membranous formation, and are termed dermal or covering bones. They are partly developed from the mesoderm of the membranous cranium, and partly from that which lies outside the entoderm of the foregut. They comprise the upper part of the occipital squama (interparietal), the squamæ and tympanic parts of the temporals, the parietals, the frontal, the vomer, the medial pterygoid plates, and the bones of the face. Some of them remain distinct throughout life, e.g., parietal and frontal, while others join with the bones of the chondrocranium, e.g., interparietal, squamæ of temporals, and medial pterygoid plates. 19

FIG. 70– Model of the chondrocranium of a human embryo, 8 cm. long. (Hertwig.) The membrane bones are not represented. (See enlarged image)

Recent observations have shown that, in mammals, the basi-cranial cartilage, both in the chordal and prechordal regions of the base of the skull, is developed as a single plate which extends from behind forward. In man, however, its posterior part shows an indication of being developed from two chondrifying centers which fuse rapidly in front and below. The anterior and posterior thirds of the cartilage surround the notochord, but its middle third lies on the dorsal aspect of the notochord, which in this region is placed between the cartilage and the wall of the pharynx.

FIG. 71– The same model as shown in Fig. 70 from the left side. Certain of the membrane bones of the right side are represented in yellow. (Hertwig.) (See enlarged image)

Note 11. In the amphioxus the notochord persists and forms the only representative of a skeleton in that animal. [back]

2. Bone

Structure and Physical Properties.—Bone is one of the hardest structures of the animal body; it possesses also a certain degree of toughness and elasticity. Its color, in a fresh state, is pinkish-white externally, and deep red within. On examining a section of any bone, it is seen to be composed of two kinds of tissue, one of which is dense in texture, like ivory, and is termed compact tissue; the other consists of slender fibers and lamellæ, which join to form a reticular structure; this, from its resemblance to lattice-work, is called cancellous tissue. The compact tissue is always placed on the exterior of the bone, the cancellous in the interior. The relative quantity of these two kinds of tissue varies in different bones, and in different parts of the same bone, according as strength or lightness is requisite. Close examination of the compact tissue shows it to be extremely porous, so that the difference in structure between it and the cancellous tissue depends merely upon the different amount of solid matter, and the size and number of spaces in each; the cavities are small in the compact tissue and the solid matter between them abundant, while in the cancellous tissue the spaces are large and the solid matter is in smaller quantity.

Bone during life is permeated by vessels, and is enclosed, except where it is coated with articular cartilage, in a fibrous membrane, the periosteum, by means of which many of these vessels reach the hard tissue. If the periosteum be stripped from the surface of the living bone, small bleeding points are seen which mark the entrance of the periosteal vessels; and on section during life every part of the bone exudes blood from the minute vessels which ramify in it. The interior of each of the long bones of the limbs presents a cylindrical cavity filled with marrow and lined by a highly vascular areolar structure, called the medullary membrane.

THE STRENGTH OF BONE COMPARED WITH OTHER MATERIALS Substance. Weight in pounds per cubic foot. Ultimate strength. Pounds per square inch.
Tension. Compression. Shear.
Medium steel 490 65,000 60,000 40,000
Granite 170 1,500 15,000 2,000
Oak, white 46 12,500 12 7,000 4,000 13
Compact bone (low) 119 13,200 18,000 11,800
Compact bone (high) …… 17,700 24,000 7,150

Periosteum.—The periosteum adheres to the surface of each of the bones in nearly every part, but not to cartilaginous extremities. When strong tendons or ligaments are attached to a bone, the periosteum is incorporated with them. It consists of two layers closely united together, the outer one formed chiefly of connective tissue, containing occasionally a few fat cells; the inner one, of elastic fibers of the finer kind, forming dense membranous networks, which again can be separated into several layers. In young bones the periosteum is thick and very vascular, and is intimately connected at either end of the bone with the epiphysial cartilage, but less closely with the body of the bone, from which it is separated by a layer of soft tissue, containing a number of granular corpuscles or osteoblasts, by which ossification proceeds on the exterior of the young bone. Later in life the periosteum is thinner and less vascular, and the osteoblasts are converted into an epithelioid layer on the deep surface of the periosteum. The periosteum serves as a nidus for the ramification of the vessels previous to their distribution in the bone; hence the liability of bone to exfoliation or necrosis when denuded of this membrane by injury or disease. Fine nerves and lymphatics, which generally accompany the arteries, may also be demonstrated in the periosteum.

Marrow.—The marrow not only fills up the cylindrical cavities in the bodies of the long bones, but also occupies the spaces of the cancellous tissue and extends into the larger bony canals (Haversian canals) which contain the bloodvessels. It differs in composition in different bones. In the bodies of the long bones the marrow is of a yellow color, and contains, in 100 parts, 96 of fat, 1 of areolar tissue and vessels, and 3 of fluid with extractive matter; it consists of a basis of connective tissue supporting numerous bloodvessels and cells, most of which are fat cells but some are “marrow cells,” such as occur in the red marrow to be immediately described. In the flat and short bones, in the articular ends of the long bones, in the bodies of the vertebræ, in the cranial diploë, and in the sternum and ribs the marrow is of a red color, and contains, in 100 parts, 75 of water, and 25 of solid matter consisting of cell-globulin, nucleoprotein, extractives, salts, and only a small proportion of fat. The red marrow consists of a small quantity of connective tissue, bloodvessels, and numerous cells (Fig. 72), some few of which are fat cells, but the great majority are roundish nucleated cells, the true “marrow cells” of Kölliker. These marrow cells proper, or myelocytes, resemble in appearance lymphoid corpuscles, and like them are ameboid; they generally have a hyaline protoplasm, though some show granules either oxyphil or basophil in reaction. A number of eosinophil cells are also present. Among the marrow cells may be seen smaller cells, which possess a slightly pinkish hue; these are the erythroblasts or normoblasts, from which the red corpuscles of the adult are derived, and which may be regarded as descendants of the nucleated colored corpuscles of the embryo. Giant cells (myeloplaxes, osteoclasts), large, multinucleated, protoplasmic masses, are also to be found in both sorts of adult marrow, but more particularly in red marrow. They were believed by Kölliker to be concerned in the absorption of bone matrix, and hence the name which he gave to them—osteoclasts. They excavate in the bone small shallow pits or cavities, which are named Howship’s foveolæ, and in these they are found lying. 4

FIG. 72– Human bone marrow. Highly magnified. (See enlarged image)

Vessels and Nerves of Bone.—The bloodvessels of bone are very numerous. Those of the compact tissue are derived from a close and dense network of vessels ramifying in the periosteum. From this membrane vessels pass into the minute orifices in the compact tissue, and run through the canals which traverse its substance. The cancellous tissue is supplied in a similar way, but by less numerous and larger vessels, which, perforating the outer compact tissue, are distributed to the cavities of the spongy portion of the bone. In the long bones, numerous apertures may be seen at the ends near the articular surfaces; some of these give passage to the arteries of the larger set of vessels referred to; but the most numerous and largest apertures are for some of the veins of the cancellous tissue, which emerge apart from the arteries. The marrow in the body of a long bone is supplied by one large artery (or sometimes more), which enters the bone at the nutrient foramen (situated in most cases near the center of the body), and perforates obliquely the compact structure. The medullary or nutrient artery, usually accompanied by one or two veins, sends branches upward and downward, which ramify in the medullary membrane, and give twigs to the adjoining canals. The ramifications of this vessel anastomose with the arteries of the cancellous and compact tissues. In most of the flat, and in many of the short spongy bones, one or more large apertures are observed, which transmit to the central parts of the bone vessels corresponding to the nutrient arteries and veins. The veins emerge from the long bones in three places (Kölliker): (1) one or two large veins accompany the artery; (2) numerous large and small veins emerge at the articular extremities; (3) many small veins pass out of the compact substance. In the flat cranial bones the veins are large, very numerous, and run in tortuous canals in the diploic tissue, the sides of the canals being formed by thin lamellæ of bone, perforated here and there for the passage of branches from the adjacent cancelli. The same condition is also found in all cancellous tissue, the veins being enclosed and supported by osseous material, and having exceedingly thin coats. When a bone is divided, the vessels remain patulous, and do not contract in the canals in which they are contained. Lymphatic vessels, in addition to those found in the periosteum, have been traced by Cruikshank into the substance of bone, and Klein describes them as running in the Haversian canals. Nerves are distributed freely to the periosteum, and accompany the nutrient arteries into the interior of the bone. They are said by Kölliker to be most numerous in the articular extremities of the long bones, in the vertebræ, and in the larger flat bones. 5

FIG. 73– Transverse section of compact tissue bone. Magnified. (Sharpey.) (See enlarged image)

Minute Anatomy.—A transverse section of dense bone may be cut with a saw and ground down until it is sufficiently thin. 6
If this be examined with a rather low power the bone will be seen to be mapped out into a number of circular districts each consisting of a central hole surrounded by a number of concentric rings. These districts are termed Haversian systems; the central hole is an Haversian canal, and the rings are layers of bony tissue arranged concentrically around the central canal, and termed lamellæ. Moreover, on closer examination it will be found that between these lamellæ, and therefore also arranged concentrically around the central canal, are a number of little dark spots, the lacunæ, and that these lacunæ are connected with each other and with the central Haversian canal by a number of fine dark lines, which radiate like the spokes of a wheel and are called canaliculi. Filling in the irregular intervals which are left between these circular systems are other lamellæ, with their lacunæ and canaliculi running in various directions, but more or less curved (Fig. 73); they are termed interstitial lamellæ. Again, other lamellæ, found on the surface of the bone, are arranged parallel to its circumference; they are termed circumferential, or by some authors primary or fundamental lamellæ, to distinguish them from those laid down around the axes of the Haversian canals, which are then termed secondary or special lamellæ. 7
The Haversian canals, seen in a transverse section of bone as round holes at or about the center of each Haversian system, may be demonstrated to be true canals if a longitudinal section be made (Fig. 74). It will then be seen that the canals run parallel with the longitudinal axis of the bone for a short distance and then branch and communicate. They vary considerably in size, some being as much as 0.12 mm. in diameter; the average size is, however, about 0.05 mm. Near the medullary cavity the canals are larger than those near the surface of the bone. Each canal contains one or two bloodvessels, with a small quantity of delicate connective tissue and some nerve filaments. In the larger ones there are also lymphatic vessels, and cells with branching processes which communicate, through the canalculi, with the branched processes of certain bone cells in the substance of the bone. Those canals near the surface of the bone open upon it by minute orifices, and those near the medullary cavity open in the same way into this space, so that the whole of the bone is permeated by a system of bloodvessels running through the bony canals in the centers of the Haversian systems. 8
The lamellæ are thin plates of bony tissue encircling the central canal, and may be compared, for the sake of illustration, to a number of sheets of paper pasted one over another around a central hollow cylinder. After macerating a piece of bone in dilute mineral acid, these lamellæ may be stripped off in a longitudinal direction as thin films. If one of these be examined with a high power of the microscope, it will be found to be composed of a finely reticular structure, made up of very slender transparent fibers, decussating obliquely; and coalescing at the points of intersection; these fibers are composed of fine fibrils identical with those of white connective tissue. The intercellular matrix between the fibers is impregnated by calcareous deposit which the acid dissolves. In many places the various lamellæ may be seen to be held together by tapering fibers, which run obliquely through them, pinning or bolting them together; they were first described by Sharpey, and were named by him perforating fibers (Fig. 75). 9

FIG. 74– Section parallel to the surface from the body of the femur. X 100. a, Haversian canals; b, lacunæ seen from the side; c, others seen from the surface in lamellæ, which are cut horizontally. (See enlarged image)

FIG. 75– Perforating fibers, human parietal bone, decalcified. (H. Müller.) a, perforating fibers in situ; b, fibres drawn out of their sockets; c, sockets. (See enlarged image)

The Lacunæ are situated between the lamellæ, and consist of a number of oblong spaces. In an ordinary microscopic section, viewed by transmitted light, they appear as fusiform opaque spots. Each lacuna is occupied during life by a branched cell, termed a bone-cell or bone-corpuscle, the processes from which extend into the canaliculi (Fig. 76). 10
The Canaliculi are exceedingly minute channels, crossing the lamellæ and connecting the lacunæ with neighboring lacunæ and also with the Haversian canal. From the Haversian canal a number of canaliculi are given off, which radiate from it, and open into the first set of lacunæ between the first and second lamellæ. From these lacunæ a second set of canaliculi is given off; these run outward to the next series of lacunæ, and so on until the periphery of the Haversian system is reached; here the canaliculi given off from the last series of lacunæ do not communicate with the lacunæ of neighboring Haversian systems, but after passing outward for a short distance form loops and return to their own lacunæ. Thus every part of an Haversian system is supplied with nutrient fluids derived from the vessels in the Haversian canal and distributed through the canaliculi and lacunæ. 11
The bone cells are contained in the lacunæ, which, however, they do not completely fill. They are flattened nucleated branched cells, homologous with those of connective tissue; the branches, especially in young bones, pass into the canaliculi from the lacunæ. 12
In thin plates of bone (as in the walls of the spaces of cancellous tissue) the Haversian canals are absent, and the canaliculi open into the spaces of the cancellous tissue (medullary spaces), which thus have the same function as the Haversian canals. 13

FIG. 76– Nucleated bone cells and their processes, contained in the bone lacunæ and their canaliculi respectively. From a section through the vertebra of an adult mouse. (Klein and Noble Smith.) (See enlarged image)

Chemical Composition.—Bone consists of an animal and an earthy part intimately combined together. 14
The animal part may be obtained by immersing a bone for a considerable time in dilute mineral acid, after which process the bone comes out exactly the same shape as before, but perfectly flexible, so that a long bone (one of the ribs, for example) can easily be tied in a knot. If now a transverse section is made (Fig. 77) the same general arrangement of the Haversian canals, lamellæ, lacunæ, and canaliculi is seen. 15
The earthy part may be separately obtained by calcination, by which the animal matter is completely burnt out. The bone will still retain its original form, but it will be white and brittle, will have lost about one-third of its original weight, and will crumble down with the slightest force. The earthy matter is composed chiefly of calcium phosphate, about 58 per cent. of the weight of the bone, calcium carbonate about 7 per cent., calcium fluoride and magnesium phosphate from 1 to 2 per cent. each and sodium chloride less than 1 per cent.; they confer on bone its hardness and rigidity, while the animal matter (ossein) determines its tenacity. 16

Ossification.—Some bones are preceded by membrane, such as those forming the roof and sides of the skull; others, such as the bones of the limbs, are preceded by rods of cartilage. Hence two kinds of ossification are described: the intramembranous and the intracartilaginous. 17
INTRAMEMBRANOUS OSSIFICATION.—In the case of bones which are developed in membrane, no cartilaginous mould precedes the appearance of the bony tissue. The membrane which occupies the place of the future bone is of the nature of connective tissue, and ultimately forms the periosteum; it is composed of fibers and granular cells in a matrix. The peripheral portion is more fibrous, while, in the interior the cells or osteoblasts predominate; the whole tissue is richly supplied with blood vessels. At the outset of the process of bone formation a little network of spicules is noticed radiating from the point or center of ossification. These rays consist at their growing points of a network of fine clear fibers and granular corpuscles with an intervening ground substance (Fig. 78). The fibers are termed osteogenetic fibers, and are made up of fine fibrils differing little from those of white fibrous tissue. The membrane soon assumes a dark and granular appearance from the deposition of calcareous granules in the fibers and in the intervening matrix, and in the calcified material some of the granular corpuscles or osteoblasts are enclosed. By the fusion of the calcareous granules the tissue again assumes a more transparent appearance, but the fibers are no longer so distinctly seen. The involved osteoblasts from the corpuscles of the future bone, the spaces in which they are enclosed constituting the lacunæ. As the osteogenetic fibers grow out to the periphery they continue to calcify, and give rise to fresh bone spicules. Thus a network of bone is formed, the meshes of which contain the bloodvessels and a delicate connective tissue crowded with osteoblasts. The bony trabeculæ thicken by the addition of fresh layers of bone formed by the osteoblasts on their surface, and the meshes are correspondingly encroached upon. Subsequently successive layers of bony tissue are deposited under the periosteum and around the larger vascular channels which become the Haversian canals, so that the bone increases much in thickness. 18

FIG. 77– Transverse section of body of human fibula, decalcified. X 250. (See enlarged image)

FIG. 78– Part of the growing edge of the developing parietal bone of a fetal cat. (After J. Lawrence.) (See enlarged image)

INTERCARTILAGINOUS OSSIFICATION.—Just before ossification begins the mass is entirely cartilaginous, and in a long bone, which may be taken as an example, the process commences in the center and proceeds toward the extremities, which for some time remain cartilaginous. Subsequently a similar process commences in one or more places in those extremities and gradually extends through them. The extremities do not, however, become joined to the body of the bone by bony tissue until growth has ceased; between the body and either extremity a layer of cartilaginous tissue termed the epiphysial cartilage persists for a definite period. 19
The first step in the ossification of the cartilage is that the cartilage cells, at the point where ossification is commencing and which is termed a center of ossification, enlarge and arrange themselves in rows (Fig. 79). The matrix in which they are imbedded increases in quantity, so that the cells become further separated from each other. A deposit of calcareous material now takes place in this matrix, between the rows of cells, so that they become separated from each other by longitudinal columns of calcified matrix, presenting a granular and opaque appearance. Here and there the matrix between two cells of the same row also becomes calcified, and transverse bars of calcified substance stretch across from one calcareous column to another. Thus there are longitudinal groups of the cartilage cells enclosed in oblong cavities, the walls of which are formed of calcified matrix which cuts off all nutrition from the cells; the cells, in consequence, atrophy, leaving spaces called the primary areolæ. 20

FIG. 79– Section of fetal bone of cat. ir. Irruption of the subperiosteal tissue. p. Fibrous layer of the periosteum. o. Layer of osteoblasts. im. Subperiosteal bony deposit. (From Quain’s “Anatomy,” E. A. Schäfer.) (See enlarged image)

At the same time that this process is going on in the center of the solid bar of cartilage, certain changes are taking place on its surface. This is covered by a very vascular membrane, the perichondrium, entirely similar to the embryonic connective tissue already described as constituting the basis of membrane bone; on the inner surface of this—that is to say, on the surface in contact with the cartilage—are gathered the formative cells, the osteoblasts. By the agency of these cells a thin layer of bony tissue is formed between the perichondrium and the cartilage, by the intramembranous mode of ossification just described. There are then, in this first stage of ossification, two processes going on simultaneously: in the center of the cartilage the formation of a number of oblong spaces, formed of calcified matrix and containing the withered cartilage cells, and on the surface of the cartilage the formation of a layer of true membrane bone. The second stage consists in the prolongation into the cartilage of processes of the deeper or osteogenetic layer of the perichondrium, which has now become periosteum (Fig. 79, ir). The processes consist of bloodvessels and cells—osteoblasts, or bone-formers, and osteoclasts, or bone-destroyers. The latter are similar to the giant cells (myeloplaxes) found in marrow, and they excavate passages through the new-formed bony layer by absorption, and pass through it into the calcified matrix (Fig. 80). Wherever these processes come in contact with the calcified walls of the primary areolæ they absorb them, and thus cause a fusion of the original cavities and the formation of larger spaces, which are termed the secondary areolæ or medullary spaces. These secondary spaces become filled with embryonic marrow, consisting of osteoblasts and vessels, derived, in the manner described above, from the osteogenetic layer of the periosteum (Fig. 80). 21
Thus far there has been traced the formation of enlarged spaces (secondary areolæ), the perforated walls of which are still formed by calcified cartilage matrix, containing an embryonic marrow derived from the processes sent in from the osteogenetic layer of the periosteum, and consisting of bloodvessels and osteoblasts. The walls of these secondary areolæ are at this time of only inconsiderable thickness, but they become thickened by the deposition of layers of true bone on their surface. This process takes place in the following manner: Some of the osteoblasts of the embryonic marrow, after undergoing rapid division, arrange themselves as an epithelioid layer on the surface of the wall of the space (Fig. 81). This layer of osteoblasts forms a bony stratum, and thus the wall of the space becomes gradually covered with a layer of true osseous substance in which some of the bone-forming cells are included as bone corpuscles. The next stage in the process consists in the removal of these primary bone spicules by the osteoclasts. One of these giant cells may be found lying in a Howship’s foveola at the free end of each spicule. The removal of the primary spicules goes on pari passu with the formation of permanent bone by the periosteum, and in this way the medullary cavity of the body of the bone is formed. 22

FIG. 80– Part of a longitudinal section of the developing femur of a rabbit. a. Flattened cartilage cells. b. Enlarged cartilage cells. c, d. Newly formed bone. e. Osteoblasts. f. Giant cells or osteoclasts. g, h. Shrunken cartilage cells. (From “Atlas of Histology,” Klein and Noble Smith.) (See enlarged image)

FIG. 81– Osteoblasts and osteoclasts on trabecula of lower jaw of calf embryo. (Kölliker.) (See enlarged image)

This series of changes has been gradually proceeding toward the end of the body of the bone, so that in the ossifying bone all the changes described above may be seen in different parts, from the true bone at the center of the body to the hyaline cartilage at the extremities. 23
While the ossification of the cartilaginous body is extending toward the articular ends, the cartilage immediately in advance of the osseous tissue continues to grow until the length of the adult bone is reached. 24
During the period of growth the articular end, or epiphysis, remains for some time entirely cartilaginous, then a bony center appears, and initiates in it the process of intracartilaginous ossification; but this process never extends to any great distance. The epiphysis remains separated from the body by a narrow cartilaginous layer for a definite time. This layer ultimately ossifies, the distinction between body and epiphysis is obliterated, and the bone assumes its completed form and shape. The same remarks also apply to such processes of bone as are separately ossified, e.g., the trochanters of the femur. The bones therefore continue to grow until the body has acquired its full stature. They increase in length by ossification continuing to extend behind the epiphysial cartilage, which goes on growing in advance of the ossifying process. They increase in circumference by deposition of new bone, from the deeper layer of the periosteum, on their external surface, and at the same time an absorption takes place from within, by which the medullary cavities are increased. 25
The permanent bone formed by the periosteum when first laid down is cancellous in structure. Later the osteoblasts contained in its spaces become arranged in the concentric layers characteristic of the Haversian systems, and are included as bone corpuscles. 26
The number of ossific centers varies in different bones. In most of the short bones ossification commences at a single point near the center, and proceeds toward the surface. In the long bones there is a central point of ossification for the body or diaphysis: and one or more for each extremity, the epiphysis. That for the body is the first to appear. The times of union of the epiphyses with the body vary inversely with the dates at which their ossifications began (with the exception of the fibula) and regulate the direction of the nutrient arteries of the bones. Thus, the nutrient arteries of the bones of the arm and forearm are directed toward the elbow, since the epiphyses at this joint become united to the bodies before those at the opposite extremities. In the lower limb, on the other hand, the nutrient arteries are directed away from the knee: that is, upward in the femur, downward in the tibia and fibula; and in them it is observed that the upper epiphysis of the femur, and the lower epiphyses of the tibia and fibula, unite first with the bodies. Where there is only one epiphysis, the nutrient artery is directed toward the other end of the bone; as toward the acromial end of the clavicle, toward the distal ends of the metacarpal bone of the thumb and the metatarsal bone of the great toe, and toward the proximal ends of the other metacarpal and metatarsal bones. 27
Parsons 14 groups epiphyses under three headings, viz.: (1) pressure epiphyses, appearing at the articular ends of the bones and transmitting “the weight of the body from bone to bone;” (2) traction epiphyses, associated with the insertion of muscles and “originally sesamoid structures though not necessarily sesamoid bones;” and (3) atavistic epiphyses, representing parts of the skeleton, which at one time formed separate bones, but which have lost their function, “and only appear as separate ossifications in early life.” 28
Note 12. Indicates stresses with the grain, i.e., when the load is parallel to the long axis of the material, or parallel to the direction of the fibers of the material. [back]
Note 13. Indicates unit-stresses across the grain, i.e., at right angles to the direction of the fibers of the material. [back]
Note 14. Jour. of Anat. and Phys., vols. xxxviii, xxxix, and xlii. [back]

3. The Vertebral Column

(Columna Vertebralis; Spinal Column).

The vertebral column is a flexuous and flexible column, formed of a series of bones called vertebræ.

The vertebræ are thirty-three in number, and are grouped under the names cervical, thoracic, lumbar, sacral, and coccygeal, according to the regions they occupy; there are seven in the cervical region, twelve in the thoracic, five in the lumbar, five in the sacral, and four in the coccygeal.

This number is sometimes increased by an additional vertebra in one region, or it may be diminished in one region, the deficiency often being supplied by an additional vertebra in another. The number of cervical vertebræ is, however, very rarely increased or diminished.

The vertebræ in the upper three regions of the column remain distinct throughout life, and are known as true or movable vertebræ; those of the sacral and coccygeal regions, on the other hand, are termed false or fixed vertebræ, because they are united with one another in the adult to form two bones—five forming the upper bone or sacrum, and four the terminal bone or coccyx.

With the exception of the first and second cervical, the true or movable vertebræ present certain common characteristics which are best studied by examining one from the middle of the thoracic region.

3a. General Characteristics of a Vertebra

A typical vertebra consists of two essential parts—viz., an anterior segment, the body, and a posterior part, the vertebral or neural arch; these enclose a foramen, the vertebral foramen. The vertebral arch consists of a pair of pedicles and a pair of laminæ, and supports seven processes—viz., four articular, two transverse, and one spinous.

FIG. 82– A typical thoracic vertebra, viewed from above.

When the vertebræ are articulated with each other the bodies form a strong pillar for the support of the head and trunk, and the vertebral foramina constitute a canal for the protection of the medulla spinalis (spinal cord), while between every pair of vertebræ are two apertures, the intervertebral foramina, one on either side, for the transmission of the spinal nerves and vessels.

Body (corpus vertebræ).—The body is the largest part of a vertebra, and is more or less cylindrical in shape. Its upper and lower surfaces are flattened and rough, and give attachment to the intervertebral fibrocartilages, and each presents a rim around its circumference. In front, the body is convex from side to side and concave from above downward. Behind, it is flat from above downward and slightly concave from side to side. Its anterior surface presents a few small apertures, for the passage of nutrient vessels; on the posterior surface is a single large, irregular aperture, or occasionally more than one, for the exit of the basi-vertebral veins from the body of the vertebra.

Pedicles (radices arci vertebræ).—The pedicles are two short, thick processes, which project backward, one on either side, from the upper part of the body, at the junction of its posterior and lateral surfaces. The concavities above and below the pedicles are named the vertebral notches; and when the vertebræ are articulated, the notches of each contiguous pair of bones form the intervertebral foramina, already referred to.

Laminæ.—The laminæ are two broad plates directed backward and medialward from the pedicles. They fuse in the middle line posteriorly, and so complete the posterior boundary of the vertebral foramen. Their upper borders and the lower parts of their anterior surfaces are rough for the attachment of the ligamenta flava.

Processes.—Spinous Process (processus spinosus).—The spinous process is directed backward and downward from the junction of the laminæ, and serves for the attachment of muscles and ligaments.

Articular Processes.—The articular processes, two superior and two inferior, spring from the junctions of the pedicles and laminæ. The superior project upward, and their articular surfaces are directed more or less backward; the inferior project downward, and their surfaces look more or less forward. The articular surfaces are coated with hyaline cartilage.

Transverse Processes (processus transversi).—The transverse processes, two in number, project one at either side from the point where the lamina joins the pedicle, between the superior and inferior articular processes. They serve for the attachment of muscles and ligaments.

Structure of a Vertebra (Fig. 83).—The body is composed of cancellous tissue, covered by a thin coating of compact bone; the latter is perforated by numerous orifices, some of large size for the passage of vessels; the interior of the bone is traversed by one or two large canals, for the reception of veins, which converge toward a single large, irregular aperture, or several small apertures, at the posterior part of the body. The thin bony lamellæ of the cancellous tissue are more pronounced in lines perpendicular to the upper and lower surfaces and are developed in response to greater pressure in this direction (Fig. 83). The arch and processes projecting from it have thick coverings of compact tissue.

FIG. 83– Sagittal section of a lumbar vertebra. (See enlarged image)

3a. 1. The Cervical Vertebræ

(Vertebræ Cervicales).

cervical vertebræ (Fig. 84) are the smallest of the true vertebræ, and can be readily distinguished from those of the thoracic or lumbar regions by the presence of a foramen in each transverse process. The first, second, and seventh present exceptional features and must be separately described; the following characteristics are common to the remaining four.

The body is small, and broader from side to side than from before backward. The anterior and posterior surfaces are flattened and of equal depth; the former is placed on a lower level than the latter, and its inferior border is prolonged downward, so as to overlap the upper and forepart of the vertebra below. The upper surface is concave transversely, and presents a projecting lip on either side; the lower surface is concave from before backward, convex from side to side, and presents laterally shallow concavities which receive the corresponding projecting lips of the subjacent vertebra. The pedicles are directed lateralward and backward, and are attached to the body midway between its upper and lower borders, so that the superior vertebral notch is as deep as the inferior, but it is, at the same time, narrower. The laminæ are narrow, and thinner above than below; the vertebral foramen is large, and of a triangular form. The spinous process is short and bifid, the two divisions being often of unequal size. The superior and inferior articular processes on either side are fused to form an articular pillar, which projects lateralward from the junction of the pedicle and lamina. The articular facets are flat and of an oval form: the superior look backward, upward, and slightly medialward: the inferior forward, downward, and slightly lateralward. The transverse processes are each pierced by the foramen transversarium, which, in the upper six vertebræ, gives passage to the vertebral artery and vein and a plexus of sympathetic nerves. Each process consists of an anterior and a posterior part. The anterior portion is the homologue of the rib in the thoracic region, and is therefore named the costal process or costal element: it arises from the side of the body, is directed lateralward in front of the foramen, and ends in a tubercle, the anterior tubercle. The posterior part, the true transverse process, springs from the vertebral arch behind the foramen, and is directed forward and lateralward; it ends in a flattened vertical tubercle, the posterior tubercle. These two parts are joined, outside the foramen, by a bar of bone which exhibits a deep sulcus on its upper surface for the passage of the corresponding spinal nerve. 15

FIG. 84– A cervical vertebra.

FIG. 85– Side view of a typical cervical vertebra.

First Cervical Vertebra.—The first cervical vertebra (Fig. 86) is named the atlas because it supports the globe of the head. Its chief peculiarity is that it has no body, and this is due to the fact that the body of the atlas has fused with that of the next vertebra. Its other peculiarities are that it has no spinous process, is ring-like, and consists of an anterior and a posterior arch and two lateral masses. The anterior arch forms about one-fifth of the ring: its anterior surface is convex, and presents at its center the anterior tubercle for the attachment of the Longus colli muscles; posteriorly it is concave, and marked by a smooth, oval or circular facet (fovea dentis), for articulation with the odontoid process (dens) of the axis. The upper and lower borders respectively give attachment to the anterior atlantooccipital membrane and the anterior atlantoaxial ligament; the former connects it with the occipital bone above, and the latter with the axis below. The posterior arch forms about two-fifths of the circumference of the ring: it ends behind in the posterior tubercle, which is the rudiment of a spinous process and gives origin to the Recti capitis posteriores minores. The diminutive size of this process prevents any interference with the movements between the atlas and the skull. The posterior part of the arch presents above and behind a rounded edge for the attachment of the posterior atlantoöccipital membrane, while immediately behind each superior articular process is a groove (sulcus arteriæ vertebralis), sometimes converted into a foramen by a delicate bony spiculum which arches backward from the posterior end of the superior articular process. This groove represents the superior vertebral notch, and serves for the transmission of the vertebral artery, which, after ascending through the foramen in the transverse process, winds around the lateral mass in a direction backward and medialward; it also transmits the suboccipital (first spinal) nerve. On the under surface of the posterior arch, behind the articular facets, are two shallow grooves, the inferior vertebral notches. The lower border gives attachment to the posterior atlantoaxial ligament, which connects it with the axis. The lateral masses are the most bulky and solid parts of the atlas, in order to support the weight of the head. Each carries two articular facets, a superior and an inferior. The superior facets are of large size, oval, concave, and approach each other in front, but diverge behind: they are directed upward, medialward, and a little backward, each forming a cup for the corresponding condyle of the occipital bone, and are admirably adapted to the nodding movements of the head. Not infrequently they are partially subdivided by indentations which encroach upon their margins. The inferior articular facets are circular in form, flattened or slightly convex and directed downward and medialward, articulating with the axis, and permitting the rotatory movements of the head. Just below the medial margin of each superior facet is a small tubercle, for the attachment of the transverse atlantal ligament which stretches across the ring of the atlas and divides the vertebral foramen into two unequal parts—the anterior or smaller receiving the odontoid process of the axis, the posterior transmitting the medulla spinalis and its membranes. This part of the vertebral canal is of considerable size, much greater than is required for the accommodation of the medulla spinalis, and hence lateral displacement of the atlas may occur without compression of this structure. The transverse processes are large; they project lateralward and downward from the lateral masses, and serve for the attachment of muscles which assist in rotating the head. They are long, and their anterior and posterior tubercles are fused into one mass; the foramen transversarium is directed from below, upward and backward.

FIG. 86– First cervical vertebra, or atlas.

FIG. 87– Second cervical vertebra, or epistropheus, from above.

Second Cervical Vertebra.—The second cervical vertebra (Fig. 87 and 88) is named the epistropheus or axis because it forms the pivot upon which the first vertebra, carrying the head, rotates. The most distinctive characteristic of this bone is the strong odontoid process which rises perpendicularly from the upper surface of the body. The body is deeper in front than behind, and prolonged downward anteriorly so as to overlap the upper and fore part of the third vertebra. It presents in front a median longitudinal ridge, separating two lateral depressions for the attachment of the Longus colli muscles. Its under surface is concave from before backward and covex from side to side. The dens or odontoid process exhibits a slight constriction or neck, where it joins the body. On its anterior surface is an oval or nearly circular facet for articulation with that on the anterior arch of the atlas. On the back of the neck, and frequently extending on to its lateral surfaces, is a shallow groove for the transverse atlantal ligament which retains the process in position. The apex is pointed, and gives attachment to the apical odontoid ligament; below the apex the process is somewhat enlarged, and presents on either side a rough impression for the attachment of the alar ligament; these ligaments connect the process to the occipital bone. The internal structure of the odontoid process is more compact than that of the body. The pedicles are broad and strong, especially in front, where they coalesce with the sides of the body and the root of the odontoid process. They are covered above by the superior articular surfaces. The laminæ are thick and strong, and the vertebral foramen large, but smaller than that of the atlas. The transverse processes are very small, and each ends in a single tubercle; each is perforated by the foramen transversarium, which is directed obliquely upward and lateralward. The superior articular surfaces are round, slightly convex, directed upward and lateralward, and are supported on the body, pedicles, and transverse processes. The inferior articular surfaces have the same direction as those of the other cervical vertebræ. The superior vertebral notches are very shallow, and lie behind the articular processes; the inferior lie in front of the articular processes, as in the other cervical vertebræ. The spinous process is large, very strong, deeply channelled on its under surface, and presents a bifid, tuberculated extremity.

FIG. 88– Second cervical vertebra, epistropheus, or axis, from the side.

FIG. 89– Seventh cervical vertebra.

The Seventh Cervical Vertebra (Fig. 89).—The most distinctive characteristic of this vertebra is the existence of a long and prominent spinous process, hence the name vertebra prominens. This process is thick, nearly horizontal in direction, not bifurcated, but terminating in a tubercle to which the lower end of the ligamentum nuchæ is attached. The transverse processes are of considerable size, their posterior roots are large and prominent, while the anterior are small and faintly marked; the upper surface of each has usually a shallow sulcus for the eighth spinal nerve, and its extremity seldom presents more than a trace of bifurcation. The foramen transversarium may be as large as that in the other cervical vertebræ, but is generally smaller on one or both sides; occasionally it is double, sometimes it is absent. On the left side it occasionally gives passage to the vertebral artery; more frequently the vertebral vein traverses it on both sides; but the usual arrangement is for both artery and vein to pass in front of the transverse process, and not through the foramen. Sometimes the anterior root of the transverse process attains a large size and exists as a separate bone, which is known as a cervical rib.

Note 15. The costal element of a cervical vertebra not only includes the portion which springs from the side of the body, but the anterior and posterior tubercles and the bar of bone which connects them (Fig. 67). [back]