ALIMENTATION 



97 



median and two lateral pouches, produces during breeding 

 season a thick fluid, called pigeon's milk, that is fed to the young. 

 The human esophagus is a tube of uniform diameter lined with 

 a stratified epithelium which receives the openings of two types 

 of glands, producing secretions having the properties of mucin 

 and serving apparently for lubrication. The esophagus joins 

 the stomach at the cardiac aperture (Fig. 63). 



Fig. 64. — Stomach region of the domestic fowl, cut open. D, duodenum; 

 G, gizzard, showing thickness of muscular wall; L, cornified lining of gizzard; 

 deeply creased; O, esophagus; P, proventriculus, showing rounded elevations of 

 its soft glandular lining. 



Stomach. — The stomach of fishes and lower vertebrates is 

 generally little more than a dilatation in the digestive tract, 

 serving as a food reservoir in which a certain amount of digestion 

 may take place. The stomach of the frog is widest at its anterior 

 (cardiac) end, where it bends slightly to the left and then con- 

 tinues to the right in almost a straight line to the narrow pyloric 

 end. Its union with the duodenum, at the pylorus, is marked by 

 a well-defined constriction, where the circular layer of the mus- 



