198 NORMAL HISTOLOGY AND ORGANOGRAPHY. 



Villus. 



of lymph tissue situated just beneath the mucous 

 epithelium. They are found along the whole ali- 

 mentary tract and in all mucous membranes. 



Peyer's Patches, or Agminated Lymph Nodules. 

 These appear as oval elevations on the mucous sur- 

 face and are collections of lymph nodules. They 

 may be three inches long or less, and one-third to 

 one-half inch broad. They number from thirty to 

 forty, and are found in the ileum and always in the 



mucous surface op- 

 posite the mesen- 

 teric attachment, 

 the long axis being 

 parallel with that 

 of the intestine. 

 Their bodies usu- 

 ally extend to the 

 circular muscle 

 layer and they 

 therefore invade 

 the submucosa. 

 Villi are either 

 stunted or altogeth- 

 er absent over these 

 patches. In early 

 youth they are very 

 prominent, while in 

 middle life they be- 

 gin to atrophy, and 

 in old age they may entirely disappear. 



The patches are the seat of ulcerations, particu- 

 larly in typhoid fever and tuberculosis. The latter 

 form transverse ulcers as the bacteria of tuberculosis 



Fig. 147. Longitudinal section of ileum 

 showing part of a Peyer's patch. 



