3a. 2. The Thoracic Vertebræ

(Vertebræ Thoracales).

The thoracic vertebræ (Fig. 90) are intermediate in size between those of the cervical and lumbar regions; they increase in size from above downward, the upper vertebræ being much smaller than those in the lower part of the region.

They are distinguished by the presence of facets on the sides of the bodies for articulation with the heads of the ribs, and facets on the transverse processes of all, except the eleventh and twelfth, for articulation with the tubercles of the ribs.

FIG. 90– A thoracic vertebra.

The bodies in the middle of the thoracic region are heart-shaped, and as broad in the antero-posterior as in the transverse direction. At the ends of the thoracic region they resemble respectively those of the cervical and lumbar vertebræ. They are slightly thicker behind than in front, flat above and below, convex from side to side in front, deeply concave behind, and slightly constricted laterally and in front.

They present, on either side, two costal demi-facets, one above, near the root of the pedicle, the other below, in front of the inferior vertebral notch; these are covered with cartilage in the fresh state, and, when the vertebræ are articulated with one another, form, with the intervening intervertebral fibrocartilages, oval surfaces for the reception of the heads of the ribs.

The pedicles are directed backward and slightly upward, and the inferior vertebral notches are of large size, and deeper than in any other region of the vertebral column.

The laminæ are broad, thick, and imbricated—that is to say, they overlap those of subjacent vertebræ like tiles on a roof. The vertebral foramen is small, and of a circular form.

The spinous process is long, triangular on coronal section, directed obliquely downward, and ends in a tuberculated extremity. These processes overlap from the fifth to the eighth, but are less oblique in direction above and below.

The superior articular processes are thin plates of bone projecting upward from the junctions of the pedicles and laminæ; their articular facets are practically flat, and are directed backward and a little lateralward and upward.

The inferior articular processes are fused to a considerable extent with the laminæ, and project but slightly beyond their lower borders; their facets are directed forward and a little medialward and downward.

The transverse processes arise from the arch behind the superior articular processes and pedicles; they are thick, strong, and of considerable length, directed obliquely backward and lateralward, and each ends in a clubbed extremity, on the front of which is a small, concave surface, for articulation with the tubercle of a rib. The first, ninth, tenth, eleventh, and twelfth thoracic vertebræ present certain peculiarities, and must be specially considered (Fig. 91).

FIG. 91– Peculiar thoracic vertebræ.

The First Thoracic Vertebra has, on either side of the body, an entire articular facet for the head of the first rib, and a demi-facet for the upper half of the head of the second rib.

The body is like that of a cervical vertebra, being broad transversely; its upper surface is concave, and lipped on either side. The superior articular surfaces are directed upward and backward; the spinous process is thick, long, and almost horizontal. The transverse processes are long, and the upper vertebral notches are deeper than those of the other thoracic vertebræ.

The Ninth Thoracic Vertebra may have no demi-facets below. In some subjects however, it has two demi-facets on either side; when this occurs the tenth has only demi-facets at the upper part.

The Tenth Thoracic Vertebra has (except in the cases just mentioned) an entire articular facet on either side, which is placed partly on the lateral surface of the pedicle.

In the Eleventh Thoracic Vertebra the body approaches in its form and size to that of the lumbar vertebræ. The articular facets for the heads of the ribs are of large size, and placed chiefly on the pedicles, which are thicker and stronger in this and the next vertebra than in any other part of the thoracic region. The spinous process is short, and nearly horizontal in direction. The transverse processes are very short, tuberculated at their extremities, and have no articular facets.

The Twelfth Thoracic Vertebra has the same general characteristics as the eleventh, but may be distinguished from it by its inferior articular surfaces being convex and directed lateralward, like those of the lumbar vertebræ; by the general form of the body, laminæ, and spinous process, in which it resembles the lumbar vertebræ; and by each transverse process being subdivided into three elevations, the superior, inferior, and lateral tubercles: the superior and inferior correspond to the mammillary and accessory processes of the lumbar vertebræ. Traces of similar elevations are found on the transverse processes of the tenth and eleventh thoracic vertebræ.

FIG. 92– A lumbar vertebra seen from the side.

3a. 3. The Lumbar Vertebræ

(Vertebræ Lumbales).

The lumbar vertebræ (Figs. 92 and 93) are the largest segments of the movable part of the vertebral column, and can be distinguished by the absence of a foramen in the transverse process, and by the absence of facets on the sides of the body. 1
The body is large, wider from side to side than from before backward, and a little thicker in front than behind. It is flattened or slightly concave above and below, concave behind, and deeply constricted in front and at the sides. The pedicles are very strong, directed backward from the upper part of the body; consequently, the inferior vertebral notches are of considerable depth. The laminæ are broad, short, and strong; the vertebral foramen is triangular, larger than in the thoracic, but smaller than in the cervical region. The spinous process is thick, broad, and somewhat quadrilateral; it projects backward and ends in a rough, uneven border, thickest below where it is occasionally notched. The superior and inferior articular processes are well-defined, projecting respectively upward and downward from the junctions of pedicles and laminæ. The facets on the superior processes are concave, and look backward and medialward; those on the inferior are convex, and are directed forward and lateralward. The former are wider apart than the latter, since in the articulated column the inferior articular processes are embraced by the superior processes of the subjacent vertebra. The transverse processes are long, slender, and horizontal in the upper three lumbar vertebræ; they incline a little upward in the lower two. In the upper three vertebræ they arise from the junctions of the pedicles and laminæ, but in the lower two they are set farther forward and spring from the pedicles and posterior parts of the bodies. They are situated in front of the articular processes instead of behind them as in the thoracic vertebræ, and are homologous with the ribs. Of the three tubercles noticed in connection with the transverse processes of the lower thoracic vertebræ, the superior one is connected in the lumbar region with the back part of the superior articular process, and is named the mammillary process; the inferior is situated at the back part of the base of the transverse process, and is called the accessory process (Fig. 93). 2

FIG. 93– A lumbar vertebra from above and behind. (See enlarged image)

FIG. 94– Fifth lumbar vertebra, from above. (See enlarged image)

The Fifth Lumbar Vertebra (Fig. 94) is characterized by its body being much deeper in front than behind, which accords with the prominence of the sacrovertebral articulation; by the smaller size of its spinous process; by the wide interval between the inferior articular processes; and by the thickness of its transverse processes, which spring from the body as well as from the pedicles. 3

3a. 4. The Sacral and Coccygeal Vertebræ

The sacral and coccygeal vertebræ consist at an early period of life of nine separate segments which are united in the adult, so as to form two bones, five entering into the formation of the sacrum, four into that of the coccyx. Sometimes the coccyx consists of five bones; occasionally the number is reduced to three.

The Sacrum (os sacrum).—The sacrum is a large, triangular bone, situated in the lower part of the vertebral column and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones; its upper part or base articulates with the last lumbar vertebra, its apex with the coccyx. It is curved upon itself and placed very obliquely, its base projecting forward and forming the prominent sacrovertebral angle when articulated with the last lumbar vertebra; its central part is projected backward, so as to give increased capacity to the pelvic cavity.

Pelvic Surface (facies pelvina).—The pelvic surface (Fig. 95) is concave from above downward, and slightly so from side to side. Its middle part is crossed by four transverse ridges, the positions of which correspond with the original planes of separation between the five segments of the bone. The portions of bone intervening between the ridges are the bodies of the sacral vertebræ. The body of the first segment is of large size, and in form resembles that of a lumbar vertebra; the succeeding ones diminish from above downward, are flattened from before backward, and curved so as to accommodate themselves to the form of the sacrum, being concave in front, convex behind. At the ends of the ridges are seen the anterior sacral foramina, four in number on either side, somewhat rounded in form, diminishing in size from above downward, and directed lateralward and forward; they give exit to the anterior divisions of the sacral nerves and entrance to the lateral sacral arteries. Lateral to these foramina are the lateral parts of the sacrum, each consisting of five separate segments at an early period of life; in the adult, these are blended with the bodies and with each other. Each lateral part is traversed by four broad, shallow grooves, which lodge the anterior divisions of the sacral nerves, and are separated by prominent ridges of bone which give origin to the Piriformis muscle. 3
If a sagittal section be made through the center of the sacrum (Fig. 99), the bodies are seen to be united at their circumferences by bone, wide intervals being left centrally, which, in the fresh state, are filled by the intervertebral fibrocartilages. In some bones this union is more complete between the lower than the upper segments. 4

FIG. 95– Sacrum, pelvic surface. (See enlarged image)

Dorsal Surface (facies dorsalis).—The dorsal surface (Fig. 96) is convex and narrower than the pelvic. In the middle line it displays a crest, the middle sacral crest, surmounted by three or four tubercles, the rudimentary spinous processes of the upper three or four sacral vertebræ. On either side of the middle sacral crest is a shallow groove, the sacral groove, which gives origin to the Multifidus, the floor of the groove being formed by the united laminæ of the corresponding vertebræ. The laminæ of the fifth sacral vertebra, and sometimes those of the fourth, fail to meet behind, and thus a hiatus or deficiency occurs in the posterior wall of the sacral canal. On the lateral aspect of the sacral groove is a linear series of tubercles produced by the fusion of the articular processes which together form the indistinct sacral articular crests. The articular processes of the first sacral vertebra are large and oval in shape; their facets are concave from side to side, look backward and medialward, and articulate with the facets on the inferior processes of the fifth lumbar vertebra. The tubercles which represent the inferior articular processes of the fifth sacral vertebra are prolonged downward as rounded processes, which are named the sacral cornua, and are connected to the cornua of the coccyx. Lateral to the articular processes are the four posterior sacral foramina; they are smaller in size and less regular in form than the anterior, and transmit the posterior divisions of the sacral nerves. On the lateral side of the posterior sacral foramina is a series of tubercles, which represent the transverse processes of the sacral vertebræ, and form the lateral crests of the sacrum. The transverse tubercles of the first sacral vertebra are large and very distinct; they, together with the transverse tubercles of the second vertebra, give attachment to the horizontal parts of the posterior sacroiliac ligaments; those of the third vertebra give attachment to the oblique fasciculi of the posterior sacroiliac ligaments; and those of the fourth and fifth to the sacrotuberous ligaments. 5

FIG. 96– Sacrum, dorsal surface. (See enlarged image)

Lateral Surface.—The lateral surface is broad above, but narrowed into a thin edge below. The upper half presents in front an ear-shaped surface, the auricular surface, covered with cartilage in the fresh state, for articulation with the ilium. Behind it is a rough surface, the sacral tuberosity, on which are three deep and uneven impressions, for the attachment of the posterior sacroiliac ligament. The lower half is thin, and ends in a projection called the inferior lateral angle; medial to this angle is a notch, which is converted into a foramen by the transverse process of the first piece of the coccyx, and transmits the anterior division of the fifth sacral nerve. The thin lower half of the lateral surface gives attachment to the sacrotuberous and sacrospinous ligaments, to some fibers of the Glutæus maximus behind, and to the Coccygeus in front. 6

FIG. 97– Lateral surfaces of sacrum and coccyx. (See enlarged image)

FIG. 98– Base of sacrum. (See enlarged image)

Base (basis oss. sacri).—The base of the sacrum, which is broad and expanded, is directed upward and forward. In the middle is a large oval articular surface, the upper surface of the body of the first sacral vertebra, which is connected with the under surface of the body of the last lumbar vertebra by an intervertebral fibrocartilage. Behind this is the large triangular orifice of the sacral canal, which is completed by the laminæ and spinous process of the first sacral vertebra. The superior articular processes project from it on either side; they are oval, concave, directed backward and medialward, like the superior articular processes of a lumbar vertebra. They are attached to the body of the first sacral vertebra and to the alæ by short thick pedicles; on the upper surface of each pedicle is a vertebral notch, which forms the lower part of the foramen between the last lumbar and first sacral vertebræ. On either side of the body is a large triangular surface, which supports the Psoas major and the lumbosacral trunk, and in the articulated pelvis is continuous with the iliac fossa. This is called the ala; it is slightly concave from side to side, convex from before backward, and gives attachment to a few of the fibers of the Iliacus. The posterior fourth of the ala represents the transverse process, and its anterior three-fourths the costal process of the first sacral segment. 7

FIG. 99– Median sagittal section of the sacrum. (See enlarged image)

FIG. 100– Coccyx. (See enlarged image)

Apex (apex oss. sacri).—The apex is directed downward, and presents an oval facet for articulation with the coccyx. 8

Vertebral Canal (canalis sacralis; sacral canal).—The vertebral canal (Fig. 99) runs throughout the greater part of the bone; above, it is triangular in form; below, its posterior wall is incomplete, from the non-development of the laminæ and spinous processes. It lodges the sacral nerves, and its walls are perforated by the anterior and posterior sacral foramina through which these nerves pass out. 9

Structure.—The sacrum consists of cancellous tissue enveloped by a thin layer of compact bone. 10

Articulations.—The sacrum articulates with four bones; the last lumbar vertebra above, the coccyx below, and the hip bone on either side. 11

Differences in the Sacrum of the Male and Female.—In the female the sacrum is shorter and wider than in the male; the lower half forms a greater angle with the upper; the upper half is nearly straight, the lower half presenting the greatest amount of curvature. The bone is also directed more obliquely backward; this increases the size of the pelvic cavity and renders the sacrovertebral angle more prominent. In the male the curvature is more evenly distributed over the whole length of the bone, and is altogether greater than in the female. 12

Variations.—The sacrum, in some cases, consists of six pieces; occasionally the number is reduced to four. The bodies of the first and second vertebræ may fail to unite. Sometimes the uppermost transverse tubercles are not joined to the rest of the ala on one or both sides, or the sacral canal may be open throughout a considerable part of its length, in consequence of the imperfect development of the laminæ and spinous processes. The sacrum, also, varies considerably with respect to its degree of curvature. 13

The Coccyx (os coccygis).—The coccyx (Fig. 100) is usually formed of four rudimentary vertebræ; the number may however be increased to five or diminished to three. In each of the first three segments may be traced a rudimentary body and articular and transverse processes; the last piece (sometimes the third) is a mere nodule of bone. All the segments are destitute of pedicles, laminæ, and spinous processes. The first is the largest; it resembles the lowest sacral vertebra, and often exists as a separate piece; the last three diminish in size from above downward, and are usually fused with one another. 14

Surfaces.—The anterior surface is slightly concave, and marked with three transverse grooves which indicate the junctions of the different segments. It gives attachment to the anterior sacrococcygeal ligament and the Levatores ani, and supports part of the rectum. The posterior surface is convex, marked by transverse grooves similar to those on the anterior surface, and presents on either side a linear row of tubercles, the rudimentary articular processes of the coccygeal vertebræ. Of these, the superior pair are large, and are called the coccygeal cornua; they project upward, and articulate with the cornua of the sacrum, and on either side complete the foramen for the transmission of the posterior division of the fifth sacral nerve. 15

Borders.—The lateral borders are thin, and exhibit a series of small eminences, which represent the transverse processes of the coccygeal vertebræ. Of these, the first is the largest; it is flattened from before backward, and often ascends to join the lower part of the thin lateral edge of the sacrum, thus completing the foramen for the transmission of the anterior division of the fifth sacral nerve; the others diminish in size from above downward, and are often wanting. The borders of the coccyx are narrow, and give attachment on either side to the sacrotuberous and sacrospinous ligaments, to the Coccygeus in front of the ligaments, and to the Glutæus maximus behind them. 16

Base.—The base presents an oval surface for articulation with the sacrum. 17

Apex.—The apex is rounded, and has attached to it the tendon of the Sphincter ani externus. It may be bifid, and is sometimes deflected to one or other side. 18

Ossification of the Vertebral Column.—Each cartilaginous vertebra is ossified from three primary centers (Fig. 101), two for the vertebral arch and one for the body. 16 Ossification of the vertebral arches begins in the upper cervical vertebræ about the seventh or eighth week of fetal life, and gradually extends down the column. The ossific granules first appear in the situations where the transverse processes afterward project, and spread backward to the spinous process forward into the pedicles, and lateralward into the transverse and articular processes. Ossification of the bodies begins about the eighth week in the lower thoracic region, and subsequently extends upward and downward along the column. The center for the body does not give rise to the whole of the body of the adult vertebra, the postero-lateral portions of which are ossified by extensions from the vertebral arch centers. The body of the vertebra during the first few years of life shows, therefore, two synchondroses, neurocentral synchondroses, traversing it along the planes of junction of the three centers (Fig. 102). In the thoracic region, the facets for the heads of the ribs lie behind the neurocentral synchondroses and are ossified from the centers for the vertebral arch. At birth the vertebra consists of three pieces, the body and the halves of the vertebral arch. During the first year the halves of the arch unite behind, union taking place first in the lumbar region and then extending upward through the thoracic and cervical regions. About the third year the bodies of the upper cervical vertebræ are joined to the arches on either side; in the lower lumbar vertebræ the union is not completed until the sixth year. Before puberty, no other changes occur, excepting a gradual increase of these primary centers, the upper and under surfaces of the bodies and the ends of the transverse and spinous processes being cartilaginous. About the sixteenth year (Fig. 102), five secondary centers appear, one for the tip of each transverse process, one for the extremity of the spinous process, one for the upper and one for the lower surface of the body (Fig. 103). These fuse with the rest of the bone about the age of twenty-five. 19

FIG. 101– Ossification of a vertebra (See enlarged image)

FIG. 102– No caption. (See enlarged image)

FIG. 103– No caption. (See enlarged image)

FIG. 104– Atlas. (See enlarged image)

FIG. 105– Axis. (See enlarged image)

FIG. 106– Lumbar vertebra. (See enlarged image)

FIG. 107– No caption. (See enlarged image)

FIG. 108– No caption. (See enlarged image)

FIG. 109– Ossification of the sacrum. (See enlarged image)

Exceptions to this mode of development occur in the first, second, and seventh cervical vertebræ, and in the lumbar vertebræ. 20

Atlas.—The atlas is usually ossified from three centers (Fig. 104). Of these, one appears in each lateral mass about the seventh week of fetal life, and extends backward; at birth, these portions of bone are separated from one another behind by a narrow interval filled with cartilage. Between the third and fourth years they unite either directly or through the medium of a separate center developed in the cartilage. At birth, the anterior arch consists of cartilage; in this a separate center appears about the end of the first year after birth, and joins the lateral masses from the sixth to the eighth year—the lines of union extending across the anterior portions of the superior articular facets. Occasionally there is no separate center, the anterior arch being formed by the forward extension and ultimate junction of the two lateral masses; sometimes this arch is ossified from two centers, one on either side of the middle line. 21

Epistropheus or Axis.—The axis is ossified from five primary and two secondary centers (Fig. 105). The body and vertebral arch are ossified in the same manner as the corresponding parts in the other vertebræ, viz., one center for the body, and two for the vertebral arch. The centers for the arch appear about the seventh or eighth week of fetal life, that for the body about the fourth or fifth month. The dens or odontoid process consists originally of a continuation upward of the cartilaginous mass, in which the lower part of the body is formed. About the sixth month of fetal life, two centers make their appearance in the base of this process: they are placed laterally, and join before birth to form a conical bilobed mass deeply cleft above; the interval between the sides of the cleft and the summit of the process is formed by a wedge-shaped piece of cartilage. The base of the process is separated from the body by a cartilaginous disk, which gradually becomes ossified at its circumference, but remains cartilaginous in its center until advanced age. In this cartilage, rudiments of the lower epiphysial lamella of the atlas and the upper epiphysial lamella of the axis may sometimes be found. The apex of the odontoid process has a separate center which appears in the second and joins about the twelfth year; this is the upper epiphysial lamella of the atlas. In addition to these there is a secondary center for a thin epiphysial plate on the under surface of the body of the bone. 22

The Seventh Cervical Vertebra.—The anterior or costal part of the transverse process of this vertebra is sometimes ossified from a separate center which appears about the sixth month of fetal life, and joins the body and posterior part of the transverse process between the fifth and sixth years. Occasionally the costal part persists as a separate piece, and, becoming lengthened lateralward and forward, constitutes what is known as a cervical rib. Separate ossific centers have also been found in the costal processes of the fourth, fifth, and sixth cervical vertebræ. 23

Lumbar Vertebræ.—The lumbar vertebræ (Fig. 106) have each two additional centers, for the mammillary processes. The transverse process of the first lumbar is sometimes developed as a separate piece, which may remain permanently ununited with the rest of the bone, thus forming a lumbar rib—a peculiarity, however, rarely met with. 24

FIG. 110– Base of young sacrum. (See enlarged image)

Sacrum (Figs. 107 to 110).—The body of each sacral vertebra is ossified from a primary center and two epiphysial plates, one for its upper and another for its under surface, while each vertebral arch is ossified from two centers. 25
The anterior portions of the lateral parts have six additional centers, two for each of the first three vertebræ; these represent the costal elements, and make their appearance above and lateral to the anterior sacral foramina (Figs. 107, 108). 26
On each lateral surface two epiphysial plates are developed (Figs. 109, 110): one for the auricular surface, and another for the remaining part of the thin lateral edge of the bone. 17 27
PERIODS OF OSSIFICATION.—About the eighth or ninth week of fetal life, ossification of the central part of the body of the first sacral vertebra commences, and is rapidly followed by deposit of ossific matter in the second and third; ossification does not commence in the bodies of the lower two segments until between the fifth and eighth months of fetal life. Between the sixth and eighth months ossification of the vertebral arches takes place; and about the same time the costal centers for the lateral parts make their appearance. The junctions of the vertebral arches with the bodies take place in the lower vertebræ as early as the second year, but are not effected in the uppermost until the fifth or sixth year. About the sixteenth year the epiphysial plates for the upper and under surfaces of the bodies are formed; and between the eighteenth and twentieth years, those for the lateral surfaces make their appearance. The bodies of the sacral vertebræ are, during early life, separated from each other by intervertebral fibrocartilages, but about the eighteenth year the two lowest segments become united by bone, and the process of bony union gradually extends upward, with the result that between the twenty-fifth and thirtieth years of life all the segments are united. On examining a sagittal section of the sacrum, the situations of the intervertebral fibrocartilages are indicated by a series of oval cavities (Fig. 99). 28

Coccyx.—The coccyx is ossified from four centers, one for each segment. The ossific nuclei make their appearance in the following order: in the first segment between the first and fourth years; in the second between the fifth and tenth years; in the third between the tenth and fifteenth years; in the fourth between the fourteenth and twentieth years. As age advances, the segments unite with one another, the union between the first and second segments being frequently delayed until after the age of twenty-five or thirty. At a late period of life, especially in females, the coccyx often fuses with the sacrum. 29
Note 16. A vertebra is occasionally found in which the body consists of two lateral portions—a condition which proves that the body is sometimes ossified from two primary centers, one on either side of the middle line. [back]
Note 17. The ends of the spinous processes of the upper three sacral vertebræ are sometimes developed from separate epiphyses, and Fawcett (Anatomischer Anzeiger, 1907, Band xxx) states that a number of epiphysial nodules may be seen in the sacrum at the age of eighteen years. These are distributed as follows: One for each of the mammillary processes of the first sacral vertebra; twelve—six on either side—in connection with the costal elements (two each for the first and second and one each for the third and fourth) and eight for the transverse processes—four on either side—one each for the first, third, fourth, and fifth. He is further of opinion that the lower part of each lateral surface of the sacrum is formed by the extension and union of the third and fourth “costal” and fourth and fifth “transverse” epiphyses. [back]

3b. The Vertebral Column as a Whole

The vertebral column is situated in the median line, as the posterior part of the trunk; its average length in the male is about 71 cm. Of this length the cervical part measures 12.5 cm., the thoracic about 28 cm., the lumbar 18 cm., and the sacrum and coccyx 12.5 cm. The female column is about 61 cm. in length.

Curves.—Viewed laterally (Fig. 111), the vertebral column presents several curves, which correspond to the different regions of the column, and are called cervical, thoracic, lumbar, and pelvic. The cervical curve, convex forward, begins at the apex of the odontoid process, and ends at the middle of the second thoracic vertebra; it is the least marked of all the curves. The thoracic curve, concave forward, begins at the middle of the second and ends at the middle of the twelfth thoracic vertebra. Its most prominent point behind corresponds to the spinous process of the seventh thoracic vertebra. The lumbar curve is more marked in the female than in the male; it begins at the middle of the last thoracic vertebra, and ends at the sacrovertebral angle. It is convex anteriorly, the convexity of the lower three vertebræ being much greater than that of the upper two. The pelvic curve begins at the sacrovertebral articulation, and ends at the point of the coccyx; its concavity is directed downward and forward. The thoracic and pelvic curves are termed primary curves, because they alone are present during fetal life. The cervical and lumbar curves are compensatory or secondary, and are developed after birth, the former when the child is able to hold up its head (at three or four months), and to sit upright (at nine months), the latter at twelve or eighteen months, when the child begins to walk.

The vertebral column has also a slight lateral curvature, the convexity of which is directed toward the right side. This may be produced by muscular action, most persons using the right arm in preference to the left, especially in making long-continued efforts, when the body is curved to the right side. In support of this explanation it has been found that in one or two individuals who were left-handed, the convexity was to the left side. By others this curvature is regarded as being produced by the aortic arch and upper part of the descending thoracic aorta—a view which is supported by the fact that in cases where the viscera are transposed and the aorta is on the right side, the convexity of the curve is directed to the left side.

Surfaces.—Anterior Surface.—When viewed from in front, the width of the bodies of the vertebræ is seen to increase from the second cervical to the first thoracic; there is then a slight diminution in the next three vertebræ; below this there is again a gradual and progressive increase in width as low as the sacrovertebral angle. From this point there is a rapid diminution, to the apex of the coccyx.

Posterior Surface.—The posterior surface of the vertebral column presents in the median line the spinous processes. In the cervical region (with the exception of the second and seventh vertebræ) these are short and horizontal, with bifid extremities. In the upper part of the thoracic region they are directed obliquely downward; in the middle they are almost vertical, and in the lower part they are nearly horizontal. In the lumbar region they are nearly horizontal. The spinous processes are separated by considerable intervals in the lumbar region, by narrower intervals in the neck, and are closely approximated in the middle of the thoracic region. Occasionally one of these processes deviates a little from the median line—a fact to be remembered in practice, as irregularities of this sort are attendant also on fractures or displacements of the vertebral column. On either side of the spinous processes is the vertebral groove formed by the laminæ in the cervical and lumbar regions, where it is shallow, and by the laminæ and transverse processes in the thoracic region, where it is deep and broad; these grooves lodge the deep muscles of the back. Lateral to the vertebral grooves are the articular processes, and still more laterally the transverse processes. In the thoracic region, the transverse processes stand backward, on a plane considerably behind that of the same processes in the cervical and lumbar regions. In the cervical region, the transverse processes are placed in front of the articular processes, lateral to the pedicles and between the intervertebral foramina. In the thoracic region they are posterior to the pedicles, intervertebral foramina, and articular processes. In the lumbar region they are in front of the articular processes, but behind the intervertebral foramina. 5

Lateral Surfaces.—The lateral surfaces are separated from the posterior surface by the articular processes in the cervical and lumbar regions, and by the transverse processes in the thoracic region. They present, in front, the sides of the bodies of the vertebræ, marked in the thoracic region by the facets for articulation with the heads of the ribs. More posteriorly are the intervertebral foramina, formed by the juxtaposition of the vertebral notches, oval in shape, smallest in the cervical and upper part of the thoracic regions, and gradually increasing in size to the last lumbar. They transmit the spinal nerves and are situated between the transverse processes in the cervical region, and in front of them in the thoracic and lumbar regions. 6

FIG. 111– Lateral view of the vertebral column. (See enlarged image)

Vertebral Canal.—The vertebral canal follows the different curves of the column; it is large and triangular in those parts of the column which enjoy the greatest freedom of movement, viz., the cervical and lumbar regions; and is small and rounded in the thoracic region, where motion is more limited. 7

Abnormalities.—Occasionally the coalescence of the laminæ is not completed, and consequently a cleft is left in the arches of the vertebræ, through which a protrusion of the spinal membranes (dura mater and arachnoid), and generally of the medulla spinalis itself, takes place, constituting the malformation known as spina bifida. This condition is most common in the lumbosacral region, but it may occur in the thoracic or cervical region, or the arches throughout the whole length of the canal may remain incomplete. 8

FIG. 112– The thorax from in front. (Spalteholz.) (See enlarged image)

CONTENTS · BIBLIOGRAPHIC RECORD · ILLUSTRATIONS · SUBJECT INDEX

4a. The Sternum

(Breast Bone) 1

FIG. 115– Anterior surface of sternum and costa cartilages. (See enlarged image)

The sternum (Figs. 115 to 117) is an elongated, flattened bone, forming the middle portion of the anterior wall of the thorax. Its upper end supports the clavicles, and its margins articulate with the cartilages of the first seven pairs of ribs. It consists of three parts, named from above downward, the manubrium, the body or gladiolus, and the xiphoid process; in early life the body consists of four segments or sternebrœ. In its natural position the inclination of the bone is oblique from above, downward and forward. It is slightly convex in front and concave behind; broad above, becoming narrowed at the point where the manubrium joins the body, after which it again widens a little to below the middle of the body, and then narrows to its lower extremity. Its average length in the adult is about 17 cm., and is rather greater in the male than in the female. 2

Manubrium (manubrium sterni).—The manubrium is of a somewhat quadrangular form, broad and thick above, narrow below at its junction with the body. 3

Surfaces.—Its anterior surface, convex from side to side, concave from above downward, is smooth, and affords attachment on either side to the sternal origins of the Pectoralis major and Sternocleidomastoideus. Sometimes the ridges limiting the attachments of these muscles are very distinct. Its posterior surface, concave and smooth, affords attachment on either side to the Sternohyoideus and Sternothyreoideus. 4

FIG. 116– Posterior surface of sternum. (See enlarged image)

FIG. 117– Lateral border of sternum. (See enlarged image)

Borders.—The superior border is the thickest and presents at its center the jugular or presternal notch; on either side of the notch is an oval articular surface, directed upward, backward, and lateralward, for articulation with the sternal end of the clavicle. The inferior border, oval and rough, is covered in a fresh state with a thin layer of cartilage, for articulation with the body. The lateral borders are each marked above by a depression for the first costal cartilage, and below by a small facet, which, with a similar facet on the upper angle of the body, forms a notch for the reception of the costal cartilage of the second rib. Between the depression for the first costal cartilage and the demi-facet for the second is a narrow, curved edge, which slopes from above downward and medialward. 5

Body (corpus sterni; gladiolus).—The body, considerably longer, narrower, and thinner than the manubrium, attains its greatest breadth close to the lower end. 6

Surfaces.—Its anterior surface is nearly flat, directed upward and forward, and marked by three transverse ridges which cross the bone opposite the third, fourth, and fifth articular depressions. 18 It affords attachment on either side to the sternal origin of the Pectoralis major. At the junction of the third and fourth pieces of the body is occasionally seen an orifice, the sternal foramen, of varying size and form. The posterior surface, slightly concave, is also marked by three transverse lines, less distinct, however, than those in front; from its lower part, on either side, the Transversus thoracis takes origin. 7

Borders.—The superior border is oval and articulates with the manubrium, the junction of the two forming the sternal angle (angulus Ludovici 19). The inferior border is narrow, and articulates with the xiphoid process. Each lateral border (Fig. 117), at its superior angle, has a small facet, which with a similar facet on the manubrium, forms a cavity for the cartilage of the second rib; below this are four angular depressions which receive the cartilages of the third, fourth, fifth, and sixth ribs, while the inferior angle has a small facet, which, with a corresponding one on the xiphoid process, forms a notch for the cartilage of the seventh rib. These articular depressions are separated by a series of curved interarticular intervals, which diminish in length from above downward, and correspond to the intercostal spaces. Most of the cartilages belonging to the true ribs, as will be seen from the foregoing description, articulate with the sternum at the lines of junction of its primitive component segments. This is well seen in many of the lower animals, where the parts of the bone remain ununited longer than in man. 8

Xiphoid Process (processus xiphoideus; ensiform or xiphoid appendix).—The xiphoid process is the smallest of the three pieces: it is thin and elongated, cartilaginous in structure in youth, but more or less ossified at its upper part in the adult. 9

Surfaces.—Its anterior surface affords attachment on either side to the anterior costoxiphoid ligament and a small part of the Rectus abdominis; its posterior surface, to the posterior costoxiphoid ligament and to some of the fibers of the diaphragm and Transversus thoracis, its lateral borders, to the aponeuroses of the abdominal muscles. Above, it articulates with the lower end of the body, and on the front of each superior angle presents a facet for part of the cartilage of the seventh rib; below, by its pointed extremity, it gives attachment to the linea alba. The xiphoid process varies much in form; it may be broad and thin, pointed, bifid, perforated, curved, or deflected considerably to one or other side. 10

Structure.—The sternum is composed of highly vascular cancellous tissue, covered by a thin layer of compact bone which is thickest in the manubrium between the articular facets for the clavicles. 11

Ossification.—The sternum originally consists of two cartilaginous bars, situated one on either side of the median plane and connected with the cartilages of the upper nine ribs of its own side. These two bars fuse with each other along the middle line to form the cartilaginous sternum which is ossified from six centers: one for the manubrium, four for the body, and one for the xiphoid process (Fig. 118). The ossific centers appear in the intervals between the articular depressions for the costal cartilages, in the following order: in the manubrium and first piece of the body, during the sixth month; in the second and third pieces of the body, during the seventh month of fetal life; in its fourth piece, during the first year after birth; and in the xiphoid process, between the fifth and eighteenth years. The centers make their appearance at the upper parts of the segments, and proceed gradually downward. 20 To these may be added the occasional existence of two small episternal centers, which make their appearance one on either side of the jugular notch; they are probably vestiges of the episternal bone of the monotremata and lizards. Occasionally some of the segments are formed from more than one center, the number and position of which vary (Fig. 120). Thus, the first piece may have two, three, or even six centers. When two are present, they are generally situated one above the other, the upper being the larger; the second piece has seldom more than one; the third, fourth, and fifth pieces are often formed from two centers placed laterally, the irregular union of which explains the rare occurrence of the sternal foramen (Fig. 121), or of the vertical fissure which occasionally intersects this part of the bone constituting the malformation known as fissura sterni; these conditions are further explained by the manner in which the cartilaginous sternum is formed. More rarely still the upper end of the sternum may be divided by a fissure. Union of the various centers of the body begins about puberty, and proceeds from below upward (Fig. 119); by the age of twenty-five they are all united. The xiphoid process may become joined to the body before the age of thirty, but this occurs more frequently after forty; on the other hand, it sometimes remains ununited in old age. In advanced life the manubrium is occasionally joined to the body by bone. When this takes place, however, the bony tissue is generally only superficial, the central portion of the intervening cartilage remaining unossified. 12

FIG. 118– Ossification of the sternum. (See enlarged image)

FIG. 119– No caption. (See enlarged image)

FIG. 120– Peculiarities. (See enlarged image)

FIG. 121– No caption. (See enlarged image)

Articulations.—The sternum articulates on either side with the clavicle and upper seven costal cartilages. 13
Note 18. Paterson (The Human Sternum, 1904), who examined 524 specimens, points out that these ridges are altogether absent in 26.7 per cent.; that in 69 per cent. a ridge exists opposite the third costal attachment; in 39 per cent. opposite the fourth; and in 4 per cent. only, opposite the fifth. [back]
Note 19. Named after the French surgeon Antoine Louis, 1723–1792. The Latin name angulus Ludovici is not infrequently mistranslated into English as “the angle of Ludwig.” [back]
Note 20. Out of 141 sterna between the time of birth and the age of sixteen years, Paterson (op. cit.) found the fourth or lowest center for the body present only in thirty-eight cases—i. e., 26.9 per cent. [back]

4b. The Ribs

(Costæ)

The ribs are elastic arches of bone, which form a large part of the thoracic skeleton. They are twelve in number on either side; but this number may be increased by the development of a cervical or lumbar rib, or may be diminished to eleven. The first seven are connected behind with the vertebral column, and in front, through the intervention of the costal cartilages, with the sternum (Fig. 115); they are called true or vertebro-sternal ribs. 21 The remaining five are false ribs; of these, the first three have their cartilages attached to the cartilage of the rib above (vertebro-chondral): the last two are free at their anterior extremities and are termed floating or vertebral ribs. The ribs vary in their direction, the upper ones being less oblique than the lower; the obliquity reaches its maximum at the ninth rib, and gradually decreases from that rib to the twelfth. The ribs are situated one below the other in such a manner that spaces called intercostal spaces are left between them. The length of each space corresponds to that of the adjacent ribs and their cartilages; the breadth is greater in front than behind, and between the upper than the lower ribs. The ribs increase in length from the first to the seventh, below which they diminish to the twelfth. In breadth they decrease from above downward; in the upper ten the greatest breadth is at the sternal extremity. 1

Common Characteristics of the Ribs (Figs. 122, 123).—A rib from the middle of the series should be taken in order to study the common characteristics of these bones. 2
Each rib has two extremities, a posterior or vertebral, and an anterior or sternal, and an intervening portion—the body or shaft. 3

Posterior Extremity.—The posterior or vertebral extremity presents for examination a head, neck, and tubercle. 4
The head is marked by a kidney-shaped articular surface, divided by a horizontal crest into two facets for articulation with the depression formed on the bodies of two adjacent thoracic vertebræ; the upper facet is the smaller; to the crest is attached the interarticular ligament. 5
The neck is the flattened portion which extends lateralward from the head; it is about 2.5 cm. long, and is placed in front of the transverse process of the lower of the two vertebræ with which the head articulates. Its anterior surface is flat and smooth, its posterior rough for the attachment of the ligament of the neck, and perforated by numerous foramina. Of its two borders the superior presents a rough crest (crista colli costœ) for the attachment of the anterior costotransverse ligament; its inferior border is rounded. On the posterior surface at the junction of the neck and body, and nearer the lower than the upper border, is an eminence—the tubercle; it consists of an articular and a non-articular portion. The articular portion, the lower and more medial of the two, presents a small, oval surface for articulation with the end of the transverse process of the lower of the two vertebræ to which the head is connected. The non-articular portion is a rough elevation, and affords attachment to the ligament of the tubercle. The tubercle is much more prominent in the upper than in the lower ribs. 6

FIG. 122– A central rib of the left side. Inferior aspect. (See enlarged image)

Body.—The body or shaft is thin and flat, with two surfaces, an external and an internal; and two borders, a superior and an inferior. The external surface is convex, smooth, and marked, a little in front of the tubercle, by a prominent line, directed downward and lateralward; this gives attachment to a tendon of the Iliocostalis, and is called the angle. At this point the rib is bent in two directions, and at the same time twisted on its long axis. If the rib be laid upon its lower border, the portion of the body in front of the angle rests upon this border, while the portion behind the angle is bent medialward and at the same time tilted upward; as the result of the twisting, the external surface, behind the angle, looks downward, and in front of the angle, slightly upward. The distance between the angle and the tubercle is progressively greater from the second to the tenth ribs. The portion between the angle and the tubercle is rounded, rough, and irregular, and serves for the attachment of the Longissimus dorsi. The internal surface is concave, smooth, directed a little upward behind the angle, a little downward in front of it, and is marked by a ridge which commences at the lower extremity of the head; this ridge is strongly marked as far as the angle, and gradually becomes lost at the junction of the anterior and middle thirds of the bone. Between it and the inferior border is a groove, the costal groove, for the intercostal vessels and nerve. At the back part of the bone, this groove belongs to the inferior border, but just in front of the angle, where it is deepest and broadest, it is on the internal surface. The superior edge of the groove is rounded and serves for the attachment of an Intercostalis internus; the inferior edge corresponds to the lower margin of the rib, and gives attachment to an Intercostalis externus. Within the groove are seen the orifices of numerous small foramina for nutrient vessels which traverse the shaft obliquely from before backward. The superior border, thick and rounded, is marked by an external and an internal lip, more distinct behind than in front, which serve for the attachment of Intercostales externus and internus. The inferior border is thin, and has attached to it an Intercostalis externus. 7

Anterior Extremity.—The anterior or sternal extremity is flattened, and presents a porous, oval, concave depression, into which the costal cartilage is received. 8

Peculiar Ribs.—The first, second, tenth, eleventh, and twelfth ribs present certain variations from the common characteristics described above, and require special consideration. 9

FIG. 123– A central rib of the left side, viewed from behind. (See enlarged image)

First Rib.—The first rib (Fig. 124) is the most curved and usually the shortest of all the ribs; it is broad and flat, its surfaces looking upward and downward, and its borders inward and outward. The head is small, rounded, and possesses only a single articular facet, for articulation with the body of the first thoracic vertebra. The neck is narrow and rounded. The tubercle, thick and prominent, is placed on the outer border. There is no angle, but at the tubercle the rib is slightly bent, with the convexity upward, so that the head of the bone is directed downward. The upper surface of the body is marked by two shallow grooves, separated from each other by a slight ridge prolonged internally into a tubercle, the scalene tubercle, for the attachment of the Scalenus anterior; the anterior groove transmits the subclavian vein, the posterior the subclavian artery and the lowest trunk of the brachial plexus. 22 Behind the posterior groove is a rough area for the attachment of the Scalenus medius. The under surface is smooth, and destitute of a costal groove. The outer border is convex, thick, and rounded, and at its posterior part gives attachment to the first digitation of the Serratus anterior; the inner border is concave, thin, and sharp, and marked about its center by the scalene tubercle. The anterior extremity is larger and thicker than that of any of the other ribs. 10

Second Rib.—The second rib (Fig. 125) is much longer than the first, but has a very similar curvature. The non-articular portion of the tubercle is occasionally only feebly marked. The angle is slight, and situated close to the tubercle. The body is not twisted, so that both ends touch any plane surface upon which it may be laid; but there is a bend, with its convexity upward, similar to, though smaller than that found in the first rib. The body is not flattened horizontally like that of the first rib. Its external surface is convex, and looks upward and a little outward; near the middle of it is a rough eminence for the origin of the lower part of the first and the whole of the second digitation of the Serratus anterior; behind and above this is attached the Scalenus posterior. The internal surface, smooth, and concave, is directed downward and a little inward: on its posterior part there is a short costal groove. 11

FIG. 124– Peculiar ribs. (See enlarged image)

FIG. 125– Peculiar ribs. (See enlarged image)

FIG. 126– Peculiar ribs. (See enlarged image)

FIG. 127– Peculiar ribs. (See enlarged image)

FIG. 128– Peculiar ribs. (See enlarged image)

Tenth Rib.—The tenth rib (Fig. 126) has only a single articular facet on its head. 12

Eleventh and Twelfth Ribs.—The eleventh and twelfth ribs (Figs. 127 and 128) have each a single articular facet on the head, which is of rather large size; they have no necks or tubercles, and are pointed at their anterior ends. The eleventh has a slight angle and a shallow costal groove. The twelfth has neither; it is much shorter than the eleventh, and its head is inclined slightly downward. Sometimes the twelfth rib is even shorter than the first. 13

Structure.—The ribs consist of highly vascular cancellous tissue, enclosed in a thin layer of compact bone. 14

Ossification.—Each rib, with the exception of the last two, is ossified from four centers; a primary center for the body, and three epiphysial centers, one for the head and one each for the articular and non-articular parts of the tubercle. The eleventh and twelfth ribs have each only two centers, those for the tubercles being wanting. Ossification begins near the angle toward the end of the second month of fetal life, and is seen first in the sixth and seventh ribs. The epiphyses for the head and tubercle make their appearance between the sixteenth and twentieth years, and are united to the body about the twenty-fifth year. Fawcett 23 states that “in all probability there is usually no epiphysis on the non-articular part of the tuberosity below the sixth or seventh rib. 15
Note 21. Sometimes the eighth rib cartilage articulates with the sternum; this condition occurs more frequently on the right than on the left side. [back]
Note 22. Anat. Anzeiger, 1910, Band xxxvi. [back]
Note 23. Journal of Anatomy and Physiology. vol. xlv. [back]

4c. The Costal Cartilages

(Cartilagines Costales)

The costal cartilages (Fig. 115) are bars of hyaline cartilage which serve to prolong the ribs forward and contribute very materially to the elasticity of the walls of the thorax. The first seven pairs are connected with the sternum; the next three are each articulated with the lower border of the cartilage of the preceding rib; the last two have pointed extremities, which end in the wall of the abdomen. Like the ribs, the costal cartilages vary in their length, breadth, and direction. They increase in length from the first to the seventh, then gradually decrease to the twelfth. Their breadth, as well as that of the intervals between them, diminishes from the first to the last. They are broad at their attachments to the ribs, and taper toward their sternal extremities, excepting the first two, which are of the same breadth throughout, and the sixth, seventh, and eighth, which are enlarged where their margins are in contact. They also vary in direction: the first descends a little, the second is horizontal, the third ascends slightly, while the others are angular, following the course of the ribs for a short distance, and then ascending to the sternum or preceding cartilage. Each costal cartilage presents two surfaces, two borders, and two extremities. 1

Surfaces.—The anterior surface is convex, and looks forward and upward: that of the first gives attachment to the costoclavicular ligament and the Subclavius muscle; those of the first six or seven at their sternal ends, to the Pectoralis major. The others are covered by, and give partial attachment to, some of the flat muscles of the abdomen. The posterior surface is concave, and directed backward and downward; that of the first gives attachment to the Sternothyroideus, those of the third to the sixth inclusive to the Transversus thoracis, and the six or seven inferior ones to the Transversus abdominis and the diaphragm. 2

Borders.—Of the two borders the superior is concave, the inferior convex; they afford attachment to the Intercostales interni: the upper border of the sixth gives attachment also to the Pectoralis major. The inferior borders of the sixth, seventh, eighth, and ninth cartilages present heel-like projections at the points of greatest convexity. These projections carry smooth oblong facets which articulate respectively with facets on slight projections from the upper borders of the seventh, eighth, ninth, and tenth cartilages. 3

Extremities.—The lateral end of each cartilage is continuous with the osseous tissue of the rib to which it belongs. The medial end of the first is continuous with the sternum; the medial ends of the six succeeding ones are rounded and are received into shallow concavities on the lateral margins of the sternum. The medial ends of the eighth, ninth, and tenth costal cartilages are pointed, and are connected each with the cartilage immediately above. Those of the eleventh and twelfth are pointed and free. In old age the costal cartilages are prone to undergo superficial ossification. 4
Cervical ribs derived from the seventh cervical vertebra (page 83) are of not infrequent occurrence, and are important clinically because they may give rise to obscure nervous or vascular symptoms. The cervical rib may be a mere epiphysis articulating only with the transverse process of the vertebra, but more commonly it consists of a defined head, neck, and tubercle, with or without a body. It extends lateralward, or forward and lateralward, into the posterior triangle of the neck, where it may terminate in a free end or may join the first thoracic rib, the first costal cartilage, or the sternum. 24 It varies much in shape, size, direction, and mobility. If it reach far enough forward, part of the brachial plexus and the subclavian artery and vein cross over it, and are apt to suffer compression in so doing. Pressure on the artery may obstruct the circulation so much that arterial thrombosis results, causing gangrene of the finger tips. Pressure on the nerves is commoner, and affects the eighth cervical and first thoracic nerves, causing paralysis of the muscles they supply, and neuralgic pains and paresthesia in the area of skin to which they are distributed: no oculopupillary changes are to be found. 5
The thorax is frequently found to be altered in shape in certain diseases. 6
In rickets, the ends of the ribs, where they join the costal cartilages, become enlarged, giving rise to the so-called “rickety rosary,” which in mild cases is only found on the internal surface of the thorax. Lateral to these enlargements the softened ribs sink in, so as to present a groove passing downward and lateralward on either side of the sternum. This bone is forced forward by the bending of the ribs, and the antero-posterior diameter of the chest is increased. The ribs affected are the second to the eighth, the lower ones being prevented from falling in by the presence of the liver, stomach, and spleen; and when the abdomen is distended, as it often is in rickets, the lower ribs may be pushed outward, causing a transverse groove (Harrison’s sulcus) just above the costal arch. This deformity or forward projection of the sternum, often asymmetrical, is known as pigeon breast, and may be taken as evidence of active or old rickets except in cases of primary spinal curvature. In many instances it is associated in children with obstruction in the upper air passages, due to enlarged tonsils or adenoid growths. In some rickety children or adults, and also in others who give no history or further evidence of having had rickets, an opposite condition obtains. The lower part of the sternum and often the xiphoid process as well are deeply depressed backward, producing an oval hollow in the lower sternal and upper epigastric regions. This is known as funnel breast (German, Trichterbrust); it never appears to produce the least disturbance of any of the vital functions. The phthisical chest is often long and narrow, and with great obliquity of the ribs and projection of the scapulæ. In pulmonary emphysema the chest is enlarged in all its diameters, and presents on section an almost circular outline. It has received the name of the barrel-shaped chest. In severe cases of lateral curvature of the vertebral column the thorax becomes much distorted. In consequence of the rotation of the bodies of the vertebræ which takes place in this disease, the ribs opposite the convexity of the dorsal curve become extremely convex behind, being thrown out and bulging, and at the same time flattened in front, so that the two ends of the same rib are almost parallel. Coincidently with this the ribs on the opposite side, on the concavity of the curve, are sunk and depressed behind, and bulging and convex in front. 7
Note 24. W. Thorburn, The Medical Chronicle, Manchester, 1907, 4th series, xiv, No. 3 [back]

5a. The Cranial Bones. 1. The Occipital Bone

(Ossa Cranii) & (Os Occipitale).

The occipital bone (Figs. 129, 130), situated at the back and lower part of the cranium, is trapezoid in shape and curved on itself. It is pierced by a large oval aperture, the foramen magnum, through which the cranial cavity communicates with the vertebral canal. 1

FIG. 129– Occipital bone. Outer surface. (See enlarged image)

The curved, expanded plate behind the foramen magnum is named the squama; the thick, somewhat quadrilateral piece in front of the foramen is called the basilar part, whilst on either side of the foramen is the lateral portion. 2

The Squama (squama occipitalis).—The squama, situated above and behind the foramen magnum, is curved from above downward and from side to side. 3

Surfaces.—The external surface is convex and presents midway between the summit of the bone and the foramen magnum a prominence, the external occipital protuberance. Extending lateralward from this on either side are two curved lines, one a little above the other. The upper, often faintly marked, is named the highest nuchal line, and to it the galea aponeurotica is attached. The lower is termed the superior nuchal line. That part of the squama which lies above the highest nuchal lines is named the planum occipitale, and is covered by the Occipitalis muscle; that below, termed the planum nuchale, is rough and irregular for the attachment of several muscles. From the external occipital protuberance a ridge or crest, the median nuchal line, often faintly marked, descends to the foramen magnum, and affords attachment to the ligamentum nuchæ; running from the middle of this line across either half of the nuchal plane is the inferior nuchal line. Several muscles are attached to the outer surface of the squama, thus: the superior nuchal line gives origin to the Occipitalis and Trapezius, and insertion to the Sternocleidomastoideus and Splenius capitis: into the surface between the superior and inferior nuchal lines the Semispinalis capitis and the Obliquus capitis superior are inserted, while the inferior nuchal line and the area below it receive the insertions of the Recti capitis posteriores major and minor. The posterior atlantoöccipital membrane is attached around the postero-lateral part of the foramen magnum, just outside the margin of the foramen. 4

FIG. 130– Occipita bone. Inner surface. (See enlarged image)

The internal surface is deeply concave and divided into four fossæ by a cruciate eminence. The upper two fossæ are triangular and lodge the occipital lobes of the cerebrum; the lower two are quadrilateral and accommodate the hemispheres of the cerebellum. At the point of intersection of the four divisions of the cruciate eminence is the internal occipital protuberance. From this protuberance the upper division of the cruciate eminence runs to the superior angle of the bone, and on one side of it (generally the right) is a deep groove, the sagittal sulcus, which lodges the hinder part of the superior sagittal sinus; to the margins of this sulcus the falx cerebri is attached. The lower division of the cruciate eminence is prominent, and is named the internal occipital crest; it bifurcates near the foramen magnum and gives attachment to the falx cerebelli; in the attached margin of this falx is the occipital sinus, which is sometimes duplicated. In the upper part of the internal occipital crest, a small depression is sometimes distinguishable; it is termed the vermian fossa since it is occupied by part of the vermis of the cerebellum. Transverse grooves, one on either side, extend from the internal occipital protuberance to the lateral angles of the bone; those grooves accommodate the transverse sinuses, and their prominent margins give attachment to the tentorium cerebelli. The groove on the right side is usually larger than that on the left, and is continuous with that for the superior sagittal sinus. Exceptions to this condition are, however, not infrequent; the left may be larger than the right or the two may be almost equal in size. The angle of union of the superior sagittal and transverse sinuses is named the confluence of the sinuses (torcular Herophili 25), and its position is indicated by a depression situated on one or other side of the protuberance. 5

Lateral Parts (pars lateralis).—The lateral parts are situated at the sides of the foramen magnum; on their under surfaces are the condyles for articulation with the superior facets of the atlas. The condyles are oval or reniform in shape, and their anterior extremities, directed forward and medialward, are closer together than their posterior, and encroach on the basilar portion of the bone; the posterior extremities extend back to the level of the middle of the foramen magnum. The articular surfaces of the condyles are convex from before backward and from side to side, and look downward and lateralward. To their margins are attached the capsules of the atlantoöccipital articulations, and on the medial side of each is a rough impression or tubercle for the alar ligament. At the base of either condyle the bone is tunnelled by a short canal, the hypoglossal canal (anterior condyloid foramen). This begins on the cranial surface of the bone immediately above the foramen magnum, and is directed lateralward and forward above the condyle. It may be partially or completely divided into two by a spicule of bone; it gives exit to the hypoglossal or twelfth cerebral nerve, and entrance to a meningeal branch of the ascending pharyngeal artery. Behind either condyle is a depression, the condyloid fossa, which receives the posterior margin of the superior facet of the atlas when the head is bent backward; the floor of this fossa is sometimes perforated by the condyloid canal, through which an emissary vein passes from the transverse sinus. Extending lateralward from the posterior half of the condyle is a quadrilateral plate of bone, the jugular process, excavated in front by the jugular notch, which, in the articulated skull, forms the posterior part of the jugular foramen. The jugular notch may be divided into two by a bony spicule, the intrajugular process, which projects lateralward above the hypoglossal canal. The under surface of the jugular process is rough, and gives attachment to the Rectus capitis lateralis muscle and the lateral atlantoöccipital ligament; from this surface an eminence, the paramastoid process, sometimes projects downward, and may be of sufficient length to reach, and articulate with, the transverse process of the atlas. Laterally the jugular process presents a rough quadrilateral or triangular area which is joined to the jugular surface of the temporal bone by a plate of cartilage; after the age of twenty-five this plate tends to ossify. 6
The upper surface of the lateral part presents an oval eminence, the jugular tubercle, which overlies the hypoglossal canal and is sometimes crossed by an oblique groove for the glossopharyngeal, vagus, and accessory nerves. On the upper surface of the jugular process is a deep groove which curves medialward and forward and is continuous with the jugular notch. This groove lodges the terminal part of the transverse sinus, and opening into it, close to its medial margin, is the orifice of the condyloid canal. 7

Basilar Part (pars basilaris).—The basilar part extends forward and upward from the foramen magnum, and presents in front an area more or less quadrilateral in outline. In the young skull this area is rough and uneven, and is joined to the body of the sphenoid by a plate of cartilage. By the twenty-fifth year this cartilaginous plate is ossified, and the occipital and sphenoid form a continuous bone. 8

Surfaces.—On its lower surface, about 1 cm. in front of the foramen magnum, is the pharyngeal tubercle which gives attachment to the fibrous raphé of the pharynx. On either side of the middle line the Longus capitis and Rectus capitis anterior are inserted, and immediately in front of the foramen magnum the anterior atlantoöccipital membrane is attached. 9
The upper surface presents a broad, shallow groove which inclines upward and forward from the foramen magnum; it supports the medulla oblongata, and near the margin of the foramen magnum gives attachment to the membrana tectoria. On the lateral margins of this surface are faint grooves for the inferior petrosal sinuses. 10

Foramen Magnum.—The foramen magnum is a large oval aperture with its long diameter antero-posterior; it is wider behind than in front where it is encroached upon by the condyles. It transmits the medulla oblongata and its membranes, the accessory nerves, the vertebral arteries, the anterior and posterior spinal arteries, and the membrana tectoria and alar ligaments. 11

Angles.—The superior angle of the occipital bone articulates with the occipital angles of the parietal bones and, in the fetal skull, corresponds in position with the posterior fontanelle. The inferior angle is fused with the body of the sphenoid. The lateral angles are situated at the extremities of the grooves for the transverse sinuses: each is received into the interval between the mastoid angle of the parietal and the mastoid part of the temporal. 12

Borders.—The superior borders extend from the superior to the lateral angles: they are deeply serrated for articulation with the occipital borders of the parietals, and form by this union the lambdoidal suture. The inferior borders extend from the lateral angles to the inferior angle; the upper half of each articulates with the mastoid portion of the corresponding temporal, the lower half with the petrous part of the same bone. These two portions of the inferior border are separated from one another by the jugular process, the notch on the anterior surface of which forms the posterior part of the jugular foramen. 13

FIG. 131– Occipital bone at birth. (See enlarged image)

Structure.—The occipital, like the other cranial the outer and inner tables, between which is the cancellous tissue or diploë; the bone is especially thick at the ridges, protuberances, condyles, and anterior part of the basilar part; in the inferior fossæ it is thin, semitransparent, and destitute of diploë. 14

Ossification (Fig. 131).—The planum occipitale of the squama is developed in membrane, and may remain separate throughout life when it constitutes the interparietal bone; the rest of the bone is developed in cartilage. The number of nuclei for the planum occipitale is usually given as four, two appearing near the middle line about the second month, and two some little distance from the middle line about the third month of fetal life. The planum nuchale of the squama is ossified from two centers, which appear about the seventh week of fetal life and soon unite to form a single piece. Union of the upper and lower portions of the squama takes place in the third month of fetal life. An occasional center (Kerckring) appears in the posterior margin of the foramen magnum during the fifth month; this forms a separate ossicle (sometimes double) which unites with the rest of the squama before birth. Each of the lateral parts begins to ossify from a single center during the eighth week of fetal life. The basilar portion is ossified from two centers, one in front of the other; these appear about the sixth week of fetal life and rapidly coalesce. Mall 26 states that the planum occipitale is ossified from two centers and the basilar portion from one. About the fourth year the squama and the two lateral portions unite, and about the sixth year the bone consists of a single piece. Between the eighteenth and twenty-fifth years the occipital and sphenoid become united, forming a single bone. 15

Articulations.—The occipital articulates with six bones: the two parietals, the two temporals, the sphenoid, and the atlas. 16
Note 25. The columns of blood coming in different directions were supposed to be pressed together at this point (torcular, a wine press). [back]
Note 26. American Journal of Anatomy, 1906, vol. v. [back]

5a. 2. The Parietal Bone

(Os Parietale)

The parietal bones form, by their union, the sides and roof of the cranium. Each bone is irregularly quadrilateral in form, and has two surfaces, four borders, and four angles. 1

FIG. 132– Left parietal bone. Outer surface. (See enlarged image)

Surfaces.—The external surface (Fig. 132) is convex, smooth, and marked near the center by an eminence, the parietal eminence (tuber parietale), which indicates the point where ossification commenced. Crossing the middle of the bone in an arched direction are two curved lines, the superior and inferior temporal lines; the former gives attachment to the temporal fascia, and the latter indicates the upper limit of the muscular origin of the Temporalis. Above these lines the bone is covered by the galea aponeurotica; below them it forms part of the temporal fossa, and affords attachment to the Temporalis muscle. At the back part and close to the upper or sagittal border is the parietal foramen, which transmits a vein to the superior sagittal sinus, and sometimes a small branch of the occipital artery; it is not constantly present, and its size varies considerably. 2
The internal surface (Fig. 133) is concave; it presents depressions corresponding to the cerebral convolutions, and numerous furrows for the ramifications of the middle meningeal vessel; 27 the latter run upward and backward from the sphenoidal angle, and from the central and posterior part of the squamous border. Along the upper margin is a shallow groove, which, together with that on the opposite parietal, forms a channel, the sagittal sulcus, for the superior sagittal sinus; the edges of the sulcus afford attachment to the falx cerebri. Near the groove are several depressions, best marked in the skulls of old persons, for the arachnoid granulations (Pacchionian bodies). In the groove is the internal opening of the parietal foramen when that aperture exists. 3

FIG. 133– Left parietal bone. Inner surface. (See enlarged image)

Borders.—The sagittal border, the longest and thickest, is dentated and articulates with its fellow of the opposite side, forming the sagittal suture. The squamous border is divided into three parts: of these, the anterior is thin and pointed, bevelled at the expense of the outer surface, and overlapped by the tip of the great wing of the sphenoid; the middle portion is arched, bevelled at the expense of the outer surface, and overlapped by the squama of the temporal; the posterior part is thick and serrated for articulation with the mastoid portion of the temporal. The frontal border is deeply serrated, and bevelled at the expense of the outer surface above and of the inner below; it articulates with the frontal bone, forming onehalf of the coronal suture. The occipital border, deeply denticulated, articulates with the occipital, forming one-half of the lambdoidal suture. 4

Angles.—The frontal angle is practically a right angle, and corresponds with the point of meeting of the sagittal and coronal sutures; this point is named the bregma; in the fetal skull and for about a year and a half after birth this region is membranous, and is called the anterior fontanelle. The sphenoidal angle, thin and acute, is received into the interval between the frontal bone and the great wing of the sphenoid. Its inner surface is marked by a deep groove, sometimes a canal, for the anterior divisions of the middle meningeal artery. The occipital angle is rounded and corresponds with the point of meeting of the sagittal and lambdoidal sutures—a point which is termed the lambda; in the fetus this part of the skull is membranous, and is called the posterior fontanelle. The mastoid angle is truncated; it articulates with the occipital bone and with the mastoid portion of the temporal, and presents on its inner surface a broad, shallow groove which lodges part of the transverse sinus. The point of meeting of this angle with the occipital and the mastoid part of the temporal is named the asterion. 5

Ossification.—The parietal bone is ossified in membrane from a single center, which appears at the parietal eminence about the eighth week of fetal life. Ossification gradually extends in a radial manner from the center toward the margins of the bone; the angles are consequently the parts last formed, and it is here that the fontanelles exist. Occasionally the parietal bone is divided into two parts, upper and lower, by an antero-posterior suture. 6

Articulations.—The parietal articulates with five bones: the opposite parietal, the occipital, frontal, temporal, and sphenoid. 7
Note 27. Journal of Anatomy and Physiology, 1912, vol. xlvi. [back]

5a. 3. The Frontal Bone

(Os Frontale)

The frontal bone resembles a cockle-shell in form, and consists of two portions—a vertical portion, the squama, corresponding with the region of the forehead; and an orbital or horizontal portion, which enters into the formation of the roofs of the orbital and nasal cavities. 1

Squama (squama frontalis).—Surfaces.—The external surface (Fig. 134) of this portion is convex and usually exhibits, in the lower part of the middle line, the remains of the frontal or metopic suture; in infancy this suture divides the bone into two, a condition which may persist throughout life. On either side of this suture, about 3 cm. above the supraorbital margin, is a rounded elevation, the frontal eminence (tuber frontale). These eminences vary in size in different individuals, are occasionally unsymmetrical, and are especially prominent in young skulls; the surface of the bone above them is smooth, and covered by the galea aponeurotica. Below the frontal eminences, and separated from them by a shallow groove, are two arched elevations, the superciliary arches; these are prominent medially, and are joined to one another by a smooth elevation named the glabella. They are larger in the male than in the female, and their degree of prominence depends to some extent on the size of the frontal air sinuses; 28 prominent ridges are, however, occasionally associated with small air sinuses. Beneath each superciliary arch is a curved and prominent margin, the supraorbital margin, which forms the upper boundary of the base of the orbit, and separates the squama from the orbital portion of the bone. The lateral part of this margin is sharp and prominent, affording to the eye, in that situation, considerable protection from injury; the medial part is rounded. At the junction of its medial and intermediate thirds is a notch, sometimes converted into a foramen, the supraorbital notch or foramen, which transmits the supraorbital vessels and nerve. A small aperture in the upper part of the notch transmits a vein from the diploë to join the supraorbital vein. The supraorbital margin ends laterally in the zygomatic process, which is strong and prominent, and articulates with the zygomatic bone. Running upward and backward from this process is a well-marked line, the temporal line, which divides into the upper and lower temporal lines, continuous, in the articulated skull, with the corresponding lines on the parietal bone. The area below and behind the temporal line forms the anterior part of the temporal fossa, and gives origin to the Temporalis muscle. Between the supraorbital margins the squama projects downward to a level below that of the zygomatic processes; this portion is known as the nasal part and presents a rough, uneven interval, the nasal notch, which articulates on either side of the middle line with the nasal bone, and laterally with the frontal process of the maxilla and with the lacrimal. The term nasion is applied to the middle of the frontonasal suture. From the center of the notch the nasal process projects downward and forward beneath the nasal bones and frontal processes of the maxillæ, and supports the bridge of the nose. The nasal process ends below in a sharp spine, and on either side of this is a small grooved surface which enters into the formation of the roof of the corresponding nasal cavity. The spine forms part of the septum of the nose, articulating in front with the crest of the nasal bones and behind with the perpendicular plate of the ethmoid. 2

FIG. 134– Frontal bone. Outer surface. (See enlarged image)

The internal surface (Fig. 135) of the squama is concave and presents in the upper part of the middle line a vertical groove, the sagittal sulcus, the edges of which unite below to form a ridge, the frontal crest; the sulcus lodges the superior sagittal sinus, while its margins and the crest afford attachment to the falx cerebri. The crest ends below in a small notch which is converted into a foramen, the foramen cecum, by articulation with the ethmoid. This foramen varies in size in different subjects, and is frequently impervious; when open, it transmits a vein from the nose to the superior sagittal sinus. On either side of the middle line the bone presents depressions for the convolutions of the brain, and numerous small furrows for the anterior branches of the middle meningeal vessels. Several small, irregular fossæ may also be seen on either side of the sagittal sulcus, for the reception of the arachnoid granulations. 3

Orbital or Horizontal Part (pars orbitalis).—This portion consists of two thin triangular plates, the orbital plates, which form the vaults of the orbits, and are separated from one another by a median gap, the ethmoidal notch. 4

FIG. 135– Frontal bone. Inner surface. (See enlarged image)

Surfaces.—The inferior surface (Fig. 135) of each orbital plate is smooth and concave, and presents, laterally, under cover of the zygomatic process, a shallow depression, the lacrimal fossa, for the lacrimal gland; near the nasal part is a depression, the fovea trochlearis, or occasionally a small trochlear spine, for the attachment of the cartilaginous pulley of the Obliquus oculi superior. The superior surface is convex, and marked by depressions for the convolutions of the frontal lobes of the brain, and faint grooves for the meningeal branches of the ethmoidal vessels. 5
The ethmoidal notch separates the two orbital plates; it is quadrilateral, and filled, in the articulated skull, by the cribriform plate of the ethmoid. The margins of the notch present several half-cells which, when united with corresponding half-cells on the upper surface of the ethmoid, complete the ethmoidal air cells. Two grooves cross these edges transversely; they are converted into the anterior and posterior ethmoidal canals by the ethmoid, and open on the medial wall of the orbit. The anterior canal transmits the nasociliary nerve and anterior ethmoidal vessels, the posterior, the posterior ethmoidal nerve and vessels. In front of the ethmoidal notch, on either side of the frontal spine, are the openings of the frontal air sinuses. These are two irregular cavities, which extend backward, upward, and lateralward for a variable distance between the two tables of the skull; they are separated from one another by a thin bony septum, which often deviates to one or other side, with the result that the sinuses are rarely symmetrical. Absent at birth, they are usually fairly well-developed between the seventh and eighth years, but only reach their full size after puberty. They vary in size in different persons, and are larger in men than in women. 29 They are lined by mucous membrane, and each communicates with the corresponding nasal cavity by means of a passage called the frontonasal duct. 6

Borders.—The border of the squama is thick, strongly serrated, bevelled at the expense of the inner table above, where it rests upon the parietal bones, and at the expense of the outer table on either side, where it receives the lateral pressure of those bones; this border is continued below into a triangular, rough surface, which articulates with the great wing of the sphenoid. The posterior borders of the orbital plates are thin and serrated, and articulate with the small wings of the sphenoid. 7

Structure.—The squama and the zygomatic processes are very thick, consisting of diploic tissue contained between two compact laminæ; the diploic tissue is absent in the regions occupied by the frontal air sinuses. The orbital portion is thin, translucent, and composed entirely of compact bone; hence the facility with which instruments can penetrate the cranium through this part of the orbit; when the frontal sinuses are exceptionally large they may extend backward for a considerable distance into the orbital portion, which in such cases also consists of only two tables. 8

Ossification (Fig. 136).—The frontal bone is ossified in membrane from two primary centers, one for each half, which appear toward the end of the second month of fetal life, one above each supraorbital margin. From each of these centers ossification extends upward to form the corresponding half of the squama, and backward to form the orbital plate. The spine is ossified from a pair of secondary centers, on either side of the middle line; similar centers appear in the nasal part and zygomatic processes. At birth the bone consists of two pieces, separated by the frontal suture, which is usually obliterated, except at its lower part, by the eighth year, but occasionally persists throughout life. It is generally maintained that the development of the frontal sinuses begins at the end of the first or beginning of the second year, but Onodi’s researches indicate that development begins at birth. The sinuses are of considerable size by the seventh or eighth year, but do not attain their full proportions until after puberty. 9

Articulations.—The frontal articulates with twelve bones: the sphenoid, the ethmoid, the two parietals, the two nasals, the two maxillæ, the two lacrimals, and the two zygomatics. 10

FIG. 136– Frontal bone at birth. (See enlarged image)

Note 28. Some confusion is occasioned to students commencing the study of anatomy by the name “sinus” having been given to two different kinds of space connected with the skull. It may be as well, therefore, to state here that the “sinuses” in the interior of the cranium which produce the grooves on the inner surfaces of the bones are venous channels which convey the blood from the brain, while the “sinuses” external to the cranial cavity (the frontal, sphenoidal, ethmoidal, and maxillary) are hollow spaces in the bones themselves; they communicate with the nasal cavities and contain air. [back]
Note 29. Aldren Turner (The Accessory Sinuses of the Nose, 1901) gives the following measurements for a sinus of average size: height, 1 1/4 inches; breadth, 1 inch; depth from before backward, 1 inch. [back]