THE SMALL INTESTINE. 163 



The nerves of the stomach are from the vagi and the sympathetic and 

 contain both medullated and nonmedullated fibres, the latter predominating. 

 After forming a subserous plexus beneath the peritoneum, they pierce the 

 external layer of longitudinal 

 muscle and between the latter 

 and the circular muscle broaden ; 

 out and unite into the plexus \ 

 myentericus or plexus of Auci 

 bach. This is an extended 

 network, with rounded angular 

 meshes, whose points of inter- 

 section are occupied by micro- v .. 

 scopic ganglia composed of small ,-x-i. 

 groups of sympathetic cells. 

 From these numerous fibres ' 



pass tO the adjoining sheets of FIG. 202. Surface view of muscular coat of stomach, 



involuntary muscle to terminate ^^^^^s^-^^^^^^r^ph^ 

 in free endings among the fibre- 

 cells (page 86). Other fibres form the intramuscular plexus and pene- 

 trate, as obliquely directed bundles, the intervening muscle to gain the 

 submucous coat. Within the latter they form the submucous plexus or 

 plexus of Meissner, which, while resembling the intramuscular network 

 in its general features, is less pronounced, finer meshed and beset with 

 smaller ganglia. Numerous nonmedullated fibres leave the submucous 

 plexus to enter the overlying tunica propria, in which some end in delicate 

 plexiform threads around the gastric glands and others in fibrils for the 

 muscularis mucosae. Medullated fibres, dendrites of sensory neurones, are 

 also present within the mucosa, where they form a subepithelial plexus after 

 losing their medullary coat. They end in minute varicose threads within 

 the tunica propria; whether some fibrils pass between the epithelial cells is 

 uncertain. 



THE SMALL INTESTINE. 



The small intestine, about 7 meters or 23 ft. in length, is convention- 

 ally divided into three parts the duodenum, the jejunum and the ileum. 

 Although typical portions of these segments can be readily distinguished 

 from one another, chiefly by the modifications of the mucous coat, the 

 transition between them is so gradual that differentiation is in places impos- 

 sible. The small intestine, as other parts of the alimentary tube below the 

 diaphragm, consists of four coats the mucous, the submucous, the muscular 

 and the serous. 



The mucous membrane, or mucosa, presents the greatest variations, 

 since its function as an absorbent surface requires an extent of area most 

 economically provided by folds and projections. These elevations of the 

 mucosa, which include plications and villi, are most marked in the upper . 

 part of the intestine, where absorption is most active, thence gradually 

 decreasing until in the terminal part, where the small intestine passes into 

 the large, they almost disappear. 



The epithelium, everywhere covering the free surface, including the villi, 

 consists of a single layer of columnar cells, whose ends next the intestinal 

 lumen are invested by a narrow and often delicately striated cuticular border. 

 The latter, present only in the fully matured cells, lacks stability and is readily 

 resolved into minute vertical rods, probably continuous with the spongio- 



