The muscles of his leg and foot are the most important by far of his original equipment with which to set about making his arch: he could no more do this out of his present muscles than a Hebrew could make bricks without clay. It is these variable and plastic structures which are most readily adapted by use in a fresh direction or increased degree. He has the great flexors of the ankle and foot in his poorly-shaped calf (this feature might be adduced as a human character and studied in this manner if it were not of so elusive a nature) and the long flexors of his four outer toes, the special long flexor of the great toe, which in his case does not of course act in the axis of the other metatarsal bones. He is lacking here in the special detached portion of the flexor accessorius, which eventually becomes of use in maintaining the arch, between the heel-bone and the tendons of certain digits. He has, in a measure, the oblique adductor muscle of the great toe and the transverse adductor muscle, more for future use perhaps than of much present value. Like all apes and monkeys he has a peroneus longus with its tendon passing across the sole from the outer border to the base of the great toe and a peroneus brevis, both of them for everting the foot and supinating it. But here again he is lacking, for he has no little peroneus tertius, which Professor Keith speaks of as a muscle “peculiar to man” and “a special evertor of the foot”—a muscle passing from the tendons of the extensors of the toes and inserted into the little toe. He has also the tibialis anticus and tibialis posticus, the latter which flexes the ankle on the leg, and the former which also flexes it and everts the foot; he has also the special extensors of the toes.
This enumeration of the bony, ligamentous, and muscular possessions of gorilla C. is enough to show that, though he has little of new tools to make, he has to modify greatly those he has learnt to use so well, so that one can almost hear him echo the words of David to Saul as to his new armour.
The problem of an arch remains to be solved by eversion instead of inversion of the foot, growth in all directions of the heel-bone, and the enlargement and straightening of the great toe, and the “setting” of the foot in a certain degree of pronation and over-extension.
Description of the Arch.
The plantar arch is double, but the longitudinal one must be chiefly considered here. It lies under the concave roof of the tarsal bones, seven in number, and the metatarsal bones, and rests in a well-formed foot in front on the heads of the latter, and behind on the inferior surface of the heel-bone. The astragalus alone of these bones in contact with those of the leg, acts like a washer to the ankle joint, and has no muscles attached to it. Three more of the tarsal bones need reference: these are the three wedge-shaped bones which have their bases on the dorsal and their apices directed towards the plantar surface. With such a set of bony tools as this, all the requisites for an arch are at hand. Let the half-tree, half-ground walker become a complete ground-walker, and in the first place the manifest increase of the action of the flexors of the leg will pull to an unusual extent on the tendo achillis and heel-bone, leading, in accordance with a well-known law, to steady enlargement of the parts near to which it is attached. The greater amount of weight thrown henceforth on the heel tends in just the same direction, indeed, to general enlargement of the whole bone. The astragalus being in No Man’s Land, so to speak, takes less part in the change than any other tarsal bone. The wedge-shaped bones are exactly so constructed as to retreat a little in a dorsal direction as the modified walking increases under the action of certain muscles which will later be mentioned. This, in conjunction with the projection backwards of the heel and the general growth of the bone, permits, as far as the bony parts go, a gradual hollowing out of the originally flat plantar surface, and the increasing eversion of the foot places more weight on the front pier of the arch, that is, the heads of the metatarsal bones. The squeezing-up process of the smaller tarsal bones contributes also to the formation of the transverse arch.
The ligaments need no new invention on his part but only a more human degree of development, and in particular the calcaneo-navicular ligament and internal lateral of the ankle undergo in the human foot great development, and the long plantar ligament, originally part of the tendon of the gastrocnemius, comes in to the aid of the arch and goes to bind it together, so that these humbler structures follow in the wake of the changing and enlarging bones.
The plantar fascia, though a powerful protective armour for the deeper parts of the sole, cannot be held to enter into the formation of the arch. The initiative in this process lies with the muscles, and, even if neither gorilla C. himself, nor his descendants, had altered the muscles of his foot and just given up climbing for walking, there were muscles strong enough and appropriate for modifying very profoundly his simian foot, though he might not have arrived at an arch. He or they might have become long-distance walkers, but never sprinters.
If the sole of the dissected foot is observed it is seen that the plantar arch lies approximately over a triangle of which the base is formed by the transverse adductor muscle of the great toe, across the heads of the metatarsal bones, and the two sides by the oblique adductor of the great toe and the short flexor of the little toe. It extends, of course, somewhat further back under the heel-bone, but this is its highest part.
In the changing foot the tibialis posticus, which was originally a flexor of the metatarsal bones, obtains a secondary attachment to the scaphoid bone, and the tibialis anticus becomes inserted anew into the internal wedge-shaped and metatarsal bones. “Both of these muscles, thus modified, help to maintain the arch of the foot. So does the tarsal part of the tendon of the tibialis posticus.” (Keith).
The three peronei muscles, especially the new peroneus tertius, attached to the little toe, are called in by increased walking to redress the balance of forces in the foot and produce that eversion, with some supination, which is essential to the arch. No arch was possible till these muscles came into some preponderance of action over the flexors, so beloved of gorilla C. The short flexor of the digits becomes modified so that its attachment to the tendons of the long flexors in the sole has its origin completely transferred to the heel-bone in man (Keith). “It can thus act more powerfully in maintaining the arch,” and finally the flexor accessorius, a muscle which cannot fail to surprise the dissector when he first penetrates into the deep layer of muscles of the sole, and which is a detached piece of the long flexor of the great-toe, becomes especially well-developed and helps to maintain the arch.