50 IMMUNITY IN HEALTH. 



Thus are produced the tonsillar crypts and the adenoid 

 fissures. The irregularity of surface probably also 

 favours the stagnation of mucus and the growth of 

 cultures of micro-organisms, and it helps to prevent the 

 emigrated lymphocytes (see pp. 68 and 69) being swept 

 away from the surface. 



The subepithelial glands develop beneath different 

 kinds of epithelium. The intestinal glands are lined by 

 columnar cells, the faucial tonsils have a covering of 

 stratified epithelium and the nasopharyngeal collec- 

 tions underlie a layer of ciliated columnar cells, or 

 stratified epithelium in parts. 



It may be characterised generally of the larger sub- 

 epithelial glands that — (1) they have a terminal blood 

 supply. Thus Peyer's patches and the appendix are 

 anti-mesenterial ; (2) they have a large surface area ; 

 (3) whether by folding of the surface or by situation 

 along a diverticulum, they are so arranged as to en- 

 courage stagnation of adjacent alimentary contents. 



