94 



GENERAL ZOOLOGY 



glands, viz., the -parotid, the sublingual, and the submaxillary 

 glands (Fig. 60). The parotid gland lies beneath the skin of the 

 side of the head near the ear and its duct (Stenson's duct) enters 

 the oral cavity by piercing the cheek near the molars of the 

 upper jaw. The sublingual gland lies between the tongue and 

 the margin of the lower jaw and opens by several ducts in the 

 floor of the oral cavity. The submaxillary gland lies inside the 

 lower jaw and its duct (Wharton's duct) has its outlet near 

 the lower incisor teeth. All of these glands are paired. 



The secretions of the salivary glands vary. In man the sub- 

 maxillary and sublingual glands 

 produce most of the mucin, the 

 substance that gives saliva its ropy, 

 mucilaginous character; whereas 

 the secretion of the parotid is more 

 watery and contains ptyalin in 

 larger amounts than the other two. 

 Ptyalin is a digestive enzyme that 

 acts upon starchy food. 



Pharynx. — The pharynx, so far as 

 alimentation is concerned, is merely 

 a passage connecting the oral cavity 

 with the esophagus. Its impor- 

 tance is in connection with respira- 

 FiG.eo.-Diagram showing loca- tion > discussed in the following 



tion of the salivary glands in Man. chapter. 



L, sublingual; s, submaxillary; p, rr ? rr<L v 



parotid; t, tongue. Esophagus.— The esophagus IS 



for the most part a tube connect- 

 ing the pharynx with the cardiac end of the stomach. In the 

 frog it is neither sharply marked off from the pharynx in front 

 nor from the stomach behind. A cross section of the frog's 

 esophagus shows the four layers that are typical of the wall of 

 the vertebrate alimentary tract. These layers, beginning at the 

 outside, are (1) serous membrane or visceral peritoneum; (2) the 

 tunica muscularis, consisting of an outer longitudinal layer and 

 inner circular layer of plain muscle, the longitudinal layer thin- 

 ning out in the stomach region; (3) the submucosa, composed of 

 connective tissue, nerves, blood and lymphatic vessels; and (4) 

 the mucosa, an epithelium, supported by connective tissue 

 (Fig. 61). Between the submucosa and mucosa of the stomach 



