INTEODUCTION 



15 



Fig. 17. 



Vertical sections of various teeth, namely, a full-grown single-rooted incisor 

 (^A), an immature incisor with widely open pulp cavity (B), an incisor of a 

 rodent, with the front face enamelled and the pulp persistently growing (C), 

 a molar tooth of man, with a low broad crown {!)), and a molar tooth of an ox, 



e, Enamel ; 



physiological function. For grasping and holding the prey, simple and 

 conical teeth serve best ; if used as weapons or to remove obstacles, they are 

 elongated and project out in front of the mouth, or become modified into 

 tusks. If the food is 

 varied and requires an 

 apparatus for grinding 

 it, then division of 

 labour and specialisa- 

 tion take place. Cer- 

 tain teeth function 

 exclusively for grasp- 

 ing and holding the 

 food, while others tear, 

 cut, crush and grind 

 it ; and as these 

 processes are gener- 

 ally carried on in 



different nartS of the '^'^^^ ^ deep crown and folds of enamel tilled with cement (E). 

 " , d, dentine ; c, cement ; p, pulp cavity. 



mouth, the anterior 



teeth are differently formed from those more remotely situated in the mouth. 



Teeth are wanting only when the food requires no comminution, as among 

 whales and ant-eaters. Uniformity of dental structure (homodont dentition), 

 resembling that of lower vertebrates, also occurs secondarily only when, as in 

 marine forms, the teeth are used solely for seizing and holding the prey. Most 

 mammals have a differentiated or heterodont dentition, the teeth of either 

 side of a jaw being repeated symmetrically on the other side. The teeth 

 situated in the premaxilla and in the front part of the lower jaw are termed 

 incisors (I). Just behind the upper incisor and directly behind the suture 

 between the premaxilla and maxilla is placed the conical single- (seldom two-) 

 rooted canine (C), corresponding to which in the lower jaw is the lower canine. 

 Behind the canine comes the series of molars or cheek-teeth, the more anterior 

 of which, being usually simpler, are termed premolars (P), and the posterior 

 the molars (M). If all these kinds of teeth are present the dentition is 

 termed complete ; and incomplete if any are lacking. 



Among toothed whales, Sirenia and most edentates, the first set of very 

 simple teeth continue in use throughout life. These comparatively few mono- 

 phyodont forms stand in marked contrast to the great majority of diphyodont 

 mammals which have two sets of teeth. The replacement of teeth, however, 

 occurs but once during life. The first set is termed milk or deciduous teeth, 

 and this is replaced by the second or permanent set. The milk teeth are also 

 differentiated as incisors, canines and molars, the first two types corresponding 

 to their successors in number and form, while a less number of milk molars 

 occur, and these correspond in form rather to the molars than to the permanent 

 premolars which replace them. There are no teeth in the milk dentition 

 ^ which precede the molars of the permanent dentition, for the so-called milk 

 molars are replaced by the permanent premolars. The first post-canine milk 

 tooth is never replaced and is usually regarded as the first premolar of the 

 adult series (Leche). Among marsupials usually only the last milk molar is 

 replaced, the anterior milk molars may have been lost. The dental succession 

 is apparently being lost, for among all the orders it takes place now at a 



