SUMMARY 105 



-caecal apex is small. The lymphoid tissue then thickens 

 the wall of this tubular mechanism. The maximum 

 amount of lymphoid tissue is associated with the 

 tubular type and occurs in rodents where the caecum is 

 of immense size and serves for the bacterial decomposi- 

 tion of cellulose. 

 Anatomical : — 



(1) Position of the subepithelial glands. The lymph 

 nodules are placed directly beneath the epithelium. 

 They are superficial even to the fibres of the muscularis 

 mucosae. 



(2) Structure of the subepithelial glands. Where the 

 follicles are crowded together the overlying epithelium 

 is folded into pits and recesses so that each lymph 

 nodule subtends the surface. This is well seen in the 

 nasopharyngeal and the faucial tonsils. There are no 

 afferent lymphatic vessels. 



(3) Distribution of the subepithelial glands. They 

 do not underlie the skin which is thick, resistant, and 

 comparatively free from pyogenic organisms on the 

 surface, but they are found in the throat and along the 

 alimentary tract, where the mucous membrane is thin, 

 permitting of absorption. Moreover they occur exactly 

 at those points where the micro-organisms flourish. 

 The circumpharyngeal ring meets the bacteria entering 

 with the food (and with the inspired air in the case of 

 mouth-breathers). The Peyer's patches, the appendix, 

 and the solitary follicles are placed around the excellent 

 incubating media in the ileum, caecum, and ascending 

 •colon, precisely those parts which are swarming with 

 organisms. The stomach, on the other hand, protected 

 from bacterial growth in its contents by the acid nature 

 of its secretion, is almost free from lymphatic tissue, 

 €xcept towards the pylorus. 



The whole anatomical arrangement of the subepi- 

 thelial lymphatic glands is such as to bring them into 



