188 MICROSCOPIC ANATOMY OF THE ORGANS. 



goblet cells which produce mucus. It is probable, however, 

 that the other cells of the glands of Lieberkiihn secrete a, spe- 

 cific substance which is a constituent of the succus entericus. 



The tunica propria consists of a reticular connective tissue 

 which contains a varying number of lymphocytes and other 

 leucocytes. These are in some places collected in masses 

 1-2 mm. in diameter, which lie either singly (solitary 

 follicles) or are grouped together (Peyer's patches or agmin- 

 ated follicles). The solitary follicles are distributed through- 

 out the whole alimentary canal, but are found especially in the 

 intestine. Their development begins always in the tunica 

 propria and extends through the whole mucosa down to the 

 muscularis mucosse. They often cause a bulging on the sur- 

 face of the epithelium, and not infrequently break through the 

 muscularis mucosse to enter the submucosa. The villi and 

 glands usually are distorted in these regions. In the submu- 

 cosa there is a smaller resistance to the growth of the follicle, 

 and it comes therefore to have a flask-like form, with the large 

 end in the submucosa and the neck in the mucosa, The struct- 

 ure of the solitary follicles is similar to that of the follicles of a 

 lymph gland. A germinal centre is always present, and the 

 newly formed lymphocytes proceed from this place out to the 

 periphery of the follicle. There they enter the lymphatics or 

 at the surface escape between the epithelial cells into the lumen 

 of the intestine. 



The Peyer's patches (Fig. 144) are met with in the ileum, 

 more particularly near its junction with the jejunum. These 

 are oval, and sometimes several centimetres in length. They 

 may consist of as many as sixty follicles lying so close to one 

 another that they usually are compressed and deformed. 

 Often adjacent follicles coalesce, so that the follicle thus 

 formed seems to have two or more germinal centres, as is seen 

 in the appendix vermiformis. The follicles reach the surface 

 of the intestine and are covered by the columnar epithelium, 

 but there are seldom found villi immediately on them. 



The submucosa is separated from the tunica propria by the 

 muscularis mucosa3, which is a thin layer of smooth muscle 



