268 



HISTOLOGY 



tion of these lymphatics is very great and their endothelium is easily 

 seen in sections. When collapsed they are hard to distinguish from the 

 surrounding reticulum. Small lateral branches and a spiral prolongation 

 of the central lymphatic have been found by injection, but these may be 

 tissue spaces into which the injected fluid has been forced. The lym- 

 phatics branch freely in the submucosa and have numerous valves. They 

 cross the muscle layers, spreading in the intermuscular tissue and the serosa, 

 and pass through the mesentery to the thoracic duct. 



Lymphoid tissue. The lymphoid tissue of the intestine occurs pri- 

 marily in the tunica propria, and in three forms diffuse lymphoid tissue, 

 solitary nodules, and aggregate nodules. Solitary nodules are seen in Fig. 

 254. The nodules are surrounded by small vessels, the lymphatics being 



A B 



FIG. 264. 



A, Surface view of the plexus myentericus of an infant. X 50. g. Groups of nerve cells; r, layer of cir- 

 cular muscle fibers recognized by their rod-shaped nuclei. B, Surface view of the plexus submucosus 

 of the same infant. X So. g, Groups of nerve cells; b, blood vessel visible through the overlying 

 tissue. 



drawn in Fig. 261, B. Blood vessels may make a similar net, and pene- 

 trate the outer portion of the nodule. The germinative centers are simi- 

 lar to those in the lymph glands. 



Aggregate nodules (Peyer's patches) are oval areas, usually from i to 

 4 cm. long but occasionally much larger, composed of from ten to sixty 

 nodules in close contact (Fig. 263). The nodules may be distinct or 

 blended in a single mass. They distort the intestinal glands with which 

 they are in relation, and immediately above the nodules the villi are partly 

 or wholly obliterated. Thus they appear as dull patches in the lining 

 of the freshly opened intestine, and may be readily seen. There are from 

 fifteen to thirty of them in the human intestine (rarely as many as fifty 

 or sixty) and they occur chiefly in the lower part or the ileum on the side 



