SMALL INTESTINE 



269 



opposite the mesentery. A few occur in the jejunum and the distal part 

 of the duodenum. In the vermiform process, diffuse aggregate nodules are 

 always present, but they do not occur elsewhere in the large intestine. 



Nerves. The small intestine is supplied by prolongations of the vagus 

 nerves, which are joined by branches of the superior mesenteric plexus 

 of the sympathetic system. The latter are regarded as the principal 

 supply. This plexus is ventral to the aorta, and sends branches through 

 the mesentery into the serosa. The manner in which they penetrate the 

 other layers, forming the myenteric plexus (Auerbach's plexus) between 

 the circular and longitudinal muscle-layers, and the submucous plexus 

 (Meissner's plexus) in the submucosa, is shown in Fig. 261, C. In surface 

 view, obtained by stripping the layers apart, these plexuses are seen in 

 Fig. 264. Their branches supply the smooth muscle fibers. From the 

 submucous plexus the nerves extend into the villi, where nerve cells have 

 been detected by the Golgi method (Fig. 261, C); it has been suspected, 

 however, that some of these "nerve cells" are portions of the reticular 

 tissue. The nerve fibers probably terminate in contact with epithelial 

 cells and provide for local reflex action, whereby the muscles contract in 

 response to stimulation of the epithelium. Most of the intestinal nerves 

 are non-medullated, but they include a few large medullated fibers said 

 to have free endings in the epithelium. 



MESENTERY AND PERITONEUM. 



The serous membrane which surrounds the intestinal tube and certain 

 other abdominal viscera is a part of the lining of the body cavity. Its 

 general relations are shown in the diagram, Fig. 

 265. After covering the ventral surface and the 

 sides of the intestinal tube, the two layers of serous 

 membrane come together to form the mesentery 

 and extend to the dorsal body wall; then, separat- 

 ing, they pass laterally as the lining of the abdom- 

 inal walls and again come together in the mid- 

 ventral line. This serous membrane, or periton- 

 eum, consequently forms a closed sac. It is divis- 

 ible into the visceral peritoneum which covers the 

 viscera, and parietal peritoneum which lines the 

 body walls. In all cases its free surface is covered 

 with a single layer of flat polygonal cells, resem- 

 bling endothelium (Fig. 266, B). Although quite 

 flat, the cells have a thin cuticular border which 



is said to be striated, and the cuticulae of adjacent cells fit together closely. 

 The lateral walls of these flat cells are connected with one another by proto- 



FIG. 265. DIAGRAM OF A 

 MESENTERY AS SEEN IN 

 CROSS SECTION OF THE 

 ABDOMEN. (After Minot.) 



a., Aorta; c. p., cavity of the 

 peritoneum; int., intes- 

 tine; mes., mesentery; p. 

 m. and v. m., parietal and 

 visceral layers of meso- 

 thelium. 



