DIGESTIVE ORGANS AND THEIR ANATOMY. 



267 



are missing and the lower teeth strike against an upper pad. 

 In the class of rodents or gnawers, which includes such 

 forms as the rat and the squirrel, the incisors are remark- 

 ably developed. They keep growing throughout life, and 

 have to be kept short by being worn off against each 

 other. In this way these teeth are kept continually very 

 sharp. It not infrequently happens that such a rodent 



breaks off one of the 

 incisors, in which case 

 the opposite incisor no 

 longer worn away, 

 grows indefinitely, and 

 may finally grow 

 around back into the 

 head of the animal and 

 cause its 'death. ;, : 



The Structure of a Tooth. 



The typical tooth, 

 such as a canine, for 

 instance, consists of 

 three parts the fang 

 imbedded in the jaw, 

 the neck, and the 

 crown projecting from 

 the gums. If such a 

 tooth be cut in two 

 longitudinally there is 

 disclosed in the center 

 a large cavity known 

 as the "pulp" cavity 

 and opening to the ex- 

 terior through the 

 fangs. At these points 



^ig. 100. VERTICAL SECTION OFAPREMOLAR OF THE nerves and blood-VCS- 



CAT. (After waideyer.) sels enter the pulp cav- 



c, pulp cavity; J, enamel; 2, dentine; 3, cement; . ^, . -, -, 



4, dental periosteum; 5, bone of lower jaw. ity. The main body of 



