266 



HISTOLOGY 



tissue. The intestine is covered externally by the tunica serosa. The 

 distribution of the vessels and nerves in these layers is as follows. 



Blood vessels. The arteries pass from the mesentery into the serosa, 

 in which their main branches tend to encircle the intestine. Smaller 

 branches from these pass across the muscle layers to the submucosa, in 

 which they subdivide freely (Fig. 261, A). In crossing the muscle layers 

 they send out branches in the intermuscular connective tissue. These 

 and the arteries of the serosa and submucosa supply the capillary networks 

 found among the muscle fibers. The capillaries are mostly parallel with 

 the muscles. From the submucosa the arteries invade the mucosa, form- 



A B C 



FIG. 261. 



A f Diagram of the blood vessels of the small intestine; the arteries appear as coarse black lines; the cap- 

 illaries as fine ones, and the veins are shaded (after Mall). B, Diagram of the lymphatic vessels (after 

 Mall). C, Diagram of the nerves, based upon Golgi preparations (after Cajal). The layers of the 

 intestine are m., mucosa; m. m., muscularis mucosae; s. m., submucosa; c. m., circular muscle; i. c., 

 intermuscular connective tissue; 1. m., longitudinal muscle; s., serosa. c. 1., central lymphatic; n., 

 nodule; s. pi., submucous plexus; m. pi., myenteric plexus. 



ing an irregular capillary network about the glands, and sending larger 

 terminal branches into the villi. There is usually a single artery for a 

 villus, and it has been described as near the center, with the veins at the 

 periphery (Fig. 261), or sometimes on one side of the villus with the vein 

 on the other. The network of blood vessels in the villi is very abundant 

 as shown in Fig. 262. The veins branch freely in the submucosa and pass 

 out of the intestine beside the arteries. The muscularis mucosae has been 

 described as forming a sphincter for the veins which penetrate it; thus it 

 may control the amount of blood within the villi. No valves occur 

 until the veins enter the tunica muscularis; there they appear, and con- 

 tinue into the collecting veins in the mesentery. They are absent from 



