Leaving the rooms in which the skeletons are shown we make our way to another in which the teeth of all kinds of animals are displayed, in order that we may ascertain what guidance they can give us regarding man's place in the animal scale. In the first place we examine the dentitions of young children; in the upper, as in the lower jaw, we observe that twenty teeth are already erupted; of the twenty, eight are cutting or incisor teeth ; four are canine or eye-teeth ; eight are grinding or molar teeth. The teeth just enumerated form the milk set. Embedded in the bone beneath the gum we see the permanent teeth in process of formation ; there are sixteen in each jaw. Of the thirty-two permanent teeth, twenty take the place of the milk teeth, the eight milk molars being replaced by teeth of a different type— the premolars. The permanent molars, twelve in number, are added to the milk set. To make room for them a growth takes place in the jaws which leads to the marked changes we see taking place in the face during childhood and adolescence. The four first molar teeth erupt about the fifth year, the four second, about the fourteenth year; the four third molars or wisdom teeth are most irregular in their appearance in us Europeans ; they may cut at any time between the twentieth and the fortieth year, and in a considerable number of people they never erupt at all.
In the anthropoids the third molars always come into use; in the males they erupt with or even before the canine teeth. The canine teeth of the male anthropoids are large and need a considerable number of months to reach their full growth after they have pierced the gum. While the permanent teeth of anthropoids are all in place by the fourteenth year, the human dentition, even in primitive races of mankind, is not complete until the twentieth year, and as we have just seen, in European and civilized races the period of completion may be delayed until quite late in life. In primitive races, as was also the case in ancient Europeans, the wisdom teeth never fail to erupt. It is only in European or civilized races that there is a marked tendency to a suppression of the last or most posterior of the molar teeth. There is also a tendency for an arrest of the growth of the jaws in the same races. Civilization appears to be exercising a deteriorating influence on human teeth and jaws. It seems as if our wisdom teeth had become unnecessary, and that in this matter our dental constitution is out of keeping with our present-day needs.
On the other hand, one does occasionally see in the skulls of native Australians and of other native races, an extra or fourth molar added. A supernumerary molar is frequently seen in the dentitions of orangs and gorillas. These extra molars do not appear to be reversions to a former condition, but are really of the nature of a progressive adaptation.
As we pass from the human specimens to those illustrating the anthropoid dentitions, noting as we go the large teeth and strong jaws of native races—such as the Australian and Oceanic aborigines—we are at once impressed with the massive jaws of man's nearest allies. They project forwards to form a decided muzzle. In the infant anthropoids, however, we note the same twenty milk teeth, arranged as in the maxillae of the human child; in the adolescent anthropoids we see the same number of permanent teeth coming into position as in human jaws. The wisdom teeth appear as soon as or even before the canine teeth. It is the canine teeth which characterize the anthropoid dentition ; instead of occupying a place in the regular series they project above their neighbours as conical tusks. They are much larger in the males than in the females ; they are especially small in the female chimpanzees; the canines of the milk set are much less prominent than the canines of the permanent set. We observe, further, that the cusps of the crowns, and the fangs of the roots of the teeth are arranged on the same plan as in man; the differences are chiefly those of detail. In the small anthropoid or gibbon the number of teeth in each set is the same as in the large anthropoids; the gibbon's teeth are much smaller, with the exception of the canine teeth, which are especially long and sharp in both males and females.
When, however, we come to examine the teeth in monkeys we see many points of difference. In the Old World monkeys the number of teeth in both the permanent and milk dentitions remains the same as in man, but the cusps of the molar teeth are arranged on a totally different principle. In the New World monkeys the cusps of the teeth are arranged as in man and the anthropoids, but the number of teeth is different. They have twelve instead of eight premolar teeth. We have reason to believe that primates had originally sixteen premolar teeth; the American monkeys have lost four, while man, the anthropoids and Old World monkeys have lost eight of the original number. Occasionally we see an extra premolar appear in the human dentition. If we were to fix man's position among animals by the evidence of his teeth alone, we should place him on one side of the great anthropoids, and the gibbon, at about the same distance, on the other side of them, but all three included in the same group. In another group of equal value to the one just mentioned must be placed the Old World monkeys, while a third would contain the American monkeys.
Having surveyed the evidence afforded by the teeth displayed in the odontological room we move to the galleries of the museum, where we shall find Hunter at his very best. The preparations are grouped to show how the various functions of the animal body are carried out—the organs for locomotion, the organs of circulation, of respiration, for reproduction and for the purposes of sense and nerve action. In every series the preparations begin with organs selected from the lowest forms of life ; each series is arranged to show the various steps which lead on from the lowest to the highest. The particular specimens we wish to examine at present are those which digest the food and absorb nutrition—the stomach, the bowel, and the various glands connected with digestion such as the liver and pancreas. We see at once that these organs are similar in shape and arrangement in man, the great anthropoids, and in the gibbon, but when we pass down the series to the organs which represent the monkeys of the Old and of the New World, we see that the arrangement of the viscera in the abdomen is different, and even the shapes of the organs are altered. It is true that in the liver of the gorilla we note deep indentations or fissures which are absent from this organ in other anthropoids and in man ; we see in this a minor but puzzling peculiarity of the gorilla's anatomy. In all these higher primate forms we note on the right side of the abdomen, just above the groin, that the small intestine ends in a capacious caecum, the name given to the dilated commencement of the great bowel. At the closed and lower end of the caecum, in the anthropoid, as in man, is attached a narrow tube—the notorious appendix vermiformis. It terminates blindly below and hangs freely within the abdomen, lying more or less behind the caecum, into which its upper end opens. Vermiform or worm-like is not an unhappy term, for in life the appendix may be seen to slowly twist and turn as its muscular coat passes into action during digestion. It varies in size according to the age of the individual and the state of digestion. In the gorilla it is of the hickness of the small finger, but twice as long; in man it is shorter and smaller, but in him its shape and size are subject to the utmost variation after puberty. Unfortunately, we do not know the uses of the appendix, but on many occasions the writer has noted in the anthropoids and in children that the contents of the caecum pass freely into it. In the gibbon it is quite usual to find within it seeds of fruit, as big as cherry stones, during active and normal digestion. So far we have never known of appendicitis occurring in anthropoids living in a state of nature, but they do become liable to this disease when kept in captivity and fed on a human diet. In monkeys the caecum terminates below in a blunt conical point, which represents the appendix, but no real narrow worm-like appendix is present. Yet it is very possible that at one time an appendix may have been present in monkeys, for in lemurs, which represent an older type than the true primates, a true appendix is found. On the other hand, it is possible that the highest group of primates and the lemurs have acquired an appendix independently. It is not necessary to make a further survey of the organs of digestion; we see that if we were to classify animals according to the abdominal viscera, we should have to place man, the great anthropoids and the small anthropoids or gibbons in a single group. Even to the detail of possessing an appendix vermiformis they are the same.