SPECIA.L ANATOMY. 



131 



alveolar cavities are developed late, and remain tln'oughout 

 life, the sixth or posterior one not appearing until tlie 

 three anterior (temporary) molars have been replaced, as 

 the following table indicates — 



Variations, however, occur as a result of breed, manage- 

 ment, &c. TJioroughhred horses date their birth from the Isf 

 January, all other breeds from the 1st May. 



The molars are developed from compound papill?R. 

 Thej are found in four rows of six each, one on either side 

 of the upper and lower jaws. Each molar presents six 

 surfaces : —The anterior is smooth, in contact with the 

 tooth in front through the medium of the thm plate, form- 

 ing the anterior boundary of the alveolar cavity. But in 

 the first molar this margin is sharp. TlhQ posterior surf ace is 

 in similar contact with the tooth behind, but in the last 

 molar is sharp. The external surface presents two grooves 

 running from above downward, of which the anterior is the 

 deepest ; the internal surface is also grooved, but less dis- 

 tinctly, the outer surface of the upper, the inner surface of the 

 lower, molar is the largest, for the superficial or grinding 

 surface in both the upper and lower molars slants obliquely 

 downwards and outwards, so that the inner part of the 

 upper comes in contact with the outer part of the lower 

 molars, a consequence of the greater width of the superior 

 than of the inferior maxilla ; so in rasping a horse's teeth 

 the rasp must be applied to the inner edge of the lower, 

 the outer edge of th3 upper, grinders. The grinding 

 surface presents the edges of the layers of the substances 

 of which the tooth is composed arranged " in the form of a 

 Gothic B," so that, as substances of different degrees of 

 durability are exposed to equal attrition, an irregular 

 surface favorable to the comminution of the food is con- 



