25 

 rack, is not always remarkable in that respect. All, 

 however, that is gathered by the lips, or grasped by 

 the incisors, passes to the molars to be comminuted 

 and mixed with the saliva previous to being swal- 

 lowed. The molars, in fact, are animal grindstones, 

 and the different degrees of hardness which the three 

 component substances display, by wearing unevenly, 

 always keep the grinding surfaces irregular or sharp. 

 The inequality of the grinding surfaces of the mo- 

 lars enables the horse to reduce the toughest fibre to 

 a pulpy mass ; but as many substances little less hard 

 than the tooth itself must frequently be ground up 

 with the food, the molars would, at first sight, appear 

 to be subject to injury, especially as they have ten 

 times the work of the incisors to perform, and the 

 senses of touch, smell, and sight, cannot operate for 

 their protection. Nature, however, has provided 

 against the danger to which they appear to be ex- 

 posed ; for if, notwithstanding the guarding agency 

 of the senses, the incisors are so often injured, the 

 molars, blindly employed, and used when the full 

 power of the horse's jaw is exerted, certainly needed 

 some provision against fracture. The crusta petrosa 

 gives all the security that could be desired ; it en- 



