DIGESTION 



293 



the ox than in the horse. In the dog, movements other 

 than vertical are ver}?- limited. In herbivora the lower jaw 

 is narrower than the upper (fig. 143), so that when the 

 molar surfaces of the upper and lower teeth meet on one side 

 they do not come into contact on the other. Mastication is 

 therefore always unilateral. In the lateral movement of 

 mastication, the outside of the lower molar teeth and the 

 inside of the upper have the greater amount of wear and 

 tear, and consequently the tables come to be oblique 

 instead of horizontal. In the horse this sometimes 

 produces long, sharp, ragged edges, which prevent the 



Fig. 1-43. — Showing relative widths of lower and upper jaw in the horse. 



proper mastication of the food and consequently lead to 

 malnutrition. 



Salivary Glands. — Three pairs of salivary glands — parotid, 

 submaxillary, and sublingual — open into the mouth. These 

 are compound tubular glands, and are well developed in 

 herbivora. The acini are lined with mucus and enzyme 

 secreting epithelium (p. 34). The parotid has the most 

 copious secretion, and except in the ox, where the submaxillary 

 is developed to an equal size, it is the largest of the three glands. 



The (Esophagus is a muscular walled tube, lined by 

 squamous epithelium. The muscles are in two layers — an 

 outer longitudinal and an inner circular. This general 



