In the last two chapters we have been considering the changes which occur in that phase of our lives which culminates at birth. Before surveying those transformations which link infancy to old age it is necessary to emphasize one or two points relating to intra-uterine life. There are really two stages in that life; by the end of the third month all the parts of the body are formed, the active process of development is over. During the last six months of intra-uterine life the changes are those of growth and of maturation. The years which follow birth form a continuation of the latter stage of fcetal life; no new structure or organ is laid down, bone and tooth formation proceed as before, and the tissue cells continue to divide and grow. By the act of birth the economy of the human body is suddenly changed. Up till then the placenta supplied the child with oxygen and nourishment from the mother's blood; birth calls the lungs suddenly into action, and the blood passages and heart undergo a rapid transformation to suit the new respiration. The stomach and bowels have to supply nourishment from food. The remarkable transformation of the lungs, heart and stomach in the newly born child are not peculiar to man; they are the common heritage of the higher mammals.
From birth onwards the body continues to change; there is no stationary period; every year leaves its mark. Shakespeare distinguished seven ages—" The infant mewling and puking in the nurse's arms "—" the whining school boy "—" the lover sighing like a furnace"—" the soldier . . . bearded like the pard "—" the justice in fair round belly . . . with eyes severe"— " the lean and slippered pantaloon "—" second childishness . . . sans teeth, sans eyes, sans taste, sans everything." His description of the age changes of the body is perhaps the most graphic and complete ever penned. Unconsciously every one of us is a student of these bodily changes; so familiar do we become with them that with a glance of the eye we estimate instinctively the age of a passer-by. Our judgment is influenced by many characters, most of all by those of the face. Before we are aware of it we have noted the skin of the face, observed its lines and wrinkles, formed an impression of its texture—in short of its probable age. Of all the tissues of the body the skin and muscles of expression which lie under the skin are the most accurate registers of years. Not one passes and leaves the skin just as it was ; every year the tender, soft, suffused, velvety covering of babyhood moves one degree towards the dry, grey, wrinkled and loose integument of the very aged. We never mistake the " baby-fat" which gives the plump rounded outlines of youth for the obesity which may come with middle age. It is true that we do not all grow up or grow old at the same rate ; youth persists in some and is delayed in others, so that in this case or that our judgment may be a little astray.
The carriage of the body counts for much ; the suppleness of joint, firmness of hand, and agility of limb are marks of youth. The old gentleman, although he walks erect, cannot mislead us any more than the old lady who seeks a means of restoring colour and smoothness to the skin of her face. Our attention is immediately arrested by an incongruity in age of features ; a man may have grey hair but his skin, his eyes, his lips and mouth tell us he is still young. Age wipes gradually the sharp outline from the lips and mouth; the eyes become a little duller and the eyelids lose something of their cleanly cut youthful shape. The form of the chest and trunk changes; the body never ceases maturing in manhood nor decaying as age is advanced. We have all studied the age changes of the body in the practical school of daily life. The medical man is well aware that tissues cannot live and not change, and that the appearances of age we have noted in the face are but symptoms of the decay that is attacking all the tissues of the body. The elastic tissue of the skin gradually loses its elasticity ; although alive—at least we believe it is—it is less alive than any other tissue of the body and is less endowed with the powers of repair. It is the first to suffer decay. The elastic and muscular coats of our arteries are at their best about twenty-five ; that is the age when the hurdler, the footballer—all who have to make sudden physical spurts—are in their prime. The brain attains its most accurate control of muscles between thirty and forty, while as an organ of thought the brain itself is at its best between forty and fifty. We see then that the systems of the body mature and age at different rates, but the collective changes in the body occur so uniformly that when an individual is brought before us we are seldom at a loss in estimating the years that have passed since birth.
Shakespeare's seven ages run their course for most people in the Psalmist's limit of threescore years and ten. How and when did man come by his span of years ? We have to seek for evidence in the light of the evolutionary theory. How does the span of life run in those animals which are allied to man—the anthropoids and monkeys ? Let us first look at the period of infancy—for that is proportioned to the longevity of the adult. Now the period of infancy may be defined as that in which the milk teeth are erupting. In man, all of the milk teeth—twenty in number—are in place by the end of the second year, but the period in many children extends six months longer. The period of human infancy then may be fixed at two years. In monkeys and in the small anthropoids, the gibbons, the milk dentition is completed before the end of the first year. In some, such as the semnopitheques, the incisor teeth are cut at birth and the dentition is completed in eight months. The gibbons are the most important for our present purpose, because we suppose them to represent a phase in the evolution of the great anthropoids and of man. The period of infancy in the gibbon is then under one year. In the great anthropoids the milk teeth begin to appear some three or four months after birth and this dentition is not complete until some time in the second year. The period of infancy in the great anthropoids, therefore, is probably under two years, but in its extent is comparable to that of man. There is evidence to show that the intra-uterine period of the gibbon is seven months and that in the great anthropoids, as in man, it is nine. We may be fairly certain, therefore, that the lengthening of the periods of fcetal and of infantile life took place during the evolution of the great anthropoid from the small—a period which we have already shown to be at least millions of years ago.