SPLEEN. 163 



determine which cells are inside and which are outside of the vessels (Fig. 

 183) . The nodules are not sharply separated from the pulp, so that lympho- 

 cytes are abundant in their vicinity. These lymphocytes enter the term- 

 inal veins and thus are removed from the spleen. In the splenic vein the 

 proportion of lymphocytes to red corpuscles is said to be seventy times as 

 great as in the splenic artery. One for every four red corpuscles has been 

 reported by tw r o investigators, but later estimates are lower. It seems evi- 

 dent that lymphocyte production is an important function of the spleen. 

 Another is the nitration of the blood passing through the pulp. As in 

 haemolymph glands granular debris is found, and there are phagocytic, 

 pigmented, and eosinophilic cells. The phagocytes are cells with large 

 round nuclei and considerable protoplasm. They vary in size, but the 

 small forms are most numerous; these are called splenic cells. Some are 

 described as multinucleate. Erythroblasts are not found in the normal 

 adult human spleen; in certain blood diseases, however, they occur in it 

 and are normal in some adult mammals, as in the skunk. They are abun- 

 dant in the spleens of human embryos. Giant cells are numerous in the 

 spleens of young animals but are seldom found in the human adult. They 

 are described as megakaryocytes. The formation of granular leucocytes, 

 which has been asserted, presumably does not occur. 



The splenic nodules are quite like the secondary nodules of lymph 

 glands. They consist of a reticular tissue framework continuous with that 

 of the pulp, but having coarser meshes. Fine elastic fibers are associated 

 with it. It contains lymphocytes, and near the central arteries germinative 

 centers are sometimes Distinct. The nodules have been regarded as vary- 

 ing in shape from time to time, being but transient accumulations of lym- 

 phocytes. 



The capsule of the spleen is divided into two layers. The outer is 

 the tunica serosa and the inner, the tunica albuginea. The serosa consists 

 of the peritoneal mesothelium which covers the spleen except around its 

 hilus, and of the underlying connective tissue. The albuginea is a dense 

 layer of connective tissue, containing elastic networks and smooth muscle 

 fibers. Similar tissue is found in the trabeculae. The muscle elements 

 are less numerous in the human spleen than in those of many animals. 

 By contraction they force blood from the pulp and cause the circulation to 

 follow more definite channels. When they are paralyzed the pulp becomes 

 filled with the blood corpuscles. 



The nerves of the spleen, from the right vagus and the coeliac sym- 

 pathetic plexus, are medullated and non-medullated fibers, chiefly the 

 latter. They form plexuses around the blood vessels (Fig. 184) and send 

 fibers into the pulp. Besides supplying the muscles of the vessels and 



