CHAP. XII.] THE VASCULAR SYSTEM. 149 



capillary network, from which the blood is carried away by one 

 or more small veins. Surrounding the mass are lymph channels 

 which are continuous with the lymphatic vessels in the tissue 

 below. 



A Peyer's patch, or " agminated gland," as it is often called, is 

 simply a collection of these follicles. A well-formed Peyer's 

 patch consists of fifty or more of these solitary follicles, ar- 

 ranged in a single layer, close under the epithelium of the 

 intestinal mucous membrane, and stretching well down into 

 the tissue beneath. These patches are circular or oval in shape, 

 and from twenty to thirty in number. They are largest and 

 most numerous in the portion of the intestine called the ileurn. 

 They increase in size during digestion. 



The tonsils are two thick masses of lymphoid tissue, placed 

 one on each side of the fauces or throat, into which they pro- 

 ject. They are covered by stratified epithelium, and their sur- 

 faces are pitted with apertures which lead into recesses or crypts 

 in the substance of the tissue. 



The spleen. The spleen differs in many important particu- 

 lars from lymphatic glands, but may be conveniently studied in 

 conjunction with them, as it resembles these glands in structure, 

 arid is possibly connected functionally with the blood. 



Like the lymphatic glands, the spleen is covered by a fibrous 

 and muscular capsule which sends fibrous bands to form a net- 

 work in the interior of the organ. In the meshes of the fibrous 

 framework lies a soft pulpy substance containing a large 

 amount of blood, and, therefore, of a deep red colour. This 

 soft, red pulp is dotted with whitish specks, which are small 

 masses of lymphoid tissue, and are called the Malpighian cor- 

 puscles of the spleen. 



The blood supplied to the spleen appears to escape from the 

 minute subdivisions of the arteries into the red pulp before 

 entering the exceedingly thin-walled veins by which it is con- 

 veyed from the gland. The pulp contains numerous red cor- 

 puscles, and many bodies which appear to be red corpuscles in 

 process of decay or destruction, and it is surmised that the red 

 corpuscles are in some way destroyed, and that additional white 

 corpuscles are formed, within the spleen. 



The spleen is covered by a portion of the peritoneum, the 

 serous membrane covering the viscera of the abdomen, and 



