CHAPTEK XVI 

 THE DIGESTIVE SYSTEM (Continued) 



. THE ALIMENTARY TRACT 



IT is convenient to consider collectively under this head the 

 pharynx, esophagus, stomach, and the small and large intestines. 

 This tract forms a continuous tube whose wall has, throughout its 

 entire extent, many common characteristics. Thus the wall in all 

 portions consists of four coats which are respectively known, from 

 within outward, as the mucous, submucous, muscular, and fibro- 

 serous. The three outermost coats are of very similar structure 

 in all portions of the tract. 



The Fibro-serous Coat. In the abdominal cavity the outermost 

 coat is derived from the peritoneum, by which the stomach and 

 intestines are invested. In the upper portion of the tract, pharynx 

 and esophagus, the serous coat is replaced by a layer of areolar 

 connective tissue which usually contains much fat. In the abdo- 

 men the homologous subserous connective tissue is covered by a 

 layer of endothelium. The connective tissue of the outer fibro- 

 serous coat contains the larger blood and lymphatic vessels whose 

 branches are distributed to the three inner coats. 



The Muscular Coat, situated next within the fibro-serous, is 

 divisible into two layers, an outer longitudinal the direction of 

 whose fibres is parallel to the long axis of the tract, and an inner 

 transverse layer whose fibres are circularly disposed. The two 

 layers are united by a thin septum of areolar connective tissue 

 which serves for the support of the larger blood vessels and lym- 

 phatics, whose capillaries are distributed to the muscular coat. 

 This septum also contains a coarse-meshed nerve plexus, consisting 

 of small anastomosing nerve trunks which are composed in large 

 part of non-medullated fibres, at whose intersections are numer- 

 ous small sympathetic ganglia. This is the nerve plexus of 

 Auerbach. 



Below the level of the junction of the middle and lower third 



271 



