TEETH. 155 



very few English horses live to reach the thirtieth year ; but to show 

 that those signs which were remarkable in the last engraving commence 

 at an earher period, below is the jaw of a twelve-year old horse, ir, 

 which the presence of all the indications that at the thirtieth year seem 

 exaggerated, may be clearly discerned in their commencement. 



JAW OP A TWELVE-YEAR OLD. 



The author must now explain the phenomena to which he has directed 

 the reader's attention. The molar teeth are not all of the like size, nor 

 of one form. The organs occupying the upper jaw are nearly, not quite, 

 double the width of those which are locaj^ed in the lower jaw. The 

 inferior molars are the grinding agents, or the active organs of mastica- 

 tion. The superior teeth are simply the passive tables upon which, or 

 against which, the food undergoes comminution. The slab is always 

 the lowest of the two in human mills ; but nature has more to provide 

 for than the mere pulverization of certain substances. "With mastication, 

 actually commences a very compound process. With the act of chewing, 

 digestion begins ; it was ordained that more than any mechanical inven- 

 tion can accomplish should be imperative to the due performance of this 

 function. The benevolence of the All-wise instituted that while his 

 creatures were promoting the healthy exercise of the appropriative ne- 

 cessity, they should likewise excite their enjoyment. Therefore when 

 pulp is masticated, the pressure of the teeth expels the juices, which fall 

 directly upon the seat of taste. "When a harder substance has to be 

 comminuted, the bulk is first shattered into fragments ; the particles, de- 

 scending upon either side of the teeth, have to be gathered up and placed 

 again between the masticatory organs. The movements of the tongue 

 and jaw excite the salivary glands ; the broken substance becomes min- 



