Peroneus Tertius.

The foot of man possesses the small peroneus tertius which was referred to as one of the evertors of the foot concerned in the construc­tion of his plantar arch. Macalister and Professor Keith both speak of it as peculiar to man, and the latter refers to it at some length,[79] the whole passage being worth quoting here. “Although the evolution of the human method of progression was attended by a profound altera­tion in the form and action of every muscle and bone with lower limbs, yet this great transforma­tion was produced without the appearance of any really new element. One new muscle—the peroneus tertius—did appear, and the history of its evolution throws an interesting sidelight on the origin of new structures. It arises by the outer fibres of the common extensor muscles of the toes being separated. In all the anthropoids the feet are so articulated at the ankle-joints that the soles are directed towards each other, and only the outer edge of the foot comes to rest on the ground when the animal tries to stand. The feet have a tendency to assume a similar position in children at birth. The advantage of a muscle, such as the peroneus tertius, is apparent in the human foot, for it tends to raise the outer border of the foot, so that the sole is properly applied to the ground. If we examine the muscles which, rising from the front of the leg, cross the ankle-joint to end on the back of the foot on the toes of fifty men, we shall find every stage in the evolution of this muscle. In one man at least it will be undeveloped; in two or perhaps three it will be represented by a part of the tendon of the extensor muscle of the little toe, which in place of ending entirely on the toe sends a part to end on the metatarsal bone of the little toe. In only forty of the fifty men will the peroneus tertius be found quite isolated from the parent muscle—the extensor communis digitorum, and to have a distinct origin from the fibula in the leg, and a separate insertion to the base of the fifth metatarsal bone in the foot. In a series of fifty specimens every stage in the isolation of this new muscle will be seen. It has never been found in any anthropoid, and is more often absent or undeveloped in African than European races.”

To this excellent account I have only to add one comment. It can hardly be an accident or without significance that this special human evertor of the foot concerned in the construc­tion of the plantar arch is “often absent or undeveloped” in African races, which are well-known in some groups to have adapted themselves to a form of foot which shows no plantar arch, being normally flat-footed. In this small field of observa­tion, a mere plot of lentils like that which Shammah defended of old, there is set forth a mimic battlefield, and it is not difficult to see that the forces at work can owe allegiance to one and one only of various commanders. The problem as to the origin of the peroneus tertius would no more attract the Mendelian than did the trousseau and approaching marriage of Caddie Jellyby attract the far-away gaze of her mother, fixed upon the world of Borria-boula-gha, and, for that matter, de Vries would hardly pay it more attention—to him it would be indifferent; whereas Weismann would have as much to say about it as about the little toe of man, which furnished for him and Herbert Spencer such fruitful material for debate many years ago. This muscle resembles the results of some of Michael Angelo’s first attempts at sculpture, thrown aside perhaps in his place of work and from time to time taken up, rough-hewn again and again and finally shaped into a form far from perfect, but with the value and teaching of a failure for him who was some day to outshine all modern rivals. If the history of this muscle be not one of initiative in evolution through the factor of use and habit the Pan-Selectionist must do the best he can with an incalculable number of “trials and errors,” and must suppose that, rather than allow this small territory to the neo-Lamarckian, a long series of man’s ancestors have been making experiments for the benefit of man’s walking power under the guidance of selection with an insignificant muscle whose only function is that of aiding in the eversion of the foot, and that in the rudimentary condition described by Professor Keith it had selective value. No one who was not committed to a dominating theory could hesitate for a moment which of the two alternative views of the origin of the peroneus tertius he would choose. Dr. Barclay Smith speaks in the paper referred to above of the extensor brevis pollicis, or minor, as a muscle of extremely late appearance, and as “peculiarly human,” and says all the evidence points to its being a segmenta­tion product of the extensor ossis metacarpi pollicis, its appearance being foreshadowed in the anthropoid by an extension of that muscle on to the proximal phalanx of the thumb.

It is not without interest to the thesis before us to read the rather bewildering story of the early life of a very insignificant muscle such as the small extensor of the thumb of man.

As illustrations of the moulding and pruning of perfected muscles it may be remembered that, as Macalister says, “portions of muscles may also become detached and degenerated so as to act as ligaments,” and “the adult muscular system of man bears everywhere traces of earlier cleavings and subsequent fusions, partial disappearances and local outgrowths.”[80] This passage recalls one in which Huxley says in watching certain phases of development you can almost see the hidden artist at work, and here the sculptor may be pictured in his chipping, trimming, rejecting and finally shaping, some creation of his brain; and from a biological point of view a vision of the processes of use and disuse may be obtained. Professor Keith also speaks often of the migrations of muscular attachments in a way which agrees with the passage quoted from Macalister.

CHAPTER XXIII.
INNERVATION OF THE HUMAN SKIN.

For at least seventy years the surface of the human skin has been the subject of so much physiological observa­tion and experiment that Professor Sherrington considers the literature connected with it to be probably greater than in any other branch of physiology. Most of this study centres round the skin as a receptive field and problems of the nervous system. It is easy to see why this should be in the case of an organ so great as the skin, covering all the other structures and organs and exposed through ages of evolution to the vicissitudes of an inconceivable number of stimuli. And one outcome of this study is to show that, metaphorically speaking, the skin is a mosaic, and not the confused and blurred produc­tion of a child of four years old who has been given a sheet of paper and a paint-box. There is order in this field, and even without calling in final causes, plan and purpose. Beside the protective function exercised by the skin it plays a large part, through its nervous endowment, in the processes by which the brain is made aware of the surrounding phenomena, thus conveying intelligence to the centre of life only less important than that of the special senses. It is maintained here that the result of the various physical stimuli, of which pain, cold, warmth and touch are the chief, is that certain functions and structures of the skin have arisen in response to them.

This is, no doubt, to beg the question of origin, and if the balance of evidence be seen to be against this view the order of events would need to be stated differently. But the position is clear, whether correct or not, and if it be shown to be erroneous it will at least have good “lighthouse value.”