LYMPHOID TISSUE, LYMPH -NODULES, AND LYMPH-GLANDS. 197 



form of round nodules, simple follicles or nodules. These are either 

 single, solitary lymph-follicles, or gathered into groups, agminated 

 lymph -nodules. They are found scattered in the mucous mem- 

 brane of the mouth, pharynx, and intestine. In lymph-nodules also 

 we find the characteristic lymph-cells and the adenoid reticulum. 

 As a rule, the former are arranged concentrically at the periphery ; 

 and in the center of the nodule the reticular tissue usually has wider 

 meshes, and the lymph-cells are less densely placed. (Fig. 160.) 

 In the center of the nodule the cells often show numerous mitoses, 

 and it is here that an active proliferation of the cells takes place. 

 The cells may either remain in the lymph-follicle or the newly 

 formed cells are pushed to the periphery of the nodule, and are then 

 swept into the circulation by the slow lymph current which circu- 

 lates between the wide meshes of the reticular connective tissue. 

 Flemming (85, II) has called that central part of the nodule con- 

 taining the proliferating cells the germ center or secondary nodule 

 (compare p. 194). The germ centers are transitory structures, and 

 are consequently found in different stages of development. They 

 may even be absent for a time. 



-Epithelium 



of intes- 

 tine. 

 a 



Fig. 160. A solitary lymph-nodule from the human colon. At a is seen the pronounced 

 concentric arrangement of the lymph-cells. 



The lymph-glands are organs of a more complicated structure, 

 but also consist of lymphoid tissue. They are situated here and there 

 in the course of the lymph-vessel and are widely distributed. Their 

 size varies greatly. In shape they are much like a bean or kidney, 

 and the indentation on one side is known as the hilum. The affer- 

 ent lymph-vessels, the vasa affercntia, enter at the convex surface 

 of the organ, while the efferent vessels, the vasa efferentia, pass out 



