3o8 



COMPARATIVE ANATOMY 



the pharynx. During this change, its lumen becomes occluded tem- 

 porarily, a new lumen arising secondarily through the union of a number 

 of vacuolar spaces. Ciliated epithelial cells occur in the esophagus until 

 relatively late (loth week) stages of development. 



History of the Esophagus. There is little to distinguish the esophagus 

 of a fish from its stomach, except the relative scarcity of glands, and the 

 fact that its muscle fibers, Hke those of the pharynx, are striated, while 



EPITHELIUM 



rUNICA PROPRIA 



MUSCULARIS MUCOSAE 



MUCOUS GLAND 

 SUBMUCOSA 





•IJl-^'-t^ .tJ%" ^tS/*? .'CIRCULAR MUSCLES 



LONGITUDINAL 

 MUSCLES 



i ADVENTITIA 



VAGUS NERVE 



Fig. 258. — The esophagus as seen in cross section. A is a section of the entire 

 esophagus. 5 is a small portion much enlarged. The five layers of tissue characteristic 

 of the entire alimentary canal are found in the esophagus. (Redrawn after Braus.) 



those of the stomach are smooth. In amphibia, the esophagus becomes 

 slightly elongated. Its considerable elongation in reptiles and mammals 

 is correlated with the elongation of the neck. In these groups, it becomes 

 constricted in diameter and most of its muscle fibers become smooth. 



The Stomach 



The stomach, lying between the esophagus and small intestine, is the 

 most expanded part of the alimentary canal. Its shape in man varies 

 greatly, depending upon the quantity of food contained. The human 

 stomach lies almost transversely across the abdominal cavity with a 



