THE SPLEEN 127 



Lower vertebrates do not show such marked lyini)h()id or^jjaiiiza- 

 tion. In the frog there are scattered and ^•ery small oval or spJierical 

 bodies composed of accumulations of lym])hocytes in a network of 

 connective tissue surrounded by a fibroelastic cai)sule. Fine 

 trabecuke may dixidc tlie structure, but medulla and cortex are not 

 differentiated. Presumably the lym])hocytes ])roduced here enter 

 lymph capillaries su])])orted in the connective tissue of the organ. 



THE HEMOLYMPH GLAND. 



Although the lym])h node appears limited to mammals, somewhat 

 modified structures, called hemolym])h nodes or glands, are found 

 in mannnals and birds. In these lymphoid organizations, which 

 structurally resemble the lymph nodes, blood instead of lymph is 

 distributed in the cortex from arterial capillaries and filters through 

 sinuses to be collected by efferent veins. Lymph vessels are 

 limited to the scanty trabecular tissue. 



THE SPLEEN. 



The spleen is the largest single lymph gland. It is similar to 

 hemal nodes, which are often called accessory spleens, in being 

 associated with the blood-vascular system instead of lymphatics. 

 Its shape and location varies in different vertebrates; among the 

 fishes it may be an irregular mass closely associated with the stomach 

 and intestinal wall; in others, as in the frog, it occurs as a spherical 

 mass in the mesentery supporting the intestine; among the mam- 

 mals it is a flattened structure near the stomach. . 



The spleen is surrounded with a capsule of fibroelastic connective 

 tissue and smooth muscle covered with mesothelium, but there is 

 no division into a distinct cortex and medulla. Trabecula? of con- 

 nective tissue and smooth muscle extend from the capsule to form 

 a coarse internal framework. A reticulum like that in the nodes 

 extends between the trabeculse and forms the inner supjiort for the 

 lymphoid tissue. The larger arteries and veins supplying the spleen 

 are carried by the trabeculse. 



Examination of slices of fresh mammalian or avian spleen re\eals 

 small, whitish masses, the white pulp, distributed throughout a red 

 tissue. Each white pulp mass is a splenic nodule (splenic corpuscle 

 or Mal])ighian corpuscle) which is composed of a mass of lympho- 

 cytes enmeshed in a reticular network, and is com])arable to a 

 solitary lymph nodule or to a cortical nodule of a lymph node. 



