30 



AGE OF THE DOMESTIC ANIMALS. 



obliquity with age, and the horizontahty of their arches in- 

 dicates approximately the degree of the alteration, — excepting, 

 of course, certain abnormal changes to be studied later, — we 

 have in the profile of tlie jaw a valuable flictor by which to 

 judge of the age of the liorse. 



Direction in Regard to the Median Line (Fig. 18). — 

 In the young horse, the crowns of the six incisor teeth widened 

 from side to side, while their roots are flattened in their trans- 



FlO. 17. 

 Arch i)f the incisors, uncovered to show their successive planes of contact. 



verse diameter, cause the teeth to take a position on the end of the 

 jaw like the ribs of an open fin. They diverge from the alveo- 

 lar cavities, in which they are imbedded in the bone, toward the 

 circumference formed by tlieir crowns. But, as age advances, 

 the crowns become worn off", and the teeth are pushed fartlier 

 and farther out of their cavities. The roots, which were at first 

 almost in contact with each other, gradually separate and widen, 

 while the circumference constantly diminishes as the teeth 



