7d. Talocrural Articulation or Ankle-joint

FIG. 354– Ligaments of the medial aspect of the foot. (Quain.) (See enlarged image)

(Articulatio Talocruralis; Tibiotarsal Articulation)

The ankle-joint is a ginglymus, or hinge-joint. The structures entering into its formation are the lower end of the tibia and its malleolus, the malleolus of the fibula, and the transverse ligament, which together form a mortise for the reception of the upper convex surface of the talus and its medial and lateral facets. The bones are connected by the following ligaments: 1
The Articular Capsule.
The Anterior Talofibular.
The Deltoid.
The Posterior Talofibular.
The Calcaneofibular.

The Articular Capsule (capsula articularis; capsular ligament).—The articular capsule surrounds the joints, and is attached, above, to the borders of the articular surfaces of the tibia and malleoli; and below, to the talus around its upper articular surface. The anterior part of the capsule (anterior ligament) is a broad, thin, membranous layer, attached, above, to the anterior margin of the lower end of the tibia; below, to the talus, in front of its superior articular surface. It is in relation, in front, with the Extensor tendons of the toes, the tendons of the Tibialis anterior and Peronæus tertius, and the anterior tibial vessels and deep peroneal nerve. The posterior part of the capsule (posterior ligament) is very thin, and consists principally of transverse fibers. It is attached, above, to the margin of the articular surface of the tibia, blending with the transverse ligament; below, to the talus behind its superior articular facet. Laterally, it is somewhat thickened, and is attached to the hollow on the medial surface of the lateral malleolus. 2

FIG. 355– The ligaments of the foot from the lateral aspect. (Quain.) (See enlarged image)

The Deltoid Ligament (ligamentum deltoideum; internal lateral ligament) (Fig. 331).—The deltoid ligament is a strong, flat, triangular band, attached, above, to the apex and anterior and posterior borders of the medial malleolus. It consists of two sets of fibers, superficial and deep. Of the superficial fibers the most anterior (tibionavicular) pass forward to be inserted into the tuberosity of the navicular bone, and immediately behind this they blend with the medial margin of the plantar calcaneonavicular ligament; the middle (calcaneotibial) descend almost perpendicularly to be inserted into the whole length of the sustentaculum tali of the calcaneus; the posterior fibers (posterior talotibial) pass backward and lateralward to be attached to the inner side of the talus, and to the prominent tubercle on its posterior surface, medial to the groove for the tendon of the Flexor hallucis longus. The deep fibers (anterior talotibial) are attached, above, to the tip of the medial malleolus, and, below, to the medial surface of the talus. The deltoid ligament is covered by the tendons of the Tibialis posterior and Flexor digitorum longus. 3
The anterior and posterior talofibular and the calcaneofibular ligaments were formerly described as the three fasciculi of the external lateral ligament of the ankle-joint. 4

The Anterior Talofibular Ligament. (ligamentum talofibulare anterius) (Fig. 355).—The anterior talofibular ligament, the shortest of the three, passes from the anterior margin of the fibular malleolus, forward and medially, to the talus, in front of its lateral articular facet. 5

The Posterior Talofibular Ligament (ligamentum talofibulare posterius) (Fig. 355).—The posterior talofibular ligament, the strongest and most deeply seated, runs almost horizontally from the depression at the medial and back part of the fibular malleolus to a prominent tubercle on the posterior surface of the talus immediately lateral to the groove for the tendon of the Flexor hallucis longus. 6

The Calcaneofibular Ligament (ligamentum calcaneofibulare) (Fig. 355).—The calcaneofibular ligament, the longest of the three, is a narrow, rounded cord, running from the apex of the fibular malleolus downward and slightly backward to a tubercle on the lateral surface of the calcaneus. It is covered by the tendons of the Peronæi longus and brevis. 7

FIG. 356– Capsule of left talocrura articulation (distended). Lateral aspect. (See enlarged image)

Synovial Membrane (Fig. 356).—The synovial membrane invests the deep surfaces of the ligaments, and sends a small process upward between the lower ends of the tibia and fibula. 8

Relations.—The tendons, vessels, and nerves in connection with the joint are, in front, from the medial side, the Tibialis anterior, Extensor hallucis proprius, anterior tibial vessels, deep peroneal nerve, Extensor digitorum longus, and Peronæus tertius; behind, from the medial side, the Tibialis posterior, Flexor digitorum longus, posterior tibial vessels, tibial nerve, Flexor hallucis longus; and, in the groove behind the fibular malleolus, the tendons of the Peronæi longus and brevis. 9
The arteries supplying the joint are derived from the malleolar branches of the anterior tibial and the peroneal. 10
The nerves are derived from the deep peroneal and tibial. 11

Movements.—When the body is in the erect position, the foot is at right angles to the leg. The movements of the joint are those of dorsiflexion and extension; dorsiflexion consists in the approximation of the dorsum of the foot to the front of the leg, while in extension the heel is drawn up and the toes pointed downward. The range of movement varies in different individuals from about 50° to 90°. The transverse axis about which movement takes place is slightly oblique. The malleoli tightly embrace the talus in all positions of the joint, so that any slight degree of side-to-side movement which may exist is simply due to stretching of the ligaments of the talofibular syndesmosis, and slight bending of the body of the fibula. The superior articular surface of the talus is broader in front than behind. In dorsiflexion, herefore, greater space is required between the two malleoli. This is obtained by a slight outward rotatory movement of the lower end of the fibula and a stretching of the ligaments of the syndesmosis; this lateral movement is facilitated by a slight gliding at the tibiofibular articulation, and possibly also by the bending of the body of the fibula. Of the ligaments, the deltoid is of very great power—so much so, that it usually resists a force which fractures the process of bone to which it is attached. Its middle portion, together with the calcaneofibular ligament, binds the bones of the leg firmly to the foot, and resists displacement in every direction. Its anterior and posterior fibers limit extension and flexion of the foot respectively, and the anterior fibers also limit abduction. The posterior talofibular ligament assists the calcaneofibular in resisting the displacement of the foot backward, and deepens the cavity for the reception of the talus. The anterior talofibular is a security against the displacement of the foot forward, and limits extension of the joint. 12
The movements of inversion and eversion of the foot, together with the minute changes in form by which it is applied to the ground or takes hold of an object in climbing, etc., are mainly effected in the tarsal joints; the joint which enjoys the greatest amount of motion being that between the talus and calcaneus behind and the navicular and cuboid in front. This is often called the transverse tarsal joint, and it can, with the subordinate joints of the tarsus, replace the ankle-joint in a great measure when the latter has become ankylosed. 13
Extension of the foot upon the tibia and fibula is produced by the Gastrocnemius, Soleus, Plantaris, Tibialis posterior, Peronæi longus and brevis, Flexor digitorum longus, and Flexor hallucis longus; dorsiflexion, by the Tibialis anterior, Peronæus tertius, Extensor digitorum longus, and Extensor hallucis proprius. 74 14
Note 74. The student must bear in mind that the Extensor digitorum longus and Extensor hallucis proprius are extensors of the toes, but flexors of the ankle; and that the Flexor digitorum longus and Flexor hallucis longus are flexors of the toes, but extensors of the ankle. [back]

7e. Intertarsal Articulations

(Articulationes Intertarseæ; Articulations of the Tarsus)

Talocalcaneal Articulation (articulatio talocalcanea; articulation of the calcaneus and astragalus; calcaneo-astragaloid articulation).—The articulations between the calcaneus and talus are two in number—anterior and posterior. Of these, the anterior forms part of the talocalcaneonavicular joint, and will be described with that articulation. The posterior or talocalcaneal articulation is formed between the posterior calcaneal facet on the inferior surface of the talus, and the posterior facet on the superior surface of the calcaneus. It is an arthrodial joint, and the two bones are connected by an articular capsule and by anterior, posterior, lateral, medial, and interosseous talocalcaneal ligaments.

The Articular Capsule (capsula articularis).—The articular capsule envelops the joint, and consists for the most part of the short fibers, which are split up into distinct slips; between these there is only a weak fibrous investment.

The Anterior Talocalcaneal Ligament (ligamentum talocalcaneum anterius; anterior calcaneo-astragaloid ligament) (Figs. 356, 359).—The anterior talocalcaneal ligament extends from the front and lateral surface of the neck of the talus to the superior surface of the calcaneus. It forms the posterior boundary of the talocalcaneonavicular joint, and is sometimes described as the anterior interosseous ligament.

The Posterior Talocalcaneal Ligament (ligamentum talocalcaneum posterius; posterior calcaneo-astragaloid ligament) (Fig. 354).—The posterior talocalcaneal ligament connects the lateral tubercle of the talus with the upper and medial part of the calcaneus; it is a short band, and its fibers radiate from their narrow attachment to the talus. 4

The Lateral Talocalcaneal Ligament (ligamentum talocalcaneum laterale; external calcaneo-astragaloid ligament) (Figs. 356, 359).—The lateral talocalcaneal ligament is a short, strong fasciculus, passing from the lateral surface of the talus, immediately beneath its fibular facet to the lateral surface of the calcaneus. It is placed in front of, but on a deeper plane than, the calcaneofibular ligament, with the fibers of which it is parallel. 5

The Medial Talocalcaneal Ligament (ligamentum talocalcaneum mediale; internal calcaneo-astragaloid ligament).—The medial talocalcaneal ligament connects the medial tubercle of the back of the talus with the back of the sustentaculum tali. Its fibers blend with those of the plantar calcaneonavicular ligament (Fig. 354). 6

FIG. 357– Coronal section through right talocrural and talocalcaneal joints. (See enlarged image)

The Interosseous Talocalcaneal Ligament (ligamentum talocalcaneum interosseum) (Figs. 357, 359).—The interosseous talocalcaneal ligament forms the chief bond of union between the bones. It is, in fact, a portion of the united capsules of the talocalcaneonavicular and the talocalcaneal joints, and consists of two partially united layers of fibers, one belonging to the former and the other to the latter joint. It is attached, above, to the groove between the articular facets of the under surface of the talus; below, to a corresponding depression on the upper surface of the calcaneus. It is very thick and strong, being at least 2.5 cm. in breadth from side to side, and serves to bind the calcaneus and talus firmly together. 7

Synovial Membrane (Fig. 360).—The synovial membrane lines the capsule of the joint, and is distinct from the other synovial membranes of the tarsus. 8

Movements.—The movements permitted between the talus and calcaneus are limited to gliding of the one bone on the other backward and forward and from side to side. 9

Talocalcaneonavicular Articulation (articulatio talocalcaneonavicularis).—This articulation is an arthrodial joint: the rounded head of the talus being received into the concavity formed by the posterior surface of the navicular, the anterior articular surface of the calcaneus, and the upper surface of the planter calcaneonavicular ligament. There are two ligaments in this joint: the articular capsule and the dorsal talonavicular. 10

The Articular Capsule (capsula articularis).—The articular capsule is imperfectly developed except posteriorly, where it is considerably thickened and forms, with a part of the capsule of the talocalcaneal joint, the strong interosseous ligament which fills in the canal formed by the opposing grooves on the calcaneus and talus, as above mentioned. 11

The Dorsal Talonavicular Ligament (ligamentum talonaviculare dorsale; superior astragalonavicular ligament) (Fig. 354).—This ligament is a broad, thin band, which connects the neck of the talus to the dorsal surface of the navicular bone; it is covered by the Extensor tendons. The plantar calcaneonavicular supplies the place of a plantar ligament for this joint. 12

Synovial Membrane.—The synovial membrane lines all parts of the capsule of the joint. 13

Movements.—This articulation permits of a considerable range of gliding movements, and some rotation; its feeble construction allows occasionally of dislocation of the other bones of the tarsus from the talus. 14

Calcaneocuboid Articulation (articulatio calcaneocuboidea; articulation of the calcaneus with the cuboid).—The ligaments connecting the calcaneus with the cuboid are five in number, viz., the articular capsule, the dorsal calcaneocuboid, part of the bifurcated, the long plantar, and the plantar calcaneocuboid. 15

The Articular Capsule (capsula articularis).—The articular capsule is an imperfectly developed investment, containing certain strengthened bands, which form the other ligaments of the joint. 16

The Dorsal Calcaneocuboid Ligament (ligamentum calcaneocuboideum dorsale; superior calcaneocuboid ligament) (Fig. 355).—The dorsal calcaneocuboid ligament is a thin but broad fasciculus, which passes between the contiguous surfaces of the calcaneus and cuboid, on the dorsal surface of the joint. 17

The Bifurcated Ligament (ligamentum bifurcatum; internal calcaneocuboid; interosseous ligament) (Fig. 355, 359).—The bifurcated ligament is a strong band, attached behind to the deep hollow on the upper surface of the calcaneus and dividing in front in a Y-shaped manner into a calcaneocuboid and a calcaneonavicular part. The calcaneocuboid part is fixed to the medial side of the cuboid and forms one of the principal bonds between the first and second rows of the tarsal bones. The calcaneonavicular part is attached to the lateral side of the navicular. 18

The Long Plantar Ligament (ligamentum plantare longum; long calcaneocuboid ligament; superficial long plantar ligament) (Fig. 358).—The long plantar ligament is the longest of all the ligaments of the tarsus: it is attached behind to the plantar surface of the calcaneus in front of the tuberosity, and in front to the tuberosity on the plantar surface of the cuboid bone, the more superficial fibers being continued forward to the bases of the second, third, and fourth metatarsal bones. This ligament converts the groove on the plantar surface of the cuboid into a canal for the tendon of the Peronæus longus. 19

The Plantar Calcaneocuboid Ligament (ligamentum calcaneocuboideum plantare; short calcaneocuboid ligament; short plantar ligament) (Fig. 358).—The plantar calcaneocuboid ligament lies nearer to the bones than the preceding, from which it is separated by a little areolar tissue. It is a short but wide band of great strength, and extends from the tubercle and the depression in front of it, on the forepart of the plantar surface of the calcaneus, to the plantar surface of the cuboid behind the peroneal groove. 20

Synovial Membrane.—The synovial membrane lines the inner surface of the capsule and is distinct from that of the other tarsal articulations (Fig. 360). 21

Movements.—The movements permitted between the calcaneus and cuboid are limited to slight gliding movements of the bones upon each other. 22
The transverse tarsal joint is formed by the articulation of the calcaneus with the cuboid, and the articulation of the talus with the navicular. The movement which takes place in this joint is more extensive than that in the other tarsal joints, and consists of a sort of rotation by means of which the foot may be slightly flexed or extended, the sole being at the same time carried medially (inverted) or laterally (everted). 23

The Ligaments Connecting the Calcaneus and Navicular.—Though the calcaneus and navicular do not directly articulate, they are connected by two ligaments: the calcaneonavicular part of the bifurcated, and the plantar calcaneonavicular. 24
The calcaneonavicular part of the bifurcated ligament is described on page 354. 25

FIG. 358– Ligaments of the sole of the foot, with the tendons of the Peronæus longus, Tibialis posterior and Tibialis anterior muscles. (Quain.) (See enlarged image)

The Plantar Calcaneonavicular Ligament (ligamentum calcaneonaviculare plantare; inferior or internal calcaneonavicular ligament; calcaneonavicular ligament) (Figs. 354, 358).—The plantar calcaneonavicular ligament is a broad and thick band of fibers, which connects the anterior margin of the sustentaculum tali of the calcaneus to the plantar surface of the navicular. This ligament not only serves to connect the calcaneus and navicular, but supports the head of the talus, forming part of the articular cavity in which it is received. The dorsal surface of the ligament presents a fibrocartilaginous facet, lined by the synovial membrane, and upon this a portion of the head of the talus rests. Its plantar surface is supported by the tendon of the Tibialis posterior; its medial border is blended with the forepart of the deltoid ligament of the ankle-joint. 26

FIG. 359– Talocalcaneal and talocalcaneonavicular articulations exposed from above by removing the talus. (See enlarged image)

The plantar calcaneonavicular ligament, by supporting the head of the talus, is principally concerned in maintaining the arch of the foot. When it yields, the head of the talus is pressed downward, medialward, and forward by the weight of the body, and the foot becomes flattened, expanded, and turned lateralward, and exhibits the condition known as flat-foot. This ligament contains a considerable amount of elastic fibers, so as to give elasticity to the arch and spring to the foot; hence it is sometimes called the “spring” ligament. It is supported, on its plantar surface, by the tendon of the Tibialis posterior, which spreads out at its insertion into a number of fasciculi, to be attached to most of the tarsal and metatarsal bones. This prevents undue stretching of the ligament, and is a protection against the occurrence of flat-foot; hence muscular weakness is, in most cases, the primary cause of the deformity. 27

Cuneonavicular Articulation (articulatio cuneonavicularis; articulation of the navicular with the cuneiform bones).—The navicular is connected to the three cuneiform bones by dorsal and plantar ligaments. 28

The Dorsal Ligaments (ligamenta navicularicuneiformia dorsalia).—The dorsal ligaments are three small bundles, one attached to each of the cuneiform bones. The bundle connecting the navicular with the first cuneiform is continuous around the medial side of the articulation with the plantar ligament which unites these two bones (Figs. 354, 355). 29

The Plantar Ligaments (ligamenta navicularicuneiformia plantaria).—The plantar ligaments have a similar arrangement to the dorsal, and are strengthened by slips from the tendon of the Tibialis posterior (Fig. 358). 30

Synovial Membrane.—The synovial membrane of these joints is part of the great tarsal synovial membrane (Fig. 360). 31

Movements.—Mere gliding movements are permitted between the navicular and cuneiform bones. 32

Cuboideonavicular Articulation.—The navicular bone is connected with the cuboid by dorsal, plantar, and interosseous ligaments. 33

The Dorsal Ligament (ligamentum cuboideonaviculare dorsale).—The dorsal ligament extends obliquely forward and lateralward from the navicular to the cuboid bone (Fig. 355). 34

The Plantar Ligament (ligamentum cuboideonaviculare plantare).—The plantar ligament passes nearly transversely between these two bones (Fig. 358). 35

The Interosseous Ligament.—The interosseous ligament consists of strong transverse fibers, and connects the rough non-articular portions of the adjacent surfaces of the two bones Fig. 360). 36

Synovial Membrane.—The synovial membrane of this joint is part of the great tarsal synovial membrane (Fig. 360). 37

Movements.—The movements permitted between the navicular and cuboid bones are limited to a slight gliding upon each other. 38

FIG. 360– Oblique section of left intertarsal and tarsometatarsal articulations, showing the synovial cavities. (See enlarged image)

Intercuneiform and Cuneocuboid Articulations.—The three cuneiform bones and the cuboid are connected together by dorsal, plantar, and interosseous ligaments. 39

The Dorsal Ligaments (ligamenta intercuneiformia dorsalia).—The dorsal ligaments consist of three transverse bands: one connects the first with the second cuneiform, another the second with the third cuneiform, and another the third cuneiform with the cuboid. 40

The Plantar Ligaments (ligamenta intercuneiformia plantaria).—The plantar ligaments have a similar arrangement to the dorsal, and are strengthened by slips from the tendon of the Tibialis posterior. 41

The Interosseous Ligaments (ligamenta intercuneiformia interossea).—The interosseous ligaments consist of strong transverse fibers which pass between the rough non-articular portions of the adjacent surfaces of the bones (Fig. 360). 42

Synovial Membrane.—The synovial membrane of these joints is part of the great tarsal synovial membrane (Fig. 360). 43

Movements.—The movements permitted between these bones are limited to a slight gliding upon each other. 44

1F. Tarsometatarsal Articulations

(Articulationes Tarsometatarseæ)

These are arthrodial joints. The bones entering into their formation are the first, second, and third cuneiforms, and the cuboid, which articulate with the bases of the metatarsal bones. The first metatarsal bone articulates with the first cuneiform; the second is deeply wedged in between the first and third cuneiforms articulating by its base with the second cuneiform; the third articulates with the third cuneiform; the fourth, with the cuboid and third cuneiform; and the fifth, with the cuboid. The bones are connected by dorsal, plantar, and interosseous ligaments.

The Dorsal Ligaments (ligamenta tarsometatarsea dorsalia).—The dorsal ligaments are strong, flat bands. The first metatarsal is joined to the first cuneiform by a broad, thin band; the second has three, one from each cuneiform bone; the third has one from the third cuneiform; the fourth has one from the third cuneiform and one from the cuboid; and the fifth, one from the cuboid (Figs. 354, 355).

The Plantar Ligaments (ligamenta tarsometatarsea plantaria).—The plantar ligaments consist of longitudinal and oblique bands, disposed with less regularity than the dorsal ligaments. Those for the first and second metatarsals are the strongest; the second and third metatarsals are joined by oblique bands to the first cuneiform; the fourth and fifth metatarsals are connected by a few fibers to the cuboid (Fig. 358).

The Interosseous Ligaments (ligamenta cuneometatarsea interossia).
—The interosseous ligaments are three in number. The first is the strongest, and passes from the lateral surface of the first cuneiform to the adjacent angle of the second metatarsal. The second connects the third cuneiform with the adjacent angle of the second metatarsal. The third connects the lateral angle of the third cuneiform with the adjacent side of the base of the third metatarsal.

Synovial Membrane (Fig. 360).—The synovial membrane between the first cuneiform and the first metatarsal forms a distinct sac. The synovial membrane between the second and third cuneiforms behind, and the second and third metatarsal bones in front, is part of the great tarsal synovial membrane. Two prolongations are sent forward from it, one between the adjacent sides of the second and third, and another between those of the third and fourth metatarsal bones. The synovial membrane between the cuboid and the fourth and fifth metatarsal bones forms a distinct sac. From it a prolongation is sent forward between the fourth and fifth metatarsal bones.

Movements.—The movements permitted between the tarsal and metatarsal bones are limited to slight gliding of the bones upon each other.

Nerve Supply.—The intertarsal and tarsometatarsal joints are supplied by the deep peroneal nerve. 7

7g. Intermetatarsal Articulations

(Articulationes Intermetatarseæ)

The base of the first metatarsal is not connected with that of the second by any ligaments; in this respect the great toe resembles the thumb. 1
The bases of the other four metatarsals are connected by the dorsal, plantar, and interosseous ligaments. 2
The Dorsal Ligaments (ligamenta basium [oss. metatars.] dorsalia) pass transversely between the dorsal surfaces of the bases of the adjacent metatarsal bones. 3

The Plantar Ligaments (ligamenta basium [oss. metatars] plantaria).—The plantar ligaments have a similar arrangement to the dorsal. 4

The Interosseous Ligaments (ligamenta basium [oss. metatars.] interossea).—The interosseous ligaments consist of strong transverse fibers which connect the rough non-articular portions of the adjacent surfaces. 5

Synovial Membranes (Fig. 360).—The synovial membranes between the second and third, and the third and fourth metatarsal bones are part of the great tarsal synovial membrane; that between the fourth and fifth is a prolongation of the synovial membrane of the cuboideometatarsal joint. 6

Movements.—The movement permitted between the tarsal ends of the metatarsal bones is limited to a slight gliding of the articular surfaces upon one another.

The heads of all the metatarsal bones are connected together by the transverse metatarsal ligament.

The Transverse Metatarsal Ligament.—The transverse metatarsal ligament is a narrow band which runs across and connects together the heads of all the metatarsal bones; it is blended anteriorly with the plantar (glenoid) ligaments of the metatarsophalangeal articulations. Its plantar surface is concave where the Flexor tendons run below it; above it the tendons of the Interossei pass to their insertions. It differs from the transverse metacarpal ligament in that it connects the metatarsal to the others.

The Synovial Membranes in the Tarsal and Tarsometatarsal Joints (Fig. 360).—The synovial membranes found in the articulations of the tarsus and metatarsus are six in number: one for the talocalcaneal articulation; a second for the talocalcaneonavicular articulation; a third for the calcaneocuboid articulation; and a fourth for the cuneonavicular, intercuneiform, and cuneo-cuboid articulations, the articulations of the second and third cuneiforms with the bases of the second and third metatarsal bones, and the adjacent surfaces of the bases of the second, third, and fourth metatarsal bones; a fifth for the first cuneiform with the metatarsal bone of the great toe; and a sixth for the articulation of the cuboid with the fourth and fifth metatarsal bones. A small synovial cavity is sometimes found between the contiguous surfaces of the navicular and cuboid bones.

7h. Metatarsophalangeal Articulations

(Articulationes Metatarsophalangeæ)

The metatarsophalangeal articulations are of the condyloid kind, formed by the reception of the rounded heads of the metatarsal bones in shallow cavities on the ends of the first phalanges. 1
The ligaments are the plantar and two collateral. 2

The Plantar Ligaments (ligamenta accessoria plantaria; glenoid ligaments of Cruveilhier).—The plantar ligaments are thick, dense, fibrous structures. They are placed on the plantar surfaces of the joints in the intervals between the collateral ligaments, to which they are connected; they are loosely united to the metatarsal bones, but very firmly to the bases of the first phalanges. Their plantar surfaces are intimately blended with the transverse metatarsal ligament, and grooved for the passage of the Flexor tendons, the sheaths surrounding which are connected to the sides of the grooves. Their deep surfaces form part of the articular facets for the heads of the metatarsal bones, and are lined by synovial membrane. 3

The Collateral Ligaments (ligamenta collateralia; lateral ligaments).—The collateral ligaments are strong, rounded cords, placed one on either side of each joint, and attached, by one end, to the posterior tubercle on the side of the head of the metatarsal bone, and, by the other, to the contiguous extremity of the phalanx. 4
The place of dorsal ligaments is supplied by the Extensor tendons on the dorsal surfaces of the joints. 5

Movements.—The movements permitted in the metatarsophalangeal articulations are flexion, extension, abduction, and adduction. 6

7i. Articulations of the Digits

(Articulationes Digitorum Pedis; Articulations of the Phalanges)

The interphalangeal articulations are ginglymoid joints, and each has a plantar and two collateral ligaments. 1
The arrangement of these ligaments is similar to that in the metatarsophalangeal articulations: the Extensor tendons supply the places of dorsal ligaments. 2

Movements.—The only movements permitted in the joints of the digits are flexion and extension; these movements are more extensive between the first and second phalanges than between the second and third. The amount of flexion is very considerable, but extension is limited by the plantar and collateral ligaments. 3

7j. Arches of the Foot

In order to allow it to support the weight of the body in the erect posture with the least expenditure of material, the foot is constructed of a series of arches formed by the tarsal and metatarsal bones, and strengthened by the ligaments and tendons of the foot.

The main arches are the antero-posterior arches, which may, for descriptive purposes, be regarded as divisible into two types—a medial and a lateral. The medial arch (see Fig. 290, page 276) is made up by the calcaneus, the talus, the navicular, the three cuneiforms, and the first, second, and third metatarsals. Its summit is at the superior articular surface of the talus, and its two extremities or piers, on which it rests in standing, are the tuberosity on the plantar surface of the calcaneus posteriorly and the heads of the first, second, and third metatarsal bones anteriorly. The chief characteristic of this arch is its elasticity, due to its height and to the number of small joints between its component parts. Its weakest part, i. e., the part most liable to yield from overpressure, is the joint between the talus and navicular, but this portion is braced by the plantar calcaneonavicular ligament, which is elastic and is thus able to quickly restore the arch to its pristine condition when the disturbing force is removed. The ligament is strengthened medially by blending with the deltoid ligament of the ankle-joint, and is supported inferiorly by the tendon of the Tibialis posterior, which is spread out in a fanshaped insertion and prevents undue tension of the ligament or such an amount of stretching as would permanently elongate it. The arch is further supported by the plantar aponeurosis, by the small muscles in the sole of the foot, by the tendons of the Tibialis anterior and posterior and Peronæus longus, and by the ligaments of all the articulations involved. The lateral arch (see Fig. 291, page 277) is composed of the calcaneus, the cuboid, and the fourth and fifth metatarsals. Its summit is at the talocalcaneal articulation, and its chief joint is the calcaneocuboid, which possesses a special mechanism for locking, and allows only a limited movement. The most marked features of this arch are its solidity and its slight elevation; two strong ligaments, the long plantar and the plantar calcaneocuboid, together with the Extensor tendons and the short muscles of the little toe, preserve its integrity.

While these medial and lateral arches may be readily demonstrated as the component antero-posterior arches of the foot, yet the fundamental longitudinal arch is contributed to by both, and consists of the calcaneus, cuboid, third cuneiform, and third metatarsal: all the other bones of the foot may be removed without destroying this arch.

In addition to the longitudinal arches the foot presents a series of transverse arches. At the posterior part of the metatarsus and the anterior part of the tarsus the arches are complete, but in the middle of the tarsus they present more the characters of half-domes the concavities of which are directed downward and medialward, so that when the medial borders of the feet are placed in apposition a complete tarsal dome is formed. The transverse arches are strengthened by the interosseous, plantar, and dorsal ligaments, by the short muscles of the first and fifth toes (especially the transverse head of the Adductor hallucis), and by the Peronæus longus, whose tendon stretches across between the piers of the arches. 4

IV. Myology

THE MUSCLES 75 are connected with the bones, cartilages, ligaments, and skin, either directly, or through the intervention of fibrous structures called tendons or aponeuroses. Where a muscle is attached to bone or cartilage, the fibers end in blunt extremities upon the periosteum or perichondrium, and do not come into direct relation with the osseous or cartilaginous tissue. Where muscles are connected with its skin, they lie as a flattened layer beneath it, and are connected with its areolar tissue by larger or smaller bundles of fibers, as in the muscles of the face. 1
The muscles vary extremely in their form. In the limbs, they are of considerable length, especially the more superficial ones; they surround the bones, and constitute an important protection to the various joints. In the trunk, they are broad, flattened, and expanded, and assist in forming the walls of the trunk cavities. Hence the reason of the terms, long, broad, short, etc., used in the description of a muscle. 2
There is considerable variation in the arrangement of the fibers of certain muscles with reference to the tendons to which they are attached. In some muscles the fibers are parallel and run directly from their origin to their insertion; these are quadrilateral muscles, such as the Thyreohyoideus. A modification of these is found in the fusiform muscles, in which the fibers are not quite parallel, but slightly curved, so that the muscle tapers at either end; in their actions, however, they resemble the quadrilateral muscles. Secondly, in other muscles the fibers are convergent; arising by a broad origin, they converge to a narrow or pointed insertion. This arrangement of fibers is found in the triangular muscles—e. g., the Temporalis. In some muscles, which otherwise would belong to the quadrilateral or triangular type, the origin and insertion are not in the same plane, but the plane of the line of origin intersects that of the line of insertion; such is the case in the Pectineus. Thirdly, in some muscles (e. g., the Peronei) the fibers are oblique and converge, like the plumes of a quill pen, to one side of a tendon which runs the entire length of the muscle; such muscles are termed unipennate. A modification of this condition is found where oblique fibers converge to both sides of a central tendon; these are called bipennate, and an example is afforded in the Rectus femoris. Finally, there are muscles in which the fibers are arranged in curved bundles in one or more planes, as in the Sphincters. The arrangement of the fibers is of considerable importance in respect to the relative strength and range of movement of the muscle. Those muscles where the fibers are long and few in number have great range, but diminished strength; where, on the other hand, the fibers are short and more numerous, there is great power, but lessened range. 3
The names applied to the various muscles have been derived: (1) from their situation, as the Tibialis, Radialis, Ulnaris, Peronæus; (2) from their direction, as the Rectus abdominis, Obliqui capitis, Transversus abdominis; (3) from their uses, as Flexors, Extensors, Abductors, etc.; (4) from their shape, as the Deltoideus, Rhomboideus; (5) from the number of their divisions, as the Biceps and Triceps; (6) from their points of attachment, as the Sternocleidomastoideus, Sternohyoideus, Sternothyreoideus. 4
In the description of a muscle, the term origin is meant to imply its more fixed or central attachment; and the term insertion the movable point on which the force of the muscle is applied; but the origin is absolutely fixed in only a small number of muscles, such as those of the face which are attached by one extremity to immovable bones, and by the other to the movable integument; in the greater number, the muscle can be made to act from either extremity. 5
In the dissection of the muscles, attention should be directed to the exact origin, insertion, and actions of each, and to its more important relations with surrounding parts. While accurate knowledge of the points of attachment of the muscles is of great importance in the determination of their actions, it is not to be regarded as conclusive. The action of the muscle deduced from its attachments, or even by pulling on it in the dead subject, is not necessarily its action in the living. By pulling, for example, on the Brachioradialis in the cadaver the hand may be slightly supinated when in the prone position and slightly pronated when in the supine position, but there is no evidence that these actions are performed by the muscle during life. It is impossible for an individual to throw into action any one muscle; in other words, movements, not muscles, are represented in the central nervous system. To carry out a movement a definite combination of muscles is called into play, and the individual has no power either to leave out a muscle from this combination or to add one to it. One (or more) muscle of the combination is the chief moving force; when this muscle passes over more than one joint other muscles (synergic muscles) come into play to inhibit the movements not required; a third set of muscles (fixation muscles) fix the limb—i. e., in the case of the limb-movements—and also prevent disturbances of the equilibrium of the body generally. As an example, the movement of the closing of the fist may be considered: (1) the prime movers are the Flexores digitorum, Flexor pollicis longus, and the small muscles of the thumb; (2) the synergic muscles are the Extensores carpi, which prevent flexion of the wrist; while (3) the fixation muscles are the Biceps and Triceps brachii, which steady the elbow and shoulder. A further point which must be borne in mind in considering the actions of muscles is that in certain positions a movement can be effected by gravity, and in such a case the muscles acting are the antagonists of those which might be supposed to be in action. Thus in flexing the trunk when no resistance is interposed the Sacrospinales contract to regulate the action of gravity, and the Recti abdominis are relaxed. 76 6
By a consideration of the action of the muscles, the surgeon is able to explain the causes of displacement in various forms of fracture, and the causes which produce distortion in various deformities, and, consequently, to adopt appropriate treatment in each case. The relations, also, of some of the muscles, especially those in immediate apposition with the larger bloodvessels, and the surface markings they produce, should be remembered, as they form useful guides in the application of ligatures to those vessels. 7

1. Mechanics of Muscle

In studying the mechanical action of muscles 77 the individual muscle cannot always be treated as a single unit, since different parts of the same muscle may have entirely different actions, as with the Pectoralis major, the Deltoid, and the Trapezius where the nerve impulses control and stimulate different portions of the muscle in succession or at different times. Most muscles are, however, in a mechanical sense units. But in either case the muscle fibers constitute the elementary motor elements. 8

FIG. 361– No caption. (See enlarged image)

The Direction of the Muscle Pull.—In those muscles where the fibers always run in a straight line from origin to insertion in all positions of the joint, a straight line joining the middle of the surface of origin with the middle of the insertion surface will give the direction of the pull (Fig. 361). If, however, the muscle or its tendon is bent out of a straight line by a bony process or ligament so that it runs over a pulley-like arrangement, the direction of the muscle pull is naturally bent out of line. The direction of the pull in such cases is from the middle point of insertion to the middle point of the pulley where the muscle or tendon is bent. Muscles or tendons of muscles which pass over more than one joint and pass through more than one pulley may be resolved, so far as the direction of the pull is concerned, into two or more units or single-joint muscles (Fig. 362). The tendons of the Flexor profundus digitorum, for example, pass through several pulleys formed by fibrous sheaths. The direction of the pull is different for each joint and varies for each joint according to the position of the bones. The direction is determined in each case, however, by a straight line between the centers of the pulleys on either side of the joint (Fig. 363). The direction of the pull in any of the segments would not be altered by any change in the position or origin of the muscle belly above the proximal pulley. 9

FIG. 362– No caption. (See enlarged image)

FIG. 363– No caption. (See enlarged image)

The Action of the Muscle Pull on the Tendon.—Where the muscle fibers are parallel or nearly parallel to the direction of the tendon the entire strength of the muscle contraction acts in the direction of the tendon. 10
In pinnate muscles, however, only a portion of the strength of contraction is efficient in the direction of the tendon, since a portion of the pull would tend to draw the tendon to one side, this is mostly annulled by pressure of surrounding parts. In bipinnate muscles this lateral pull is counterbalanced. If, for example, the muscle fibers are inserted into the tendon at an angle of 60 degrees (Fig. 364), it is easy to determine by the parallelogram of forces that the strength of the pull along the direction of the tendon is equal to one-half the muscle pull. 11
T = tendon, m = strength and direction of muscle pull. 12
t = component acting in the direction of the tendon. 13
φ = angle of insertion of muscle fibers into tendon. 14
cos φ = t/m cos ∠ 60° = 0.50000 15
0.5 = t/m t = 1/2 m 16
If < φ = 72° 30' cos = 1/3
< φ = 41° 20' cos = 3/4
< φ = 90° cos = 0
< φ = 0° cos = 1

The more acute the angle φ, that is the smaller the angle, the greater the component acting in the direction of the tendon pull. At 41° 20’ three-fourths of the pull would be exerted in the direction of the tendon and at 0° the entire strength. On the other hand, the greater the angle the smaller the tendon component; at 72° 30’ one-third the muscle strength would act in the direction of the tendon and at 90° the tendon component would be nil. 17

FIG. 364– No caption. (See enlarged image)

The Strength of Muscles.—The strength of a muscle depends upon the number of fibers in what is known as the physiological cross-section, that is, a section which passes through practically all of the fibers. In a muscle with parallel or nearly parallel fibers which have the same direction as the tendon this corresponds to the anatomical cross-section, but in unipinnate and bipinnate muscles the physiological cross-section may be nearly at right angles to the anatomical cross-section as shown in Fig. 365. Since Huber has shown that muscle fibers in a single fasciculus of a given muscle vary greatly in length, in some fasciculi from 9 mm. to 30.4 mm., it is unlikely that the physiological cross-section will pass through all the fibers. Estimates have been made of the strength of muscles and it is probable that coarse-fibered muscles are somewhat stronger per square centimeter of physiological cross-section than are the fine-fibered muscles. Fick estimates the average strength as about 10 kg. per square cm. This is known as the absolute muscle strength. The total strength of a muscle would be equal to the number of square centimeters in its physiological cross-section x 10 kg. 18

FIG. 365– A, fusiform; B, unipinnate; C, bipinnate; P.C.S., physiological cross-section. (See enlarged image)

The work Accomplished by Muscles.—For practical uses this should be expressed in kilogrammeters. In order to reckon the amount of work which a muscle can perform under the most favorable conditions it is necessary to know (1) its physiological cross-section (2) the maximum shortening, and (3) the position of the joint when the latter is obtained. 19
Work = lifted weight x height through which the weight is lifted; or 20
Work = tension x distance; tension = physiological cross-section x absolute muscle strength. 21
If a muscle has a physiological cross-section of 5 sq. cm. its tension strength = 5 x 10 or 50 kg. If it shortens 5 cm. the work = 50 x .05 = 2.5 kilogrammeters. If one determines then the physiological cross-section and multiplies the absolute muscle strength, 10 kg. by this, the amount of tension is easily obtained. Then one must determine only the amount of shortening of the muscle for any particular position of the joint in order to determine the amount of work the muscle can do, since work = tension x distance. 22
The tension of a muscle is, however, not constant during the course of contraction but is continually decreasing during contraction. It is at a maximum at the beginning and gradually decreases. 23
This can be illustrated by the work diagram Fig. 366. A M D (ordinate) = tension.
A V X (abscissa) = shortening.
A D = tension of muscle in extended or antagonistic position.
A V = amount of actual shortening.
A M = tension in midposition = absolute muscle strength.
D V = shows how the tension sinks from maximum (in the extended position of the muscle) where it is about double that in the midposition (M) to nothing on complete contraction.
Δ A D V = work diagram, in reality the hypothenose is not straight but has a concave curve. The Δ has the same area as the rectangle A M M’ V.
A M = the average tension.
Work = A M x A V kilogrammeters if the size of the ordinate as expressed in kilograms and the abscissa in meters.
24

FIG. 366– No caption. (See enlarged image)

Although the muscle works with a changing tension, yet the accomplishment is the same as if it were contracting with the tension of the midposition. 25
In reality the amount of work is somewhat greater since even in extreme contraction the muscle still retains a certain amount of tension so that the maximum amount of work is more nearly like A D X. We know that a muscle may have an extreme actual shortening of about 80 per cent. of its length when the tendon of insertion is cut. 26
The trapezoid A D S V represents more nearly the amount of work, but since there are only approximate values and A D S V is not much larger than A M M’ V, we may use the latter. 27
Only the tension and amount of shortening are needed to determine the amount of work of the muscle. Neither the lever arm nor the fiber angle in pinnate muscles need be considered. 28
The diagram Fig. 367 shows that the lever arm is of no importance for determining the amount of work the muscle performs. 29
J B and J B1 = two bones jointed at J. C D and E F = the direction of the pull of two muscles of equal cross-section, each having a muscle tension of 1000 gms. 30
The centers of the attachments are such that perpendiculars J c and J e to C D and E F are equal to 40 and 23 mm. respectively, J c = 40 mm. and J e = 23 mm. The static moments are equal to 1000 x 40 and 1000 x 23, therefore the first muscle can hold a much larger load (L) on the bone J B1 at H1 (100 mm. from J) than the second muscle whose load can be designated as L1. 31
Equilibrium exists for the first muscle if

L x 100 = 1000 x 40 or L = 1000 x 40/100 = 400 gms. 32
For the second muscle L1 x 100 = 1000 x 23.

L1 = 1000 x 23/100 = 230 gms. 33
If we suppose J B to be fixed and J B1 to move in the plane of the paper about J and the muscle C D to shorten 5 mm. C d = C D − 5 mm. and with the tension of 1000 gms., J B1 will take the position J B2 and the load (L) will be lifted from H1 to H2. 34
If the second muscle likewise shortens 5 mm. then E f = E F — 5 mm., and with the tension of 1000 gms. the bone J B1 will take the position J B3 and the weight or load (L1) will be lifted from H1 to H3. The question now is to prove that the work done is the same in both cases, namely, 5 x 1000 grammillimeters. If so, 400 x H1 H2 = 230 x H1 H3 = 5000 grammillimeters. 35
Since the two radii C d and C d’ are very long as compared with the arc d d’ we may consider this short arc as a line ?? to C D at d’, likewise the arc f f’ may be considered as a straight line ?? to E F. In the same manner we can consider the short arcs F f, D d, H1 H2 and H1 H3 ?? to the line J B1. The sides D d’ and F f’ of the Δ D d d’ and F f f’ are each 5 mm. 36
The lever arm D J = 60 mm. and J F = 30 mm. 37

FIG. 367– No caption. (See enlarged image)

The Δ D d d’ is similar to the Δ D c J 38
hence D d : 5 :: 60 : 40 D d = 300/40
also H1 H2 : D d :: 100 : 60
H1 H2 : 300/40 :: 100 : 60 H1 H2 = 300/24

hence F f : 5 :: 30 : 23 F f = 150/23
also H1 H3 : F f :: 100 : 30
H1 H3 :150/23::100:30 H1 H3 = 1500/69
… 400 x 300/24 = 230 x 1500/69 = 5000

Thus we see that the work of the two muscles depends on the size of the contraction and on the tension and not on the lever arm in very small contractions or in the summation of such contractions and therefore for large contractions. In the first muscle a large load is moved through a short distance and in the second muscle a lighter load is moved through a greater distance. 39
The amount of work accomplished by pinnate muscles is not dependent upon the angle of insertion of the muscle fibers into the tendon, as will be seen by the following diagram Fig. 368. 40
T’ T = direction of the tendon pull.
w a = direction of muscle fiber before contraction.
m’ = direction of muscle fiber after contraction.
v = amount of contraction.
m = tension of the muscle.
φ = angle of insertion of muscle fiber.
t = tendon component = m x cos φ = the weight carried by the tendon to balance the muscle tension.
d = distance tendon is drawn up.
(1) m x v = work done by the muscle fiber.
(2) t x d = work done by the movement of the tendon.

If we consider the distance v as being very short then the line b c can be dealt with as though it were perpendicular to a c. 41

then v = d x cos φ or d = v/cos φ
since t = m x cos φ or m = t/cos φ
m x v = t/cos φ x d x cos φ = t x d 42
If this is true for very minute contractions it is likewise true for a series of such contraction and hence for larger contractions. 43
If we assume that φ = 60°, m = 10 kg. and v = 5 mm., the work done by the contracting muscle fiber = m v or 10 x 5 kilogrammillimeters. 44

FIG. 368– No caption. (See enlarged image)

cos ∠ 60° = 1/2; hence t = 1/2 m; and d = v/1/2 = 2 v; 1/2 m = 5 kg.; and 2 v = 10 mm. hence t d = 50 kilogrammillimeters or the work done by the movement of the tendon in lifting the load of 5 kg. a distance of 10 mm., and is exactly the same as that done by the muscle fiber. The load on the tendon is but one-half the tension of the muscle, but the distance through which the load is lifted is twice that of the amount of shortening of the muscle.
If φ = 41° 20’ then cos φ = 3/4
hence t = 3/4 m and d = 4/3 v and t d = m v 45
In pinnate muscles, then, we have the rather unexpected condition in which the same amount of movement of the tendon can be accomplished with less contraction of the muscle than in muscles where the fibers have the same direction as the tendon. 46

The Action of Muscles on Joints.—If we consider now the action of a single muscle extending over a single joint in which one bone is fixed and the other movable, we will find that muscle pull can be resolved into two components, a turning component and a friction or pressure component as shown in Fig. 369. 47

FIG. 369– No caption. (See enlarged image)

D F = the fixed bone from which the muscle takes its origin. 48
D K = the movable bone. 49
O I = a line from the middle of origin to the middle of insertion. 50
I M = size and direction of the muscle pull. 51
If the parallelogram is constructed with I t and M b ⊥ to D K, then I t = the turning component and I b = the component which acts against the joint. 52
The size of the two components depends upon the insertion angle φ. The smaller this angle the smaller the turning component, and the nearer this angle φ is to 90° the larger the turning component. 53
I t = I M x sin φ 54
I b = I M x cos φ 55
If φ = 90° cos φ = 0, sine φ = 1 hence I b = 0 and I t = I m 56
If φ = 0° cos φ = 1, sine φ = 0 hence I b = 1 and I t = 0 57
With movements of the bone D K the angle of insertion is continually changing, and hence the two components are changing in value. 58

FIG. 370– No caption. (See enlarged image)

If, for example, the distance from origin 0 to the joint D is greater than from D to I, as in the Brachialis or Biceps muscles, the turning component increases until the insertion angle φ = 90°, which is the optimum angle for muscle action, while the pressure component gradually decreases. If the movement continues beyond this point the turning component gradually decreases and the pressure component changes into a component which tends to draw the two bones apart and which gradually increases as shown in Fig. 370. 59
When the bone D K is in such a position that the insertion angle φ = 41° 20’ the pressure component = 3/4 I m and the turning component 1/4 I m, at 60° the two components are equal, at 90° the pressure component = 0 and the turning component = I M and at 131° 21’ the pressure component has been converted into a pulling component = 1/4 I M and the turning component = 3/4 I M. 60

FIG. 371– No caption. (See enlarged image)

If, for example, the distance from the origin O to the joint D is less than the distance from the insertion I to the joint D, as in the Brachioradialis muscle, the insertion angle increases with the flexion but never reaches 90°. The turning component gradually increases to a certain point and then slowly decreases as shown in Fig. 371, while the pressure component gradually decreases and then slowly increases. It always remains large and its action is always in the direction of the joint. 61

Levers.—The majority of the muscles of the body act on bones as the power on levers. Levers of the III class are the most common, as the action of the Biceps, and the Brachialis muscles on the forearm bones. Levers of the I Class are found in movements of the head where the occipito-atlantal joint acts as the fulcrum and the muscles on the back of the neck as the power. Another common example is 62

FIG. 372– No caption. (See enlarged image)

the foot when one raises the body by contracting the Gastrocnemius and Soleus. Here the ankle-joint acts as the fulcrum and the pressure of the toes on the ground as the weight. This is frequently, though wrongly, considered a lever of the II Class. If one were to stand on one’s head with the legs up and with a weight on the plantar surface of the toes, it is easy to see that we would have a lever of the I Class if the weight were raised by contraction of the Gastrocnemius muscle. The confusion has arisen by not considering the fact that the fulcrum and the power in all three classes of levers must have a common basis of action, as shown in Fig. 372. 63
If the fulcrum rests on the earth the power must either directly or indirectly push from the earth or be attached to the earth either by gravity or otherwise if it pulls toward the earth. If the power were attached to the weight no lever action could be obtained. 64
There are no levers of the II Class represented in the body. 65
Note 75. The muscles and fasciæ are described conjointly, in order that the student may consider the arrangement of the latter in his dissection of the former. It is rare for the student of anatomy in this country to have the opportunity of dissecting the fasciæ separately; and it is for this reason, as well as from the close connection that exists between the muscles and their investing sheaths, that they are considered together. Some general observations are first made on the anatomy of the muscles and fasciæ, the special descriptions being given in connection with the different regions. [back]
Note 76. Consult in this connection the Croonian Lectures (1903) on “Muscular Movements and Their Representation in the Central Nervous System.” by Charles E. Beevor, M.D. [back]
Note 77. R. Fick. Bd. ii, in Bardeleben’s Handbuch der Anatomie des Menschen. [back]

2. Development of the Muscles

Both the cross-striated and smooth muscles, with the exception of a few that are of ectodermal origin, arise from the mesoderm. The intrinsic muscles of the trunk are derived from the myotomes while the muscles of the head and limbs differentiate directly from the mesoderm.

The Myotomic Muscles.—The intrinsic muscles of the trunk which are derived directly from the myotomes are conveniently treated in two groups, the deep muscles of the back and the thoraco-abdominal muscles.

The deep muscles of the back extend from the sacral to the occipital region and vary much in length and size. They act chiefly on the vertebral column. The shorter muscles, such as the Interspinales, Intertransversarii, the deeper layers of the Multifidus, the Rotatores, Levatores costarum, Obliquus capitis inferior, Obliquus capitis superior and Rectus capitis posterior minor which extend between adjoining vertebræ, retain the primitive segmentation of the myotomes. Other muscles, such as the Splenius capitis, Splenius cervicis, Sacrospinalis, Semispinalis, Multifidus, Iliocostalis, Longissimus, Spinales, Semispinales, and Rectus capitis posterior major, which extend over several vertebræ, are formed by the fusion of successive myotomes and the splitting into longitudinal columns.

The fascia lumbo-dorsalis develops between the true myotomic muscles and the more superficial ones which migrate over the back such as the Trapezius, Rhomboideus, and Latissimus.

The anterior vertebral muscles, the Longus colli, Longus capitis, Rectus capitis anterior and Rectus capitis lateralis are derived from the ventral part of the cervical myotomes as are probably also the Scaleni.

The thoraco-abdominal muscles arise through the ventral extension of the thoracic myotomes into the body wall. This process takes place coincident with the ventral extension of the ribs. In the thoracic region the primitive myotomic segments still persist as the intercostal muscles, but over the abdomen these ventral myotomic processes fuse into a sheet which splits in various ways to form the Rectus, the Obliquus externus and internus, and the Transversalis. Such muscles as the Pectoralis major and minor and the Serratus anterior do not belong to the above group.

The Ventrolateral Muscles of the Neck.—The intrinsic muscles of the tongue, the Infrahyoid muscles and the diaphragm are derived from a more or less continuous premuscle mass which extends on each side from the tongue into the lateral region of the upper half of the neck and into it early extend the hypoglossal and branches of the upper cervical nerves. The two halves which form the Infrahyoid muscles and the diaphragm are at first widely separated from each other by the heart. As the latter descends into the thorax the diaphragmatic portion of each lateral mass is carried with its nerve down into the thorax and the laterally placed Infrahyoid muscles move toward the midventral line of the neck.

Muscles of the Shoulder Girdle and Arm.—The Trapezius and Sternocleidomastoideus arise from a common premuscle mass in the occipital region just caudal to the last branchial arch; as the mass increases in size it spreads downward to the shoulder girdle to which it later becomes attached. It also spreads backward and downward to the spinous processes, gaining attachment at a still later period.

The Levator scapulæ, Serratus anterior and the Rhomboids arise from premuscle tissue in the lower cervical region and undergo extensive migration.

The Latissimus dorsi and Teres major are associated in their origin from the premuscle sheath of the arm as are also the two Pectoral muscles when the arm bud lies in the lower cervical region.

The intrinsic muscles of the arm develop in situ from the mesoderm of the arm bud and probably do not receive cells or buds from the myotomes. The nerves enter the arm bud when it still lies in the cervical region and as the arm shifts caudally over the thorax the lower cervical nerves which unite to form the brachial plexus, acquire a caudal direction.

The Muscles of the Leg.—The muscles of the leg like those of the arm develop in situ from the mesoderma of the leg bud, the myotomes apparently taking no part in their formation.

The Muscles of the Head.—The muscles of the orbit arise from the mesoderm over the dorsal and caudal sides of the optic stalk.
The muscles of mastication arise from the mesoderm of the mandibular arch. The mandibular division of the trigeminal nerve enters this premuscle mass before it splits into the Temporal, Masseter and Pterygoideus. 14
The facial muscles (muscles of expression) arise from the mesoderm of the hyoid arch. The facial nerve enters this mass before it begins to split, and as the muscle mass spreads out over the face and head and neck it splits more or less incompletely into the various muscles. 15
The early differentiation of the muscular system apparently goes on independently of the nervous system and only later does it appear that muscles are dependent on the functional stimuli of the nerves for their continued existence and growth. Although the nervous system does not influence muscle differentiation, the nerves, owing to their early attachments to the muscle rudiments, are in a general way indicators of the position of origin of many of the muscles and likewise in many instances the nerves indicate the paths along which the developing muscles have migrated during development. The muscle of the diaphragm, for example, has its origin in the region of the fourth and fifth cervical segments. The phrenic nerve enters the muscle mass while the latter is in this region and is drawn out as the diaphragm migrates through the thorax. The Trapezius and Sternocleidomastoideus arise in the lateral occipital region as a common muscle mass, into which at a very early period the nervus accessorius extends and as the muscle mass migrates and extends caudally the nerve is carried with it. The Pectoralis major and minor arise in the cervical region, receive their nerves while in this position and as the muscle mass migrates and extends caudally over the thorax the nerves are carried along. The Latissimus dorsi and Serratus anterior are excellent examples of migrating muscles whose nerve supply indicates their origin in the cervical region. The Rectus abdominis and the other abdominal muscles migrate or shift from a lateral to a ventrolateral or abdominal position, carrying with them the nerves. 16
The facial nerve, which early enters the common facial muscle mass of the second branchial or hyoid arch, is dragged about with the muscle as it spreads over the head and face and neck, and as the muscle splits into the various muscles of expression, the nerve is correspondingly split. The mandibular division of the trigeminal nerve enters at an early time the muscle mass in the mandibular arch and as this mass splits and migrates apart to form the muscles of mastication the nerve splits into its various branches. 17
The nerve supply then serves as a key to the common origin of certain groups of muscles. The muscles supplied by the oculomotor nerve arise from a single mass in the eye region; the lingual muscles arise from a common mass supplied by the hypoglossal nerve. 18

Striped or Voluntary Muscle.—Striped or voluntary muscle is composed of bundles of fibers each enclosed in a delicate web called the perimysium in contradistinction to the sheath of areolar tissue which invests the entire muscle, the epimysium. The bundles are termed fasciculi; they are prismatic in shape, of different sizes in different muscles, and are for the most part placed parallel to one another, though they have a tendency to converge toward their tendinous attachments. Each fasciculus is made up of a strand of fibers, which also run parallel with each other, and are separated from one another by a delicate connective tissue derived from the perimysium and termed endomysium. This does not form the sheath of the fibers, but serves to support the bloodvessels and nerves ramifying between them. 19
A muscular fiber may be said to consist of a soft contractile substance, enclosed in a tubular sheath named by Bowman the sarcolemma. The fibers are cylindrical or prismatic in shape (Fig. 373), and are of no great length, not exceeding, as a rule, 40 mm. Huber 78 has recently found that the muscle fibers in the adductor muscle of the thigh of the rabbit vary greatly in length even in the same fasciculus. In a fasciculus 40 mm. in length the fibers varied from 30.4 mm. to 9 mm. in length. Their breadth varies in man from 0.01 to 0.1 mm. As a rule, the fibers do not divide or anastomose; but occasionally, especially in the tongue and facial muscles, they may be seen to divide into several branches. In the substance of the muscle, the fibers end by tapering extremities which are joined to the ends of other fibers by the sarcolemma. At the tendinous end of the muscle the sarcolemma appears to blend with a small bundle of fibers, into which the tendon becomes subdivided, while the muscular substance ends abruptly and can be readily made to retract from the point of junction. The areolar tissue between the fibers appears to be prolonged more or less into the tendon, so as to form a kind of sheath around the tendon bundles for a longer or shorter distance. When muscular fibers are attached to skin or mucous membranes, their fibers become continuous with those of the areolar tissue. 20

FIG. 373– Transverse section of human striped muscle fibers. x 255. (See enlarged image)

FIG. 374– Striped muscle fibers from tongue of cat. x 250. (See enlarged image)

The sarcolemma, or tubular sheath of the fiber, is a transparent, elastic, and apparently homogeneous membrane of considerable toughness, so that it sometimes remains entire when the included substance is ruptured. On the internal surface of the sarcolemma in mammalia, and also in the substance of the fiber in frogs, elongated nuclei are seen, and in connection with these is a little granular protoplasm. 21
Upon examination of a voluntary muscular fiber by transmitted light, it is found to be marked by alternate light and dark bands or striæ, which pass transversely across the fiber (Fig. 374). When examined by polarized light the dark bands are found to be doubly refracting (anisotropic), while the clear stripes are singly refracting (isotropic). The dark and light bands are of nearly equal breadth, and alternate with great regularity; they vary in breadth from about 1 to 2μ. If the surface be carefully focussed, rows of granules will be detected at the points of junction of the dark and light bands, and very fine longitudinal lines may be seen running through the dark bands and joining these granules together. By treating the specimen with certain reagents (e. g., chloride of gold) fine lines may be seen running transversely between the granules and uniting them together. This appearance is believed to be due to a reticulum or network of interstitial substance lying between the contractile portions of the muscle. The longitudinal striation gives the fiber the appearance of being made up of a bundle of fibrils which have been termed sarcostyles or muscle columns, and if the fiber be hardened in alcohol, it can be broken up longitudinally and the sarcostyles separated from each other (Fig. 375.) The reticulum, with its longitudinal and transverse meshes, is called sarcoplasm. 22

FIG. 375– A. Portion of a medium-sized human muscular fiber. Magnified nearly 800 diameters. B. Separated bundles of fibrils, equally magnified. a, a. Larger, and b, b, smaller collections. c. Still smaller. d, d. The smallest which could be detached. (See enlarged image)

In a transverse section, the muscular fiber is seen to be divided into a number of areas, called the areas of Cohnheim, more or less polyhedral in shape and consisting of the transversely divided sarcostyles, surrounded by transparent sarcoplasm (Fig. 373). 23

FIG. 376– Diagram of a sarcomere. (After Schäfer.) A. In moderately extended condition. B. In a contracted condition. k, k. Membranes of Krause. H. Line or plane of Hensen. S.E. Poriferous sarcous element. (See enlarged image)

Upon closer examination, and by somewhat altering the focus, the appearances become more complicated, and are susceptible of various interpretations. The transverse striation, which in Fig. 374 appears as a mere alternation of dark and light bands, is resolved into the appearance seen in Fig. 375, which shows a series of broad dark bands, separated by light bands, each of which is divided into two by a dark dotted line. This line is termed Dobie’s line or Krause’s membrane (Fig. 376, k), because it was believed by Krause to be an actual membrane, continuous with the sarcolemma, and dividing the light band into two compartments. In addition to the membrane of Krause, fine clear lines may be made out, with a sufficiently high power, crossing the center of the dark band; these are known as the lines of Hensen (Fig. 376, H). 24
Schäfer has worked out the minute anatomy of muscular fiber, particularly in the wing muscles of insects, which are peculiarly adapted for this purpose on account of the large amount of interstitial sarcoplasm which separates the sarco-styles. In the following description that given by Schäfer will be closely followed. 25
A sarcostyle may be said to be made up of successive portions, each of which is termed a sarcomere. The sarcomere is situated between two membranes of Krause and consists of (1) a central dark part, which forms a portion of the dark band of the whole fiber, and is named a sarcous element. This sarcous element really consists of two parts, superimposed one on the top of the other, and when the fiber is stretched these two parts become separated from each other at the line of Hensen (Fig. 376, A). (2) On either side of this central dark portion is a clear layer, most visible when the fiber is extended; this is situated between the dark center and the membrane of Krause, and when the sarcomeres are joined together to form the sarcostyle, constitutes the light band of the striated muscular fiber. 26
When the sarcostyle is extended, the clear intervals are well-marked and plainly to be seen; when, on the other hand, the sarcostyle is contracted, that is to say, when the muscle is in a state of contraction, these clear portions are very small or they may have disappeared altogether (Fig. 376, B). When the sarcostyle is stretched to its full extent, not only is the clear portion well-marked, but the dark portion—the sarcous element—is separated into its two constituents along the line of Hensen. The sarcous element does not lie free in the sarcomere, for when the sarcostyle is stretched, so as to render the clear portion visible, very fine lines, which are probably septa, may be seen running through it from the sarcous element to the membrane of Krause. 27
Schäfer explains these phenomena in the following way: He considers that each sarcous element is made up of a number of longitudinal channels, which open into the clear part toward the membrane of Krause but are closed at the line of Hensen. When the muscular fiber is contracted the clear part of the muscular substance is driven into these channels or tubes, and is therefore hidden from sight, but at the same time it swells up the sarcous element and widens and shortens the sarcomere. When, on the other hand, the fiber is extended, this clear substance is driven out of the tubes and collects between the sarcous element and the membrane of Krause, and gives the appearance of the light part between these two structures; by this means it elongates and narrows the sarcomere. 28
If this view be true, it is a matter of great interest, and, as Schäfer has shown, harmonizes the contraction of muscle with the ameboid action of protoplasm. In an ameboid cell, there is a framework of spongioplasm, which stains with hematoxylin and similar reagents, enclosing in its meshes a clear substance, hyaloplasm, which will not stain with these reagents. Under stimulation the hyaloplasm passes into the pores of the spongioplasm; without stimulation it tends to pass out as in the formation of pseudopodia. In muscle there is the same thing, viz., a framework of spongioplasm staining with hematoxylin—the substance of the sarcous element—and this encloses a clear hyaloplasm, the clear substance of the sarcomere, which resists staining with this reagent. During contraction of the muscle—i.e., stimulation—this clear substance passes into the pores of the spongioplasm; while during extension of the muscle—i.e., when there is no stimulation—it tends to pass out of the spongioplasm. 29
In this way the contraction is brought about: under stimulation the protoplasmic material (the clear substance of the sarcomere) recedes into the sarcous element, causing the sarcomere to widen out and shorten. The contraction of the muscle is merely the sum total of this widening out and shortening of these bodies. 30

Vessels and Nerves of Striped Muscle.—The capillaries of striped muscle are very abundant, and form a sort of rectangular network, the branches of which run longitudinally in the endomysium between the muscular fibers, and are joined at short intervals by transverse anastomosing branches. In the red muscles of the rabbit dilatations occur on the transverse branches of the capillary network. The larger vascular channels, arteries and veins, are found only in the perimysium, between the muscular fasciculi. Nerves are profusely distributed to striped muscle. Their mode of termination is described on page 730. The existence of lymphatic vessels in striped muscle has not been ascertained, though they have been found in tendons and in the sheaths of the muscles. 31
Ossification of muscular tissue as a result of repeated strain or injury is not infrequent. It is oftenest found about the tendon of the Adductor longus and Vastus medialis in horsemen, or in the Pectoralis major and Deltoideus of soldiers. It may take the form of exostoses firmly fixed to the bone—e.g., “rider’s bone” on the femur—or of layers or spicules of bone lying in the muscles or their fasciæ and tendons. Busse states that these bony deposits are preceded by a hemorrhagic myositis due to injury, the effused blood organizing and being finally converted into bone. In the rarer disease, progressive myositis ossificans, there is an unexplained tendency for practically any of the voluntary muscles to become converted into solid and brittle bony masses which are completely rigid. 32
Note 78. Anat. Rec., 1916, 11. [back]

3. Tendons, Aponeuroses, and Fasciæ

Tendons are white, glistening, fibrous cords, varying in length and thickness, sometimes round, sometimes flattened, and devoid of elasticity. They consist almost entirely of white fibrous tissue, the fibrils of which have an undulating course parallel with each other and are firmly united together. When boiled in water tendon is almost completely converted into gelatin, the white fibers being composed of the albuminoid collagen, which is often regarded as the anhydride of gelatin. They are very sparingly supplied with bloodvessels, the smaller tendons presenting in their interior no trace of them. Nerves supplying tendons have special modifications of their terminal fibers, named organs of Golgi.

Aponeuroses are flattened or ribbon-shaped tendons, of a pearly white color, iridescent, glistening, and similar in structure to the tendons. They are only sparingly supplied with bloodvessels.

The tendons and aponeuroses are connected, on the one hand, with the muscles, and, on the other hand, with the movable structures, as the bones, cartilages ligaments, and fibrous membranes (for instance, the sclera). Where the muscular fibers are in a direct line with those of the tendon or aponeurosis, the two are directly continuous. But where the muscular fibers join the tendon or aponeurosis at an oblique angle, they end, according to Kölliker, in rounded extremities which are received into corresponding depressions on the surface of the latter, the connective tissue between the muscular fibers being continuous with that of the tendon. The latter mode of attachment occurs in all the penniform and bipenniform muscles, and in those muscles the tendons of which commence in a membranous form, as the Gastrocnemius and Soleus.

The fasciæ are fibroareolar or aponeurotic laminæ, of variable thickness and strength, found in all regions of the body, investing the softer and more delicate organs. During the process of development many of the cells of the mesoderm are differentiated into bones, muscles, vessels, etc.; the cells of the mesoderm which are not so utilized form an investment for these structures and are differentiated into the true skin and the fasciæ of the body. They have been subdivided, from the situations in which they occur, into superficial and deep.

The superficial fascia is found immediately beneath the integument over almost the entire surface of the body. It connects the skin with the deep fascia, and consists of fibroareolar tissue, containing in its meshes pellicles of fat in varying quantity. Fibro-areolar tissue is composed of white fibers and yellow elastic fibers intercrossing in all directions, and united together by a homogeneous cement or ground substance, the matrix.

The cells of areolar tissue are of four principal kinds:

(1) Flattened lamellar cells, which may be either branched or unbranched. The branched lamellar cells are composed of clear cytoplasm, and contain oval nuclei; the processes of these cells may unite so as to form an open network, as in the cornea. The unbranched cells are joined edge to edge like the cells of an epithelium; the “tendon cells,” presently to be described, are examples of this variety.

(2) Clasmatocytes, large irregular cells characterized by the presence of granules or vacuoles in their protoplasm, and containing oval nuclei.

(3) Granule cells (Mastzellen), which are ovoid or spheroidal in shape. They are formed of a soft protoplasm, containing granules which are basophil in character.

(4) Plasma cells of Waldeyer, usually spheroidal and distinguished by containing a vacuolated protoplasm. The vacuoles are filled with fluid, and the protoplasm between the spaces is clear, with occasionally a few scattered basophil granules. 6

FIG. 377– Subcutaneous tissue from a young rabbit. Highly magnified. (Schäfer.) (See enlarged image)

In addition to these four typical forms of connective-tissue corpuscles, areolar tissue may be seen to possess wandering cells, i.e., leucocytes which have emigrated from the neighboring vessels; in some instances, as in the choroid coat of the eye cells filled with granules of pigment (pigment cells) are found.

The cells lie in spaces in the ground substance between the bundles of fibers, and these spaces may be brought into view by treating the tissue with nitrate of silver and exposing it to the light. This will color the ground substance and leave the cell-spaces unstained.
Fat is entirely absent in the subcutaneous tissue of the eyelids, of the penis and scrotum, and of the labia minora. It varies in thickness in different parts of the body; in the groin it is so thick that it may be subdivided into several laminæ. Beneath the fatty layer there is generally another layer of superficial fascia, comparatively devoid of adipose tissue, in which the trunks of the subcutaneous vessels and nerves are found, as the superficial epigastric vessels in the abdominal region, the superficial veins in the forearm, the saphenous veins in the leg and thigh, and the superficial lymph glands. Certain cutaneous muscles also are situated in the superficial fascia, as the Platysma in the neck, and the Orbicularis oculi around the eyelids. This fascia is most distinct at the lower part of the abdomen, perineum, and extremities; it is very thin in those regions where muscular fibers are inserted into the integument, as on the side of the neck, the face, and around the margin of the anus. It is very dense in the scalp, in the palms of the hands, and soles of the feet, forming a fibro-fatty layer, which binds the integument firmly to the underlying structures. 9
The superficial fascia connects the skin to the subjacent parts, facilitates the movement of the skin, serves as a soft nidus for the passage of vessels and nerves to the integument, and retains the warmth of the body, since the fat contained in its areolæ is a bad conductor of heat. 10
The deep fascia is a dense, inelastic, fibrous membrane, forming sheaths for the muscles, and in some cases affording them broad surfaces for attachment. It consists of shining tendinous fibers, placed parallel with one another, and connected together by other fibers disposed in a rectilinear manner. It forms a strong investment which not only binds down collectively the muscles in each region, but gives a separate sheath to each, as well as to the vessels and nerves. The fasciæ are thick in unprotected situations, as on the lateral side of a limb, and thinner on the medial side. The deep fasciæ assist the muscles in their actions, by the degree of tension and pressure they make upon their surfaces; the degree of tension and pressure is regulated by the associated muscles, as, for instance, by the Tensor fasciæ latæ and Glutæus maximus in the thigh, by the Biceps in the upper and lower extremities, and Palmaris longus in the hand. In the limbs, the fasciæ not only invest the entire limb, but give off septa which separate the various muscles, and are attached to the periosteum: these prolongations of fasciæ are usually spoken of as intermuscular septa. 11
The Fasciæ and Muscles may be arranged, according to the general division of the body, into those of the head and neck; of the trunk; of the upper extremity; and of the lower extremity. 12