346 



THE DIGESTIVE SYSTEM 



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 abdomen the homologous subserous 

 connective tissue is covered by a layer of mesothelium. The connective 

 tissue of the outer fibroserous coat contains the larger blood and lym- 

 phatic vessels whose branches are distributed to the three inner coats. 

 The Muscular Coat. The muscular coat, situated next within the 

 fibroserous, is divisible into two layers, an outer longitudinal the direc- 



tion of whose fibers is parallel 

 to the long axis of the canal, 

 and an- inner transverse layer 

 whose fibers are circularly dis- 

 posed. 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 con- 

 tains a coarse-meshed nerve 

 plexus, consisting of small an- 

 astomosing nerve trunks which 

 are composed in large part of 



non-medullated fibers, at whose intersections are numerous small sym- 

 pathetic ganglia, the myenteric (Auerbach's) plexus and ganglion. 



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

 esophagus, and including the musculature of the stomach and intestines, 

 the muscle is entirely of the non-striated or smooth variety. In the 

 pharynx and upper third of the esophagus, the striated or voluntary 

 type of muscle is exclusively found. In the mid-portion of the esophagus 

 both striated and non-striated muscle occur in varying proportions, oc- 

 casional striated fibers being found even in the lower third of the 

 organ. 



The Submucous Coat. The submucous coat consists of loose 

 areolar tissue, and serves for the support of the larger blood and 

 lymphatic vessels which supply this coat and the mucosa. A second 

 plexus of nerve fibers, similar in structure to the intramuscular plexus, 

 is found in the deeper layers of the submucosa, and is known as the 



FIG. 325. SURFACE VIEW OF AUERBACH'S 

 INTRAMUSCULAR NERVE PLEXUS, FROM THE 

 ESOPHAGUS OF A CAT. 



Methylene blue. X 40 to 50. (After DeWitt.) 



