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.

The order of events then is: first, increased and altered muscular function; second, growth of bones and adjustment; third, binding together of these by new or modified ligaments. If it were possible to separate in this way the age-long formation of such a living tool as the human foot, this is the order in which alone, I submit, the sequence of events can be placed. It is a convenient, because simple and plain example of initiative in evolution, and I cannot say how much I owe to Professor Keith’s teaching on the subject.[71]

CHAPTER XXII.
MUSCLES.

A work of great value to the biologist has been written by one whose work has led him in the widening path of human physiology and its very title is instinct with meaning. The Integrative action of the Nervous System may not aid the systematist or the student of genetics, but for insight into formative powers, where the former can but record facts and find no interpreta­tion, such a work is of supreme importance. When the plant sealed its fate and enclosed itself in a cell-wall and abandoned a life of movement, it was foreordained that its rival would be that cell and its descendants which could adopt a free life, and that the future of the world would lie at the proud foot of that conqueror who could command and mobilize the resources of a nervous system. And, as we know, it has fallen to man to receive the rewards of this promise and potency of a higher life. If one seeks to understand the steps by which man has arrived at his primacy it can only be by the highway of nervous progress, however much the tracing of certain connecting or collateral paths may throw light on contributing causes. So that man’s place in Nature is nearly synonymous with the structural evolution of his brain, as Huxley has shown in his clear and simple manner. Even if man is to remain still an animal Melchisedec for generations to come, or to put it lower, a foundling, no future discoveries that can be imagined will disprove Huxley’s declara­tion, “Evolution is no longer an hypothesis, but an historical fact.” And yet if man has become adapted to his world, and, in it, crowned with glory and honour by the unfolding of some original complexity, or as the result of some fortunate mutations in the distant past, the human brain, with its cranial capacity of nearly three times the number of cubic centimetres to that of the gorilla, has been making false claims to a paramountcy over all factors in the wonderful initiative of fresh capacities and their mobilisa­tion for conquest. Nothing less than such a “claim” was understood by the ancients, and, though metaphysics had to supply the lack of anatomy and physiology, it has always been held that mind was lord of matter, and now scientific research has told us why. But no one, even the most hard-shelled scholastic, can refuse to the brain organ its predominant share in the making of man. This is seen even in the frigid sphere of science by the difference of interest there is shown between any great discovery bearing on the evolution of man, or on some new lower animal form. When Sir. H. H. Johnston astonished zoologists in 1901 by his discovery and proof of the existence of an archaic large mammal which had been interned for an incalculable time in the Semliki Forest, the thrill felt at that historic meeting passed off very soon when the leading British biologist had monographed the Okapi, settled its name and surname and introduced it into text-books. This is never the fate of such as Pithecanthropus or Eoanthropus dawsoni, or of the more recent genealogical theory and researches as to arboreal man. The call of these studies of man’s evolution is felt by all, and the difference in the two branches of biology may account for what must have struck many others, that is the neglect of adding the blue ribbon of science to the honours of the discoverer of the Okapi.

These few trite remarks as to the importance of the nervous system in the making of man have been introduced here, though they bear more closely on the next two chapters, because this importance comes in at every stage of the present treatment of the origin of modifications in muscle.