168 



THE DIGESTIVE SYSTEM 



The Mammalian Esophagus. — Connecting the pharynx with the 

 stomach is a tube with the characteristic four coats of the digestive 

 canal ah-eady outhned. (Fig. lOG.) In the region of the neck the 

 adventitia consists of loose fibroelastic tissue connecting the esoph- 

 agus with neighboring structures in the neck. As it passes through 

 the thorax this outer coat becomes a serosa (visceral peritoneum), 

 which is composed of an outer thin mesothelial membrane and 

 an underlying thin layer of connective tissue. The muscularis 

 mav have striated \'oluntary muscle fibers in the upper region of 



Fig. 105. — Diagram of a cross-section of the esophagus of a lizard. Note the ruga- 

 like extensions of the submucosa into the lumen. These are covered with stratified 

 columnar epithelium with many goblet cells. The submucosa supports blood vessels. 

 The muscularis coat is represented by a well-defined circular sheath. The longi- 

 tudinal coat is represented by relatively few muscle cells. 



the esophagus, a mixture with smooth muscle in the middle region, 

 and finally a lower portion composed entirely of smooth invohmtary 

 muscle. Its composition is variable, however, for in some animals 

 striated muscle may be found reaching to the stomach. A very 

 loose fibroelastic connective tissue forms the submucosa which unites 

 the muscularis with the mucosa. The submucosa contains large 

 blood vessels and numerous mucous glaufls whose ducts o])en through 

 the mucosa. A muscularis muco.sa marks the beginning of the 

 mucosa, except in the upper })()rtions of the eso])hagus where it 

 may be missing. Within the nuiscularis nuicosa is the fibroelastic 

 tissue of the tunica propria, which projects as papilhe-like extensions 



