242 THE LYMPHATIC SYSTEM 



tissue its branches form a capillary plexus whose vessels open into the 

 blood sinuses. All the sinuses, peripheral and secondary, communicate 

 with each other, and from them the blood is ultimately collected into 

 two or more thin-walled veins. In the center of the gland these vessels 

 unite to form an efferent vein which passes out at the hilum. 



DEVELOPMENT OF LYMPH NODES 



Lymph nodes arise through the invasion of primary lymphatic capillary 

 plexuses by lymphocytes. The first lymph nodes arise in the regions of the 

 axilla and groin during the third month of development. Such areas be- 

 come circumscribed by the development of a capsule from the surrounding 

 mesenchyma. The capsular tissue is continued into the developing node 

 in the form of trabeculse, terminating in a dense network of delicate reticu- 

 lar fibers. Hydrostatic conditions probably determine the formation of a 

 peripheral lymph sinus. The retention of certain channels (internodular 

 and. medullary sinuses) between the peripheral sinus and the efferent lym- 

 phatics at the hilum is likewise probably determined mainly by the opera- 

 tion of like factors, brought into play through the appearance of cortical 

 nodules. These nodules arise as regions of proliferative activity of lympho- 

 cytes. The node has meanwhile early become invaded at a point which 

 becomes the hilum by a vascular and nerve supply. Nodules arise as ac- 

 cumulations of proliferating lymphocytes about the cortical arterial twigs. 

 Hemolymph nodes apparently arise in a manner similar to the origin of 

 ordinary lymph nodes, and become only secondarily modified. The reticu- 

 lar tissue of lymph nodes may in part arise from the capillary endotbelium. 



The function of lymph nodes is the production of lymphocytes, which 

 become phagocytic leukocytes. Besides having a leukopoietic role, lymph 

 nodes probably function also as centers for the dissolution of worn-out 

 blood elements, in which process phagocytosis predominates, the lympho- 

 cytes being in part assisted by the endothelial cells of the capillaries. 

 Lympboid aggregations also serve as 'lymph filters,' the phagocytes removing 

 from the lymph bacteria and other noxious products. 



THE TONSILS 



The Faucial Tonsils (Palatine Tonsils; Amygdala) .The tonsils 

 consist of a mass of lymphoid tissue which projects slightly from either 

 side into the cavity of the fauces, and is covered by a layer of stratified 

 epithelium continuous with that which lines the oral and pharyngeal 



