The changes in the head and neck so far described are those of infancy, youth and early manhood or womanhood, they are growth changes. Not a word has been said of the changes which set in with old age. There is no stationary interval between the first and the last. By the time the final form of the head has been determined by the processes of growth the changes of decay set in. In some, these are marked by obesity; in many they are emphasized by an absorption of the natural fat, which leaves the skin loose and the skeleton, muscles and veins apparent. With advancing years the muscles of mastication undergo a process of atrophy and the fat round them is absorbed; the temples therefore become hollow, and the bony processes of the skull become surface markings. With the loss of teeth the jaws atrophy, the cheeks fall in, the mouth becomes shapeless. With the atrophy of the jaws and muscles of mastication the neck becomes thin and its skin lies in folds.
We are now to examine a number of changes which occur in the human body preparatory to the assumption of the upright posture. In the latter part of infancy we see the body becoming modified for walking. During the first year the lower extremities cannot support the body, not because they are not strong enough, but because the brain has not yet gained control of the muscles which are needed to balance the body on the feet. Yet Dr. Louis Robinson has shown that a newly born child has sufficient strength in its grasp to support the weight of the body from the hands. The newly born monkey clings to its mother as she takes plunging leaps from tree to tree, by grasping the hair on the under surface of her body with its fingers and toes. The anthropoid, like the human infant, has short legs and cannot extend them into a line with the body. As the infant attempts to stand and walk, the legs are slightly bent at the knees and flexed at the hips. Presently, as the period of childhood is entered and the art of walking is acquired, the lower limbs grow at a faster rate than the rest of the body and peculiar changes occur at the hip and knee joints, which allow the limbs to be completely extended in line with the body, and to be used in standing and walking. While the chick can run the minute it escapes from the shell, the human child has to learn laboriously even the art of standing. In the newly born child the lower limbs form only two-fifths of the standing height; in the adult they form a half or a little more—according to the individual and to the race. We shall obtain a better insight into these remarkable adaptations for walking if we take the combined length of the trunk and head as a standard of measurement. In the human foetus at the sixth month of development the lower limbs are only 55 per cent, of the head-trunk length ; at birth they are, in the average individual, 62 per cent., and in the adult 102 per cent, of the head and trunk measured in a straight line. If we now turn to see how the anthropoids compare with man we find that their lower limbs have about the same relationship to the head-trunk length as in the newly born child—varying from 50 to 70 per cent.—but no special growth takes place in their lower extremities soon after birth as is the case in children. The lower limbs of the adult gibbon are 78 per cent, of its head-trunk length, those of the gorilla 66 per cent., those of the orang 58 per cent. Thus we see that the lower limbs of a child at birth have the same proportionate length as the anthropoid. The peculiarity of man is the rapid growth which takes place after birth and adapts the limbs for standing and walking.
At the same time as the lower limbs are undergoing a special growth in the child another change is taking place in the loins or waist of the body. No one who has closely watched the crawling child trying to stand up, can have failed to notice that it not only raises itself at the hinge or joint between the body and the thighs, but that the trunk is also bent backwards at the loins. The lumbar part of the backbone, which supports the loins between the pelvis below and the chest above, is made up of five vertebrae in man. This part of the spinal column supports and balances the upper part of the trunk. Now in all the great anthropoids the lumbar part of the spinal column is very short; the pelvis is closely knit to the expanded chest. The significance of this feature of the anthropoid spinal column is very apparent when these animals are seen swinging along, sustaining their weight as much from their hands as supporting it by their feet and legs, or even more. They do not habitually support and balance their bodies on the lumbar part of the spine as we do. In the newly born child the lumbar part is 27 per cent, of the total length of the spinal column— the same as in the adult chimpanzee. As the child learns to walk the lumbar region rapidly elongates, so that in adult man it forms 32 per cent, of the total length of his vertebral pillar. Here again we see a peculiar feature of man's body being developed in early childhood— a feature which adapts him to the upright posture. The short lumbar segment is an anthropoid character; it is replaced by a long segment which, as regards the higher primates, is peculiar to man.
These remarkable changes in the loins and lower limbs in early childhood, which have been thus cursorily described, are growth changes which fit the human body for man's peculiar method of locomotion. We recognize how perfectly these adaptations have been evolved from the easy and jaunty manner in which young people carry themselves. When forty years of age or thereabouts is reached, however, the suppleness of the joints and the easy co-ordination of the muscles begin to show some degree of impairment. As old age approaches the muscles shrink from thigh and calf ; the knees become bent, the hip joints stiff and incapable of full extension, the back bends and shrinks, while the muscles and joints of the spine fail to maintain the trunk and head in a position of easy balance. Old age wrecks the mechanism of the upright posture for every one of us. Yet in youth it seems such an easy thing to stand or walk.
The chief structural changes which occur after birth are concerned with the growth of the brain and the adaptation of the human body to upright plantigrade progression. These represent the most recently acquired characters of man. It is very apparent, too, that a complete human life is made up of two distinct periods—one in which growth is predominant and one in which decay is predominant; where the one period ends— about the twenty-fifth year—the other begins.