THE SPLEEN 



163 



placed by an investment of adenoid tissue. In the spleen pulp of 

 some animals the adenoid tissue forms a complete investment of 

 considerable thickness with occasional slender fusiform enlarge- 

 ments. In other mammals and in man this sheath is incomplete, 

 but here and there forms ovoid accumulations of lymphoid tissue,, 

 the Malpighian corpuscles, which inclose the arterial twigs. These 



FIG. 159. THREE MALPIGHIAN CORPUSCLES OF THE HUMAN SPLEEN. 



Each lymphoid nodule contains one or more arterioles ; a branch of one of these is seen 

 in longitudinal section. Hcmatein and eosin. Photo, x 338. 



lymphatic follicles are eccentrically placed as regards the artery, 

 and are most frequently situated at the bifurcations of the vessel. 

 They differ from the lymphatic follicles of other organs in that 

 they frequently contain no germinal center, and invariably possess 

 one or more small arteries with distinct muscular walls, which are 

 rarely ever situated in the axis of the corpuscle. The Malpighian 

 corpuscle is therefore characteristic of the spleen. 



