Very probably the reader is aware that in some people, especially those who are kept standing or moving about for hours on a stretch, such as postmen and policemen and nurses, the arch of the foot is apt to break down, producing the condition of flat foot. Only in man is the foot firmly arched ; it must serve him as a firm lever as he steps off one foot to take another stride. Now when flat foot develops, the break down of the arch occurs at the mid-tarsal joint—the one which we saw was flexible in the four-footed monkeys and which became firmer as the erect posture was assumed. People with flat foot do not dare to step off their toes because the lever of the foot is destroyed by a loosening or breaking-up of the mid-tarsal joint. It would not be true to describe flat foot as a relapse to a pronograde condition, yet in bending at the mid-tarsal joint the flat foot and the monkey's foot are alike in that they both allow bending to take place.
It has already been noted that man is apt to emphasize those features of his body which mark him off from lower animals and from other races of mankind. The calf of the leg is such a feature; it is highly developed in the white and yellow races—the European and Mongolian. In no dark-skinned race is the calf prominently developed, yet it is in the negroid races we so often see individuals who carry themselves gracefully. Indeed, men who have well developed calves often walk with a jerky step. Anthropoids, like negroid races, have an ill-developed calf, but before pronouncing whether such a feature is Simian or not, it will be well to enquire into its nature. The calf of the leg is made up of two muscles—the gastrocnemius and soleus; they act on the heel. The only difference between man and the anthropoid apes is, that in the former the soleus muscle is larger and has extended the area of its origin to the tibia, while in the latter it is confined to the fibula, but occasionally one sees a tibial origin in the gorilla. There is another difference; while in man the muscle ends in a tendon— the tendo Achillis—some inches above the heel, in the anthropoids the muscular fibres almost reach the heel, the tendon being short. The heel is the lever for the muscles of the calf; if it be long or set horizontally then a less amount of muscle is required to lift the body on the toes ; if the heel is short, and especially if it be set obliquely, as is the case when the arch of the foot—the instep— is high, then the heel forms a less powerful lever, and a greater amount of muscle is required to lift it. Now it is notorious that the heel of the negro is long and as his foot has not a high arch, it appears as if it projected backwards to a marked degree. With such a powerful lever the calf-muscles need not be large; they act easily and steadily and give a graceful step. In Europeans the heel is short, and it is bent downwards to form the posterior pillar of the arch of the foot. It forms a less powerful lever and hence the need of greater calf-muscles. It will thus be seen that the history of the calves of our legs is wrapped up in the evolution of the orthograde posture. The anthropoid's leg is a step in advance of the leg of a pronograde ape; man's leg is an equally great advance on the anthropoid condition.
We have thus examined traces of a former pronograde posture in many parts of man's body—in the abdomen, thorax, neck, shoulder, arm, hand, foot and leg; we will bring this chapter to a conclusion by citing one other instance—this time taken from within the body just above the region of the hip joint. In that region of the human body there is found the remains of a muscle named the small psoas—in contradistinction to the great one which bends the thigh. This small muscle is often absent in man, and if present it is small and mainly composed of tendinous or fibrous tissue. It springs from the backbone in the region of the loins, and ends on the brim of the pelvis, behind the outer wall of the lower part of the abdomen. In anthropoids it is not much better developed than in man, but in pronograde apes it is robust and strong. The four-footed apes employ this muscle to flex the pelvis on the spine, its action being well seen as the animal bends its body preparatory to a jump. With the disappearance of this action in the upright posture, the need of the small psoas muscle disappears. Its persistence in a vestigial condition in man and anthropoids shows that they have passed through a pronograde stage.
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 in the 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 side light on the origin of new structures. It arises by the outer fibres of the common extensor muscle 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 in 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 he 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 in European races.
It can be very well understood that the extension of the human thigh, so that it is brought into line with the trunk, must lead to a considerable alteration in the structures of the groin. There can be no doubt that the extension of the thigh has weakened the human groin, and made man more liable to suffer from ruptures or hernias than any other animal. The escape of the contents of the abdomen— which constitutes a hernia—is liable to occur at two points : (1) at the point where the testicles perforate at the groin—just before birth; (2) by another passage which is formed at the inner side of the great vessels of the thigh. The second passage is only found in man, and is due to the manner in which the human thigh is attached to the pelvis. The first opening is found in nearly all mammals, but in man—owing to the width of the thigh and groin—the passage made by the testicle is not so well protected by suitable closing or sphincter muscles as in other mammals. Thus man's upright posture has been obtained at a sacrifice; the modification of the groin has weakened the abdominal wall at the root of the thigh and made him liable to a malady which cripples the strength of many men. The ligament which crosses the groin—known as Poupart's ligament— is peculiar to man, but the complex of structures out of which it has been evolved is ' very apparent in the groins of the anthropoid apes.