We come now to the period of adolescence, which we may regard as defined by the eruption of the permanent teeth. In man this period may be reckoned as extending from the fifth to the twenty-second year, but we have already seen that eruption of the third molars or wisdom teeth may be long delayed in civilized peoples and may appear at an earlier date in primitive races. In monkeys the adolescent period extends from the end of the second to the end of the fifth year; in gibbons, so far as the evidence available gives us warrant to make a definite statement— from the end of the second to the sixth year. In the great anthropoids the permanent teeth commence to erupt in the fourth year and are all in place before the end of the fourteenth year. In the anthropoids then, the period of adolescence extends to about the fourteenth year, whereas in man it is prolonged to the twenty-second. As to the natural span of life of monkeys and of anthropoids we know nothing, except from animals kept in captivity, but assuming the periods of infancy and adolescence to form one-third of the total life, we may suppose a gibbon at eighteen, an anthropoid at forty-two, and a man at sixty-six to be aged individuals. So far as the evidence goes then it appears that long life in the higher primates came with the evolution of a large body, but that man—at least civilized man—appears to have extended his span of life. Reliable observers assert that the more uncivilized races, such as the natives of Australia, show at forty-two the age change of a European at sixty-two. The facts at our disposal indicate that longevity is an old inheritance of the human stock and that modern man is longer lived than his forerunners.
One of the most remarkable changes of infancy is the rapid growth of the head. Everyone has noted the large head of early childhood, especially when compared with the slender neck which unites it to the trunk. The rate of growth in the head during the period of infancy is altogether out of proportion to the increase in the rest of the body. The facial part of the head does not share in this rapid increase; it is the cranial part— the part containing the brain, which undergoes a rapid expansion. The explanation lies in the mushroom-like growth of the infant's brain. It is of the utmost importance that the organ which is to guide the child through the intricacies of human life should be formed at the earliest possible date. By the end of the second year the child's brain has attained more than half of its adult size; by the end of the fourth year over 80 per cent, of its nerve tissue is already present. Indeed we may say that when a child of five goes to school all its nerve cells are formed and in place ; the increase which follows relates to a growth in the size of the child's body, for we know that growth in the size of the body is accompanied by an increase of brain tissue—an increase which is not connected with the higher faculties. It is extremely important to recognize such an element in the human brain and it may be called for convenience the " corporeal concomitant." The rapid increase of the cranial capacity is a character of the human infant. The brain of the newly born gorilla, which is only slightly smaller than that of a child at birth, is already 65 per cent, of its adult size; the remainder of its growth is probably due to the addition of the " corporeal concomitant." From birth onwards, the anthropoid brain continues to increase at almost a uniform rate until adult years are reached ; there is no spurt in growth such as we see in the brain of the human infant. The peculiarity of the human brain, then, is its rapid growth in infancy and early childhood. A child at five has only reached that point in the growth of the brain which the anthropoid has attained at birth. Man then is peculiar in that his brain continues to grow rapidly after birth, and in the great expansion of the head in infancy and childhood we see one of the latest phases in human evolution.
The age changes in the face and neck are ruled by other conditions. The face, from an anatomist's point of view, is really part of the apparatus of mastication. It is an accident, as it were, that the face has to accommodate the eyes and the nose. The facial part of the head is a bony scaffolding for the upper and lower jaw on which the teeth are set. As the milk teeth come into place during infancy, the face, still retaining its cherubic roundness of outline, increases gradually in size and strength. During adolescence, as the permanent teeth come into use one after another, the jaws themselves grow to accommodate the erupting molars or chewing teeth ; the facial scaffolding is enlarged to support them, while the muscles which move the lower jaw undergo a rapid growth. Every one must have observed that the features of the boy or of the girl, are replaced by bolder and rougher outlines as the last of the teeth come into position. The muscles of mastication which arise on the bony outworks on the side of the skull need stouter supports. The bony ridges of the cheek and skull become emphasized and the features rapidly change. The growth of the neck does not proceed at the same pace as that of the head. Most of us may recall, if our memories carry us back to the details of boyhood life, that every second or third year we took a larger size in collars, while we wore hats of the same size for a number of years on end. The rapid growth of the head took place during infancy and early childhood. At first sight it is not apparent why the neck should keep time in its growth with the muscles of mastication. It is so, however, and the reason becomes apparent when one remembers that the skull, which serves as a fulcrum for the muscles of mastication, must be steadied when these are in action. The skull is balanced and fixed by the muscles of the neck. It will be observed that it is the men with the wide, squarely-set, massive jaws and cheeks who have their heads deeply implanted in their necks. We describe them as strong, bull-necked men— people of a strong will and with a healthy appetite. In anthropoids we find the jaws massive, the muscles of mastication so large that they envelop the skull, and with heads hafted to exceedingly thick and muscular necks. It is as adult years are reached that we see the most marked changes occur in the fixation of the head and in the size of jaws.