There is a danger of becoming tedious were I to press on the reader the technical evidence to be found in man's body which indicates a change in posture. Yet there is an advantage in approaching the study of the human body in the manner I have adopted, for certain functional peculiarities are brought before us which otherwise would escape observation. This is especially the case as regards the shoulder region of our bodies ; we are square-shouldered ; the military man loves to emphasize this feature by wearing epaulettes. Indeed, the fashionable tailor frequently makes good any deficiency in his customers' shoulders by the use of a little padding. The shoulders of pronograde apes are set quite differently to ours ; they are pressed against the flattened sides of the chest, the sharp, keel-like sternum along the lower margin of the body projecting downwards between and beneath them. The muscles are so arranged as to advance and retract the shoulders as the animal runs along. With the assumption of the upright posture in the gibbon, the shoulder is swung round to the side of the body, and the thorax becomes widened from side to side. The shoulder thus assumes the same position as in man, but owing to the fact that the arms are Used for suspending the body and for swinging it from branch to branch, the arrangement of muscles between the shoulder and body is peculiar in the gibbon. It is quite clear that when we stand up our shoulders would tend to droop unless they were supported by the muscles which fix them to the head, neck, and spine. Hence there is a slight difference in the arrangement of the shoulder muscles of man, seeing that his arms are no longer used for locomotion as they are in the gibbon and great anthropoids. Now the question we want to answer is the following:—Is there any evidence in the musculature of the human shoulder which indicates that man at any stage of evolution used his arms as anthropoids and as monkeys do ? There is. Occasionally a muscle is found in man known as the lifter of the clavicle or collar bone (levator clavicular) which passes from the neck to the shoulder. It is invariably present in pronograde monkeys, in which it advances the shoulder in running; it becomes modified in its size and attachments in the orthograde apes ; in man it has almost disappeared. We cannot account for the occasional presence of this muscle in man except on the theory of a pronograde stage in man's evolution.
I might cite a number of vestigial muscles in the human arm which are well developed in apes. Medical students are aware of the fibrous remnant of a muscle (the latissimo-condyloideus) which is found in the posterior wall of the armpit, uniting a large muscle passing from the back (latissimus dorsi), and another passing from the shoulder into the arm (long head of the triceps). In apes this muscle, which is a mere vestige in man, is a source of strength in climbing. Another instance is seen in connexion with the biceps muscle which bends the forearm on the upper arm. In gibbons this muscle is used for bending the arm as the animals swing along with an ease that a human gymnast might well envy. It is therefore strong and is provided with two extra parts or heads. Now it is a curious fact that these extra heads for the biceps not unfrequently appear in man, indeed one of them, the inner, may be seen in ten per cent, of bodies. It is possible that man and the gibbon may have acquired these extra heads independently, but this is improbable when we take into account the great number of characters they have in common. The third instance I am going to cite of a vestigial or rudimentary muscle in the human arm is the one known as the palmaris longus. In one person out of ten it is quite absent, but the chances are in favour of the reader being able to detect its tendon'on his own wrist. If he will look at the front of the wrist, while holding his palm and fingers in a stretched position he will probably see and feel a small cord below the skin passing from the forearm into the palm exactly in the middle of the wrist. In the arms of monkeys he would find this muscle well developed and performing a very useful function. It must be remembered that the monkey uses its hand both as a hand and a foot. The palmaris longus acts on the palm, especially on the skin and pads of the palm, which are rough with papillae to give firmness of grasp on the trees. The foot-like action disappears from the hand with the assumption of the upright posture, and hence in the anthropoid apes and in man, the palmaris loses its function and becomes very small or may be quite absent.
Beneath the calf of the leg there is an exact counterpart of the palmaris longus. This muscle, the plantaris longus, is also vestigial; it is often little more than a white tendinous cord, having no muscular belly; in five per cent, of men it is altogether absent. The anthropoids are similar to man in this respect; in the gorilla it has almost disappeared. In all pronograde apes it is well developed, and instead of ending on the heel as in man, passes into the sole of the foot, where its tendon spreads out to form a stout membrane (the plantar fascia) under the skin of the sole. As a monkey runs along on all fours, its heel will be observed to be turned upwards off the ground ; there is a supple joint—the mid-tarsal joint—just in front of the ankle, which allows the hinder part of the foot to be bent easily upwards. A change in posture, such as is seen in anthropoids, is accompanied by a stiffening of the mid-tarsal joint; the tarsal part of the foot is enlarged to provide a more firm support for the weight of the anthropoid's upright body. The heel is prolonged backwards and the heel is not bent upwards as in monkeys, but the whole sole is applied flatly to the branch as the anthropoid passes along it in an erect or semi-erect posture. When the heel is thus applied to the ground in consequence of the orthograde posture it presses against the tendon of the plantaris; indeed, the heel grows through the tendon, thus cutting off the muscular part in the leg from the tendinous part in the foot. This condition is seen in the legs of man and of the anthropoids, and is proof that all of them have passed through a pronograde stage.