290 



COMPARATIVE PHYSIOLOGY. 



to wear, the other constituents appear on the upper surface of 

 the crown (Figs. 238, 289, 240). 



It follows that the former are better adapted for tearing, 

 the latter for grinding, as the different components wear un- 

 equally and leave the top of the crown rough, so that the upper 

 and lower jaws of a ruminant are like two millstones, (Fig. 

 241). 



It also follows that in the horse and in ruminants the age 

 may be learned with considerable accuracy from the condition 

 of wear of the teeth and as the incisors are most readily ex- 

 amined they are taken as the chief indicators of the age of 

 the animal. 



In nearly all animals are found the deciduous or milk teeth 

 succeeded by the permanent teeth. This arises as a necessity 

 from the growth of the jaw and the need of stronger teeth, either 

 as weapons of defense and attack or in order the more effectu- 

 ally to secure and prepare food. The permanent teeth are also 

 more numerous than the milk teeth. 



The dentition of our domestic animals may be expressed 



•s^ 



molars, «=s.= 42. 



\V 



premolars, 



3-3 



3-3 

 2-2 

 2-2 

 2-2 

 3^ 

 3-3 

 3-3 

 3-3 

 3-3 

 3-3 

 0-0 

 0-0 



3-3 

 3-3 



4-4 

 3-3 

 3-3 

 3-3 

 3-3 

 3-3 

 31^ = 34. 



= 32. 



:44. 



:32. 



= 40. 



The latter is the representation of the mUk dentition. The 

 mare is without canines (" tushes "). 



It will be noticed that in the ox incisors and canines do not 

 appear in the upper jaw, though they are represented by embry- 

 onic rudiments. 



The table above and that on page 296 (after Leyh) give a 

 large amount of information in a small space, and are illus- 

 trated by the accompanying figures : 



