638 



Comparative Morphology of Chordates 



Part of an excretory duct. 



A crescent consisting of 

 eight serous cells. 



Tangential 

 section of serous 

 cells. 



Mucous cells and 



thick membrana 



propria . 



Connective 

 tissue. 



Fig. 484. Section of a human sublingual gland. (X252.) (Courtesy, Bremer: 

 "Text-Book of Histology," Philadelphia, The Blakiston Company.) 



Pharynx and Esophagus 



The peculiarities of the mammalian pharynx have been described 

 in connection with the account of the respiratory passages (p. 610). 

 The esophagus varies in length according to that of the neck. It is 

 relatively narrow in mammals, consistent with the fact that they, in 

 contrast to most other vertebrates, ordinarily swallow food in small 

 portions. The cat ingests a mouse in numerous morsels, but so small a 

 snake as a copperhead swallows a mouse, or even a rat, entire, and a 

 python may swallow a whole pig. Such habits require a wide, or at 

 least a highly distensible, gullet. 



Stomach 



The stomach exhibits many varieties of form. In most mammals 

 it is a simple, more or less elongated, saclike enlargement of the 

 digestive tube, the cardiac region of it (next to the esophagus) being 

 of greater diameter than the pyloric region. The long axis of the 

 stomach commonly extends more or less nearly transversely to the 

 long axis of the body, the cardiac end being at the left. In many mam- 

 mals the wall of the stomach acquires constrictions which divide the 

 organ into two, three, or even four successive communicating compart- 

 ments. This complication of the stomach is most marked in cetaceans 

 and the cud-chewing (ruminant) ungulates. In the porpoise and most 

 whales the stomach is divided into three or four major chambers, and 



