214 



HISTOLOGY. 



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

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

 Figs. 232 and 244. The latter shows how the nodule which arises in the 

 propria may extend through the muscularis mucosae and spread in the 

 submucosa, thus being as a whole, flask shaped or pyriform. A per- 

 ipheral section of such a nodule may present only the part beneath the mus- 

 cularis mucosae. The nodules are surrounded by small vessels, the lym- 

 phatics being drawn in Fig. 239, B. Blood vessels may make a similar 

 net, and penetrate the outer portion of the nodule. The germinative cen- 

 ters are similar to those in the lymph glands. 



FIG. 242. 



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

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

 mucosus of the same infant. X 50. g, Groups of nerve cells; b, blood vessel visible through the 

 overlying tissue. 



Aggregate nodules [Peyer's patches] are oval macroscopic areas, 

 usually from i to 4 cms. long but occasionally much larger, composed of 

 from 10 to 60 nodules placed side by side (Fig. 241). The nodules may 

 be distinct or blended by intervening lymphoid tissue. 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. There are 

 from 15 to 30 of them in the human intestine (rarely as many as 50 or 60) 

 and they occur chiefly in the lower part of the ileum on the side opposite 

 the mesentery. A few occur in the jejunum and the distal part of the duo- 

 denum. In the vermiform process diffuse aggregate nodules are always 

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



