This section is from the book "The Manufacture Of Boots And Shoes: Being A Modern Treatise Of All The Processes Of Making And Manufacturing Footgear", by F. Y. Golding. Also available from Amazon: The Manufacture Of Boots And Shoes.
The foot owes its peculiarities of form to its bony structure; the muscles round off the angular structure. They are usually but little developed, and, being situated in masses, it is difficult to isolate and describe distinctly the precise action of the individual muscles, so that a brief description of the several muscles will serve our purpose. There are four layers of muscles in the sole of the foot, and two on the upper part of the bones. After the skin of the plantar region and the fatty tissue has been removed, an expansion of fibrous tissue known as the plantar fascia is visible. If this be taken away the first layer of muscles is exposed, consisting of Abductor pollicis (14), Flexor brevis digitorum (18), and the Abductor minimi digiti (16) (Fig. 22). The second layer, situated under the first, consists of the tendons of the Flexors longus digitorum (11) and Proprius pollicis (12). On the outer side of the foot, the tendon of the Peroneus longus (5) passes beneath the Flexor accessories (20). To complete the layer the muscles Flexor accessorius (20) and the lumbricales (19) must be named (Fig. 23). The third plantar layer consists of the tendon of Tibialis posticus (10), the Flexor brevis pollicis (15), the Adductor pollicis (21), the Flexor brevis minimi digiti (17), and, running across the foot, the Transversus pedis (22). The sheath of the Peroneus longus (5), and the "plantar ligament, are also found in this layer (Fig. 24). The fourth layer (Fig. 27), consists of three inter-ossei (23), one on the inner side of the second toe, and the others one each on the inner side of the third and fourth toes.
They draw to the central line XY, which in this treatise will be called the "central muscular action line," or the " line of muscular action." The first layer (Fig. 26) on the dorsal surface consists of the tendons of the Tibialis anticus (1), Extensor proprius pollicis (2), Extensor longus digitorum (3), and the Tertius peroneus (4). The muscles of the Extensor brevis digitorum (13), after passing under the Extensor longus digitorum (3), divide into four tendons, and aid in the extension of the toes. The second layer (Fig. 25) consists of four inter-ossei (23a), fixed on the outer side of the second, third, and fourth toes, and draw from the" central muscular action line " XY, and one on inner side of second toe drawing to line XY.
Muscles of the Foot. A. Dorsal; B. Plantar (internal, external, central).
A. Dorsal Group.
13. Extensor brevis digitorum. First layer. 23a. Inter-ossei dorsal (4). Second layer.
B. Plantar Group.
14. Abductor pollicis. Internal first layer.
15. Flexor brevis pollicis. Internal third layer.
16. Abductor minimi digiti. External first layer.
17. Flexor brevis minimi digiti. External third layer.
18. Flexor brevis digitorum. Central first layer.
19. Lumbricales. Central second layer.
20. Flexor accessorius. Central second layer.
21. Adductor pollicis. Central third layer.
22. Transversus pedis. Central third layer.
23. Inter-ossei plantar (3). Fourth layer.
13. The Extensor brevis digitorum arises in the upper outer side of the heel-bone, and, broadening out, it passes under the Extensor longus digitorum, when it divides into four tendons that go forward and are inserted in the bases of the first phalanges. Its action is to aid the extension of the toes and to counteract the tendency of obliquity of the Extensor longus digitorum.
14. The Abductor pollicis rises on the inner posterior region of the os calcis, and is inserted in the first phalanx of the great toe. Its action is to abduct the big toe away from the central line of the foot to the imaginary line that forms the centre of the body. By this action the great toes would be brought closer together.
15. The Flexor brevis pollicis comes from the second row of the tarsus, and is inserted to the base of the first phalanx.
16. The Abductor minimi digiti arises from the outside of the os calcis, and goes forwards to the external side of the first phalanx of the little toe. Its action is to draw the little toe away from the middle line of the foot.
17. The Flexor brevis minimi digiti has origin in the sheath of the Peroneus longus and the base of the fifth metatarsal bone, and is inserted in the first phalanx of the little toe. Its action is to flex the little toe.
18. The Flexor brevis digitorum, from the heel-bone and the plantar fascia, draws down the toes, and is inserted in the second phalanges of the four toes.
19. The Four Lumbricales are affixed to the inner side of the four toes. Their action is to draw the toes in to the inner side of the foot.
20. The Flexor accessorius extends from the os calcis to the second, third, and fourth toes. In contraction it counteracts the obliquity of the Flexor longus digitorum, hence its name.
21. The Adductor pollicis arises from the sheath of the Peroneus longus and the third and fourth metatarsals, and is inserted in the first phalanx of the great toe on the outer side. Its action is to adduct, or draw, the great toe to the central line of the foot.
22. The Transversus pedis goes across the foot, and is inserted in the phalanx of the great toe. Its office is to adduct, or draw, the big toe to the line of the foot termed the "line of muscular action."
23. The Three Plantar inter-ossei are situated between the bones of the toes on the inner side, and draw to the central line the three outer toes.
23a. The Four Inter-ossei, on the dorsal surface of the foot, are situated on the outer side of the bones of the toes, and draw the third and fourth toes away from the central line of muscular action. The two inter-ossei on either side of the second toe draw away from the axis of the toe either to the outer or inner side of the foot respectively.
Nerves.The foot is provided with two kinds of nerves - - those that supply the skin with sensory branches, and the other sort that give motor impressions to the muscles. The posterior tibial and the anterior tibial nerves come from the sciatic nerve, the former giving branches to the muscles in passing down to the inner side of the ankle. The posterior tibial then divides into external plantar nerves and internal plantar nerves, that supply the toes and sole of the foot. The anterior tibial nerves supply the dorsum of the foot as well as the outer side of the leg.
Arteries, etc. - The chief arteries of the foot are those situated on the dorsal and plantar surfaces. The plantar consists of two - inside and outside. The veins of the foot are of two kinds - surface and deep. They follow the direction of the arteries.
The Skin consists of two layers - the epidermis and dermis; the dermis, or true skin, is made of fibrous tissue, interposed with blood-vessels and nerves. The skin on the sole of the foot possesses no sebaceous glands that keep the skin oily and soft. Sweat glands are, however, numerous. Under the skin are found pads of fat, at the heel and toes especially.
When standing, the weight of the body is transferred to both pillars of the arch; the inner side of the arch sinks a little, and the outer side nearly touches the ground. The foot thus forms a firm basis of support, becoming slightly longer and wider.
Walking is performed as follows : (1) by lifting one leg from the ground and bending the knee, which throws the weight on one foot; (2) by raising the other leg on the ball of the great toe and bending the body forward, throwing the weight on to the leg first extended. During the action of walking one foot or the other is always on the ground (Fig. 28).
Running is a kind of very quick walking, only, both feet for short periods are from the ground together.