406 



GENERAL ZOOLOGY 



squirrel and its allies the condyles are parallel with the long 

 diameter of the head, thus allowing some backward and 

 forward motion of the lower jaw. The advantages of these 

 special adaptations of the structure of teeth and jaws to the 



life of a gnawing animal like 

 the squirrel are obvious. 



The ducts of four pairs of 

 salivary glands open into the 

 mouth. A muscular flap, 

 called the soft palate, to dis- 

 tinguish it from the roof of 

 the mouth, or hard palate, 

 separates the mouth from the 

 pharynx (Fig. 215). Embed- 

 ded in the soft tissues of the 

 soft palate are the tonsils, two 

 small oval bodies, the func- 

 tion of which is unknown. 

 The nostrils open posteriorly 

 into the pharynx. The Eu- 

 stachian tubes from the ears 

 enter the pharynx at the 

 sides. 



A short, straight esophagus 

 (Fig. 215) leads to the sac- 

 like stomach, passing through 

 a muscular partition called 

 the diaphragm, which separates the heart and lungs in the 

 thoracic cavity from the organs of the abdominal cavity. 

 In the first fold of the intestine is the pancreas, an extended 

 mass of spongy tissue roughly suggesting a bunch of grapes. 

 The liver is large and is divided into several lobes. The in- 

 testine is very long and much coiled. A clearly marked 



1 From Flower and Lydekker's Mammals. 



Fig. 214. Sections of teeth 



A, incisor, or tusk, of the elephant ; 



B, human incisor during develop- 

 ment ; C, human incisor completely 

 formed ; D, human molar ; E, molar 

 of ox ; 1, enamel ; 2, cement ; 3, den- 

 tine; .4, pulp cavity 1 



