THE ALIMENTARY CANAL 167 



The innermost coat, the mucous membrane or the tunica nnicosa, 

 is com])()se(l of the hnint:; e])itlielium resting on a l)asement mem- 

 brane l)eio\v which is fibroeiastic connective tissue called the lamina 

 proi)ria or tunica ])roi)ria. A thin, double layer of smooth muscle, 

 the muscularis mucosae, is generally i)resent and nuirks the external 

 boundary of the mucosa. 



Adjoining the mucosa there is a region of looser fibroeiastic 

 connective tissue, the tunica submucosa, rich in blood vessels and 

 nerves. The symi)athetic ner\-e plexus carried in this region is 

 called Meissner's or the submucosal ])lexus. Surrounding the sub- 

 mucosa externally there is a coat of muscle tissue, the tunica mus- 

 cularis, consisting generally of an inner circular layer and an outer 

 longitudinal layer separated by a narrow region of connective tissue 

 which carries a sympathetic nerve plexus called Auerbach's jjlexus 

 or the myenteric plexus. The outermost coat of the wall, the 

 tunica adventitia, is composed of fibroeiastic connective tissue. 

 This structural plan shows greatest variation in the mucosa of the 

 different regions, es])ecially as regards the glandular development. 



The Esophagus. — 77?f Ampldbian Esophagus.— This part of the 

 digestive tract has the typical four coats. The submucosa extends 

 into a number of long folds carrying the mucosa into the lumen. 

 The epithelium of the mucosa is usually of the stratified ciliated 

 columnar type with niunerous mucous cells, although the esophagus 

 of Amblystoma has a mucosal epithelium which resembles the 

 pseudostratified variety, and in Cryptobranchus it is more nearly 

 stratified cuboidal. The muscularis coat is usually represented by 

 a prominent circular sheath, but the longitudinal muscle sheath 

 is in many cases represented by separate bundles. The ad^'entitia 

 connects externally with the skeletal muscles of the neck region. 

 In the submucosa of certain portions of the frog esophagus large 

 compound serous glands occur. 



The Reptilian Esophagus.— A study of the esophagus of the 

 lizard (Fig. 105), snake, turtle, and alligator demonstrates that the 

 submucosal folds extending into the lumen are i)resent here also. 

 The mucosal epithelium is in general of the stratified columnar 

 variety. The columnar zone cells ma\^ be ciliated and there may 

 be many goblet cells present. The muscularis coat is represented 

 by a distinct circular coat aufl a longitudinal coat formed by 

 separate strands of smooth muscle cells. The adventitia is of 

 loose connective tissue which sujjports })lood vessels, nerves, and 

 lymphatics. 



