CHAPTER VIII. 



THE LYMPHATIC SYSTEM. 



The lymph vessels develop in a manner similar to, if not as 

 outgrowths of the embryonic venous system. In either case a 

 system of closed endothelial tu})es is formed and at intervals develop 

 vahes, as in the case of the veins. According to Drinker and 

 Field, "the lymph capillaries are complete vessels, whose content 

 is identical with the fluid outside them. The barrier presented 

 by their walls is extremely slight. It serves to guide their contents 

 into hea\ier walled channels from which escape is difficult, and by 

 means of which return to the blood is accomplished. Caj)illary 

 lymph and tissue Huid are thus considered to exist in a conunon 

 reservoir, and to this the blood cai)illaries make addition of fluid 

 and by resorption withdraw it." In the formation of the larger 

 vessels, mesenchyme cells surrounding the endothelium develop a 

 connective-tissue wall in which some smooth muscle also differen- 

 tiates. 



The smallest })lindly ending divisions of these vessels, the lymph 

 capillaries, collect fluid from spaces between the fibers and cells of 

 connective tissue. This fluid passes from the capillaries into the 

 smallest lymph \'essels, thence into larger and larger branches, w^hich 

 ultimately join the venous system in the region of the heart. Lymph 

 resembles the blood in having a plasma containing free cells, the 

 lymphocytes, but lacks erythrocytes and granulocytes. 



At intervals along the lymphatic system, active mesenchymal 

 elements develop into lymphoid organs in which lymphocytes are 

 produced and are added to the colorless, fluid h-mph. Lymphoid 

 organizations are especially well developed in mammals where 

 lymphocyte production is carried on apart from the production of 

 other myeloid cells. Among birds and reptiles, and in the anijihih- 

 ians to a lesser extent, lymphocytes may be formed in lymphoid 

 tissue scattered in ^'arious parts of the body, but the spleen is the 

 major lymph organ in these forms. In mannnals, particularly, 

 there is a Aariety of such organizations, all characterized by the 

 presence of one or more lymph nodules. 



