(C) The Anatomy of Apes (cont.)
The Teeth of the Ape
The teeth of apes consist, as in man, of incisors, canines, premolars, and molars; but the series of teeth no where forms so perfect an arch as in man, the opposite series of grinding teeth tending to become more parallel.
No ape has the teeth placed in one uninterrupted series in each jaw, as is the case in the human species, but there is always a small interval (diastema) between each upper canine and the adjacent incisor, and between each lower canine and the adjacent premolar. This condition is due to the excessive size of the canines, the interspaces giving passage to the apices of these teeth. This prolongation of the canines into tusk-like weapons of offence and defence (especially developed in the males), makes a great difference between the aspect of the dentition in apes and man.
The number of the teeth is the same as in man in the Simiadae. The Cebidae have an additional premolar on each side of each jaw, and the Hapalinae, besides this, have a true molar the less.
The incisors are always nearly vertical save in the Pithecinoe, when their apices project strongly forward. The canines are always considerably longer than the incisors, except in the genus Hapale, where the lower incisors equal the canines in length.
The premolars differ structural from the molars much as in man, save that the first lower premolar may be modified in shape to give passage to the upper canine, as is specially to be seen in Cynocephalus.
The grinding surface of the molars consists generally of two transverse ridges, each end of each ridge projecting more than the intermediate part, and so giving rise to four tubercles. In Simia and Troglodytes, however, we find in the upper molars an additional structure, which also exists in man. This is a ridge which runs obliquely from the front inner tubercle (or cusp), outwards and backwards to the hind outer tubercle. In the rest of the Simiadae this ridge is wanting, but it reappears in Ateles and Mycetes amongst the Cebidae. In the Hapalinae the tubercles of the molars are more produced and sharp-pointed, in harmony with their decidedly insectivorous habits. The last lower molar may be reduced or much enlarged as compared with the others. Thus is Cercopithecus talapoin it has but three tubercles, while in the Macaci and Cynocephali it is very large, and has five well-developed cusps.
The number of milk teeth is as in man, save that the Cebidae have an additional milk molar. In general the canines are the last teeth to be cut of the permanent dentition. Their cutting sometimes causes such constitutional disturbance as to produce convulsion and death. In the gibbons, however, the canines accompany, if they do not precede, the appearance of the hindmost molar, while in the orang they at least sometimes make their appearance before that grinder.
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