614 



THE EXTERIOR OF THE HORSE. 



formation which, appearing upon the table at a given period in the form of a 

 transverse yellowish band in front of and behind the central enamel, constitute 

 what is called the dental star. 



To speak accurately, there are two of these dental stars, but the posterior, 

 always less distinct, most frequently passes unperceived. Nevertheless, in cer- 

 tain teeth it is seen quite plainly. 



Fig. 286 represents the median longitudinal sections of pincers from horses 

 of different ages, which show the gradual diminution of the internal dental 

 cavity. It will be seen, too, in the very old teeth, that the extremity of the root 

 is covered by an abundant formation of radical cement directly applied upon 

 the surface of the ivory, from the fact of the disappearance of the enamel. On 

 that account these teeth, when the wear has reached this level, never display the 

 whiteness which they did primitively. 



The dentine is a substance less hard than the enamel, but much more resistant 

 to wear than the bone or the cement. A microscopic examination of desiccated 

 or macerated teeth shows that it is penetrated by a multitude of extremely fine 

 canaliculi, which extend in a radiating manner from, the internal cavity of the 

 pulp to terminate, after anastomosing, with one another, under the deep surface 

 of the enamel in the lacunar spaces, called the interglobular spaces of Czermak. 



Fig. 287.— Transverse section of an inferior right pincer, showing the different layers constituting 

 the tooth, with their relative thickness. (Magnified five diameters.) 



A, anterior face. 



B, posterior face. 



C, peripheral cement. 

 C", central cement. 

 E, peripheral enamel. 



£', central enamel. 



/, ivory (external layers); I', ivory darker 



in color surrounding the central enamel 



(internal layers). 



Such are the principal details which it is essential to understand before pro- 

 ceeding farther. They may also be examined in transverse sections analogous to 

 those of Fig. 287. 



Let us now see what modifications the free extremity of the permanent 

 incisors undergo under the influence of wear and tear. 



