Muscles of the leg.—The arrangement of the skeleton of the leg (tibia and fibula) is such that we might expect a priori to find four muscular masses, one on each of the surfaces of the skeleton, but as the internal surface of the tibia is subcutaneous (2, Fig. 84), is not covered by any muscle, and is overlapped by the anterior and posterior fleshy masses, it forms a long flat surface, slightly hollowed, extending from the inner side of the knee to the internal malleolus. The leg, therefore, presents for our study only three groups of muscles, occupying respectively the anterior or antero-external, the external or peroneal, and the posterior regions of the limb.

I. Anterior muscles (Fig. 84).—Arranged in the space which separates the fibula from the tibia, the muscles on the front of the leg are three in number, called, from within outwards, the tibialis anticus, the extensor proprius hallucis, and the extensor longus digitorum.

The tibialis anticus (3, Fig. 84) arises from the external surface of the tibia, and from the deep fascia over it (page 146), and descends obliquely downwards and inwards in the form of a prismatic or fusiform fleshy mass, of which the lower end gradually narrows, to be replaced by a strong tendon in the lower third of the leg. This tendon, inclining more and more to the inner side (2, Fig. 86), passes obliquely over the anterior surface of the lower end of the tibia, in front of the internal malleolus. It glides beneath the anterior annular ligament of the ankle, and reaches the inner part of the dorsum of the foot (3, Fig. 86), where it is inserted into the internal cuneiform, and the base of the first metatarsal bone. It flexes the foot on the leg, and inverts it, since it draws the dorsal surface of the foot towards the anterior surface of the leg, while at the same time it turns the foot inwards, and slightly raises its inner border. This muscle, during contraction, shows externally all the details of its shape—in the leg a fleshy mass which slightly overlaps the anterior border of the tibia or shin, and in front of the ankle an oblique cord marking clearly the direction of the tendon.

The extensor proprius hallucis (5, Fig. 84) is concealed at its origin between the tibialis anticus and the extensor longus digitorum. Only its tendon appears superficially (2, Fig. 85) in the lower third of the front of the leg, on the outer side of the tendon of the tibialis anticus. It passes beneath the annular ligament of the ankle and along the inner part of the dorsal surface of the foot (4, Fig. 86), to be inserted into the base of the terminal phalanx of the great toe. When the great toe is forcibly turned up during extension this tendon is clearly shown throughout its entire course. In a normal foot this tendon occupies a straight line from the middle of the ankle-joint to the great toe, and the inner border of the foot and great toe should be parallel to it. In other words, the great toe should be in a line with the inner border of the foot (Fig. 52, page 151).

The extensor longus digitorum (4, Fig. 84) arises from the external tuberosity of the tibia, on the outer side of the tibialis anticus, and from the upper three-fourths of the anterior surface of the fibula. It descends vertically, and terminates in a tendon divided into slips, which remain associated together (3, Fig. 85) to pass beneath the anterior annular ligament. Immediately afterwards, these slips spread out like a fan (4, Fig. 85) in the form of four tendons, which pass over the dorsum of the foot to be inserted into the second and third phalanges of the four outer toes. Each tendon behaves like the corresponding extensor tendon in the hand. It forms a membranous expansion on the dorsum of the first phalanx, joined by the tendons of the extensor brevis digitorum, lumbricals, and interossei muscles, and completes the capsule of the meta-tarso-phalangeal joint. Thereafter each tendon splits into three parts—one central and two lateral. The central slip is inserted into the intermediate phalanx, and the two lateral slips, reunited, are attached to the base of the terminal phalanx. Associated with the extensor longus digitorum is the peroneus tertius, an essentially human muscle, which has a common origin with the long extensor. Its tendon accompanies those of the last-named muscle beneath the annular ligament, and is directed outwards on the dorsum of the foot, to be inserted into the base of the fifth metatarsal bone. With the tibialis anticus the long extensor of the toes and peroneus tertius flex the foot on the leg, while at the same time the common extensor extends the toes on the foot. In this action it shows the prominence of its fleshy belly especially at the middle of the leg, and the tendons on the back of the foot are raised up like diverging cords. The peroneus tertius raises the outer border of the foot in such movements as skating and dancing, and it is only then that its prominence shows beneath the skin. Usually it is but little marked.

II. External muscles (Fig. 85).—These are two in number, placed on the outer side of the fibula, and called the peroneal muscles. They are distinguished as the peroneus longus and peroneus brevis. They completely cover the outer surface of the fibula, the peroneus longus arising from the upper two-thirds, the peroneus brevis from the lower two-thirds of the bone. The peroneus longus is behind and above, the peroneus brevis in front and below. Their tendons descend together and pass beneath the external annular ligament; that of the peroneus brevis lying next the bone and grooving the back of the external malleolus. Winding round the malleolus as on a pulley, they are directed forwards on to the outer border of the foot. Then only the tendons separate from each other, to be inserted into the foot at two points diametrically opposite. One (that of the peroneus brevis) is directed horizontally forwards, on the outer surface of the foot (10, Fig. 85), to be inserted into the base or posterior extremity of the fifth metatarsal bone; while the other, that of the peroneus longus, is directed obliquely forwards and downwards so as to reach the sole of the foot, beneath which it passes, lying in the groove of the cuboid bone; it then runs obliquely across the sole of the foot, from the outer towards the inner border, deeply hidden by the plantar muscles and ligaments, and is finally inserted into the posterior extremity of the first metatarsal bone and the under surface of the internal cuneiform bone.

These two muscles, but especially the peroneus brevis, extend and turn the foot outwards, while at the same time they raise its external border; so that their action is the reverse of that of the tibialis anticus. But the peroneus longus has another important function which explains why the muscle becomes prominent whenever a particular effort is demanded of the foot, as, for example, when it is carried forward in dancing, or in the act of imparting motion to an object. This muscle, owing to the arrangement of its tendon, which passes like the string of a bow across the hollow of the sole of the foot, acts by deepening this hollow; it therefore raises the plantar arch, marked on the dorsal surface by an increase of the curve of the foot.

III. Posterior muscles (Figs. 82, 87).—The back of the leg is thick and fleshy, and is formed by numerous and powerful muscles, divided into two groups: the superficial group, which we shall describe in detail, and the deep group, for which a few words will be sufficient.

The superficial group is formed by the gastrocnemius, plantaris, and soleus muscles.

The gastrocnemius muscle (γαδτήρ, belly; κυήμη, leg) is responsible for the surface form of the prominence of the calf. It consists of two large, fleshy masses known as the inner and outer heads, which arise respectively from the upper part of the inner and outer condyles of the femur on their outer aspect. Overlapped by the insertions of the hamstring muscles, the two heads of the gastrocnemius form the lower boundaries of the popliteal space. They converge at its lower angle to be inserted (separately) into a broad membranous tendon, which forms the beginning of the tendo Achillis (6, Fig. 87). At their origins, each head possesses a tendon which covers its superficial surface. At the insertion the tendon passes upwards for a considerable distance on their deep surfaces. At its insertion into the tendo Achillis, each fleshy head presents a rounded lower border; and it is to be particularly noted that the inner head of the muscle usually descends to a level lower than that of the outer head.

This muscle bends the knee and extends the foot on the leg. Through the tendo Achillis it acts on the calcaneum or bone of the heel so as to raise the heel and cause the living model, if erect, to rise on the toes. Now the constitution of the muscle is such that its external form is quite different when it is in repose and when it is in contraction. Each head of the muscle, as already stated, possesses at its origin a membranous tendon, which expands over the outer part of the surface, but the inner part of each belly is fleshy. In a state of repose the two halves of the muscle unite in the same rounded and prominent shape, so that we cannot distinguish the part covered by the aponeurotic expansion of tendon from the part formed by free muscular fibres. But when the living model rises on the balls of the toes, or in any other movement produced by a powerful contraction of the muscle, we see the free fleshy part of each head swell up more strongly than the part covered and tied down by the aponeurosis. At that moment, therefore, the whole convex form of the calf presents a slight ovoid surface on each side, and a long vertical prominence in the middle line. This prominence is produced by the mesial muscular parts of the two heads of the muscle, which approach each other during contraction, and unite their double mass in a single median prominence. Fig. 82, by the differences in shading, enables us to distinguish the aponeurotic from the muscular parts, and to observe the important details of shape into the study of which we have entered. The clear or aponeurotic parts correspond to the two flat surfaces already mentioned, and the shaded or fleshy parts correspond to the median prominence, with this difference, that in the calf in contraction, this median prominence is smoother than in Fig. 82, the two halves which compose it being merged into a single mass, except at the angular space above the point where they bound the popliteal space.

We will now return to what we have already said (see page 267) relative to the posterior region of the knee, examined in the living model in extension of the leg. If the model rises on the balls of the toes it is no longer possible to speak of the popliteal space as a hollow at the back of the knee. Under these conditions, the prominences (already studied) of the biceps, semi-membranosus, and gastrocnemius almost join each other, and the plantaris muscle, of which we will speak immediately, helps to fill up the space; the region of the popliteal space forms in reality a prominence, and the posterior surface of the knee is marked in its central portion by a strong muscular projection of which it is not possible to understand the cause except by an attentive study of the muscles named.

The soleus muscle, so called because its form has been compared to that of a sole (Latin—solea), is placed beneath the gastrocnemius, which overlaps it, more on the inner (7, Fig. 86) than on the outer border (12, Fig. 85) of the calf. In its action it is simple. It only acts on the ankle-joint, as a powerful extensor of the foot. Arising from the back of the fibula and from the tibia, its fibres are inserted below into the deep surface of the tendo Achillis to within two inches of the heel.

The tendo Achillis is a strong band of fibrous tissue occupying the lower part of the back of the leg. Membranous above, where it is continued upwards on the deep surface of the bellies of the gastrocnemius muscle, it becomes narrower and thicker below as it approaches the heel. Broadening out slightly at its lower end, it is inserted into the inferior half of the posterior surface of the calcaneum, forming the prominence of the heel. In a muscular leg the fibres of the soleus may be shown superficially, bulging beyond the tendo Achillis at its outer and inner borders, below the outline of the gastrocnemius, and more on the outer than on the inner side (5, Fig. 82, page 263).

The plantaris is a small, insignificant muscle of which the fleshy origin (6, Fig. 82) is from the outer condyle of the femur, under cover of the outer head of the gastrocnemius muscle. Its small fleshy belly is succeeded by a long, thin tendon (7, Fig. 82), which descends obliquely between the gastrocnemius and the soleus to reach the inner border of the tendo Achillis, along which it descends (10, Fig. 86). Sometimes it is united with this tendon early in its course, and sometimes it reaches the calcaneum, or it may join the internal annular ligament at the ankle.

The deep muscles of the back of the leg are not visible on the model except on the inner side above the ankle. Here, on the inner side of the tendo Achillis (8, Fig. 82), is placed a series of tendons, proceeding in the interval between the internal malleolus and the calcaneum to the foot. The muscles are the tibialis posticus, the flexor longus digitorum, and the flexor longus hallucis. The fleshy bellies of these muscles, deeply hidden beneath the superficial muscles, arise from the posterior surface of the tibia, from the fibula, and from the interosseous membrane. Their tendons descend obliquely over the back of the internal malleolus, where those of the tibialis posticus and the common flexor of the toes alone are visible (the tendon of the flexor proprius hallucis being almost entirely hidden beneath the tendo Achillis). From this they are reflected over the internal malleolus and pass into the sole of the foot, bound down on the inner side of the ankle by the dense band of the internal annular ligament. The tibialis posticus is almost immediately inserted by radiating bands into most of the tarsal and metatarsal bones (12, Fig. 86), and the other two tendons reach the toes, where they are inserted in the same manner as we have already described for the tendons of the flexor longus pollicis and the deep flexor of the fingers in the hand.

Muscles of the foot.—The muscles of the foot include not only the muscles of the plantar region, corresponding to the palm of the hand, but also the tendons on the dorsum of the foot, and a special extensor muscle, the extensor brevis digitorum.

The tendons of muscles on the dorsum of the foot have already been described (p. 270). The extensor brevis digitorum muscle (11, Fig. 84, and 14, Fig. 85) consists of a short, fleshy belly placed obliquely on the dorsum of the foot, and extending from behind inwards and forwards. It origin is rounded and attached to the upper surface of the calcaneum in the cavity or canal of the tarsus (page 155), external to the astragalus. From this origin it is directed forwards and inwards; becoming broader, it passes beneath the tendons of the common extensor and immediately divides into four muscular slips, each of which has a tendon attached to it. These tendons pass under those of the common extensor, so as to form with them a figure with lozenge-shaped spaces, and proceed to the four inner toes to be inserted, in the case of the great toe, into the base of the first phalanx; uniting, in the case of the second, third, and fourth toes, with the expansions of the extensor tendons on the dorsum of the phalanges. This muscle aids in the extension of the toes, and by its obliquity counteracts the action of the long extensor. When it contracts, the portion of the muscle situated on the outer side of the tendons of the long extensor become very prominent, not being tied down by anything, and this prominence is still clearer because behind it is a depression, more or less evident, corresponding to the calcaneo-astragaloid cavity of the tarsus.

We will not describe at length the muscles of the sole of the foot with regard to form. The foot derives most of its characteristics from its bony structure; the muscles of the sole round off by their presence the angles of the skeleton and fill up the cavities, but do not modify essentially the bony forms. On the other hand, these numerous muscles are in general but poorly developed, and they are to a large extent united into common masses, which makes it difficult, except for the skilled anatomist, to isolate and recognise distinctly each fleshy belly. It is therefore useless for an artist to enter into a detailed study of these muscles, which reproduce in their general arrangement the distribution of the muscles of the hand. We will content ourselves, then, by saying that the sole of the foot, like the palm of the hand, possesses three muscular masses—1st. An internal group (16, Fig. 86), belonging to the great toe, formed by the abductor hallucis, which arises from the calcaneum, and by the flexor brevis, the adductor transversus, and adductor obliquus hallucis, which arise from the anterior bones of the tarsus and from the metatarsus; 2nd. An external group (15, Fig. 86), belonging to the little toe, formed by the abductor, which arises from the calcaneum, and the flexor brevis, from the cuboid; 3rd. Finally, a central mass, formed by the flexor brevis digitorum, the long flexor tendons, lumbricals, and interossei—which in a sense repeat what we have said regarding the muscles of the same name, more easily studied, in the hand.