FOOD AND DIGESTION. 375 



replacing the epithelium, as is also the case with some of the lymphoid 

 follicles of the tonsil. 



Peyer's glands are surrounded by lymphatic sinuses which do not 

 penetrate into their interior; tho interior is, however, traversed by a 

 very rich blood capillary plexus. If the vermiform appendix of a rab- 

 bit, which consists largely of Peyer's glands, be injected with blue by 

 pressing the point of a fine syringe into one of the lymphatic sinuses, 

 the Peyer's glands will appear as grayish white spacer surrounded by 

 blue; if now the arteries of the same be injected with red, the grayish 

 patches will change to red, thus proving that they are surrounded by 

 lymphatic spaces but penetrated by blood-vessels. The lacteals passing 

 out of the villi communicate with the lymph sinuses round Peyer's 

 glands. It is to be noted that Peyer's patches are largest and most 

 prominent in children and young persons. 



Vilh.ThQ Villi (figs. 260, 262, and 263) are confined exclusively to 

 the mucous membrane of the small intestine. They are minute vascu- 

 lar processes, from a line ^g to -J of an inch (.5 to 3 mm.) in length, 

 covering the surface of the mucous membrane, and giving it a peculiar 

 velvety, fleecy appearance. Krause estimates them at fifty to ninety 

 in number in a square line at the upper part of the small intestine, and 



Ffe. 261. Agminate follicles, or Peyer's patch, in the state of distention. x 5. (Boehm.) 



at forty to seventy in the same area at the lower part. They vary in 

 form even in the same animal, and differ according as the lymphatic 

 vessels or lacteals which they contain are empty or full; being usually, 

 in the former case, flat and pointed at their summits, in the latter cylin- 

 drical or clavate. 



Each villus consists of a small projection of mucous membrane; its 

 interior is supported throughout by fine adenoid tissue, which forms 

 the framework or stroma in which the other constituents are contained. 



The surface of the villus is clothed by columnar epithelium, which. 

 rests on a fine basement membrane; while within this are found, reck- 

 oning from without inward, blood-vessels, fibres of the muscularis mu- 



