352 TBE DIGESTIVE APPARATUS IN MAMMALIA. 



(infundibulum) is shallow ; and that tliey are not constantly pushed out- 

 wards from their cavities, their growth ceasing when they begin to be used. 

 When the replacing teeth appear, they do so a little behind the temporary 

 ones, whose shedding they cause by gradually destroying their roots, which 

 at last become only a long and very thin shell of dentine. 



The follicle in which the incisor teeth are developed shows only two 

 papillae ; one for the secretion of the dentine, lodged in the internal cavity 

 of the tooth, and hollowed into a cup-shape at its free extremity ; the other 

 is contained in the external cul-de-saa (Fig. 156, a. b. c). 



TusKS, Fangs, or Canine Teeth.' — " The tusks of Solipeds only exist in 

 the male ; it being quite exceptional to meet with them in the female, and 

 even then they are rarely so strong as in the male. 



" These teeth are four in number, and are placed one at each side of the 

 jaws, a little behind the incisors, to which the lower canines are much 

 nearer than the upper. Between them and the first molar there is left a 

 considerable space, which constitutes the bar of the inferior jaw. 



" The free portion of the tusk, slightly curved and thrown outwards, 

 particularly in the lower jaw, offers two faces : an external and an internal, 

 separated from one another by two sharp borders inclined to the inner side, 

 and meeting in a point at the extremity of the tooth. The external face, 

 slightly rounded, presents a series of fine strise, longitudinal and parallel. 



" 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 fi'ee portion, bears 

 internally a cavity analogous to that of the root of the incisors, and 

 like it, this diminishes and finally disappears as it advances in age ; but it 

 is always relatively larger, because of the absence of the infnndibulum 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 snafile; 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 Eigot, 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 

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

 little cement. 



"The disposition of the developing follicle is in harmony with the 

 simplicity 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 longi- 

 tudinal ridge, on which are moulded the ridge and grooves oh the internal 

 face of the toflth." 



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

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



' The quotations included within inverted commas are from M. Lecdq's ' Traitd de 

 rExte'rieui du Cheval et des Prinoipaux Animaux Domestiques.' 



