420 THE DIGESTIVE APPARATUS IN MAMMALIA. 



" The internal face has a conical eminence in its middle, whose point is 

 directed towards that of the tooth, and is separated from each border by a 

 deep groove. 



" The fang of the tusk, more curved than the free portion, has internally a 

 cavity analogous to that of the root of the incisors, and like it, this diminishes 

 and tinally disappears as it advances in age ; but it is always relatively larger, 

 because of the absence of the infundibulum in the canine teeth. 



" The form we have described for the tusks, is that which they present while 

 still young. As the Horse grows older they lose their whiteness, and become 

 worn in an irregular manner, and this most frequently by the action of the bit 

 or snaffle ; for the difference in position of these teeth in the two jaws does not 

 allow of friction between them. 



" The canine teeth are not shed, and grow but once. Some veterinarians, 

 and among them Forthomme and Rigot, have witnessed instances in which they 

 were replaced ; but the very rare exceptions cannot make us look upon these 

 teeth as liable to be renewed. We must not, however, confound with these 

 exceptional cases the shedding of a small spicula or point, which, in the majority 

 of Horses, precedes the eruption of the real tusks. 



" The structure of these teeth is much simpler than that of the incisors ; 

 consisting, as they do, of a central mass of dentine hollowed by the pulp cavity, 

 and covered by an external layer of enamel, on which is deposited a little 

 cement. 



" The arrangement of the developing folHcle is in harmony with the sim- 

 plicity of structure of the tusks ; at the bottom there is a simple and conical 

 papilla for the internal cavity ; on the inner wall, a double longitudinal ridge, 

 on which are moulded the ridge and grooves on the internal face of the tooth." 



Molar Teeth. — " The molars are twenty-four in number — six in each side 

 of each jaw. There are also sometimes supplementary molars met with in front 

 of the true ones, and which may be four in number ; but these are small teeth, 

 having but little resemblance to the others, are most frequently shed with the 

 first deciduous molar, and are not replaced. 



" Generally considered, the molar arches have not the same disposition in 

 both jaws. Wider apart in the superior one, they form a slight curve, with the 

 convexity outwards. In the inferior jaw, on the contrary, the two arches 

 separate in the form of a V towards the back of the mouth. Instead of coming 

 in contact by level surfaces, the molars meet by inclined planes, and in such a 

 way that the internal border is higher than the external in the inferior molars, 

 while the opposite is the case in the superior. 



" Like the incisors, each molar presents for study a free and a fixed portion. 

 The free portion, nearly square in the upper molars, longer than wide in the 

 lower, shows at the external surface of the former two longitudinal grooves, the 

 anterior of which is the deepest, and which are continued on the encased portion. 

 This is not so with the inferior molars, which have but one narrow, and fre- 

 quently an indistinct, groove. 



" The internal face in both jaws only shows one groove, and that but little 

 marked ; it is placed backwards in the upper molars, and is most apparent 

 towards the root. 



" The anterior and posterior faces are in contact with the corresponding faces 

 of the adjoining molars, except at the extremities of the arches, where the 

 isolated face is converted into a narrow border. 



