THE LYMPHATIC SYSTEM. 



939 



Cross-section of small lymph-node, injected to show rich vascular 



supply. X 10. 



within the node are destitute of such folds. The passage of the lymph through the 

 nodes is retarded by the reticulum' within the sinuses, thus favoring the entrance ot 

 the young lymphocytes from the surrounding lymphoid, tissue into the sluggishly 

 circulating fluid. Germ-centres, the particular foci for the production of the lympho- 

 cytes, usually are present within the cortical nodules, but are nqt found within 

 medullary cords. 



The blood-vessels for the nutrition of the lymph-nodes are numerous. Entering 

 at the hilum , they divide into arterioles which follow the trabeculae, giving of! smaller 

 branches that pene- 

 trate the medullary Fig. 792. 

 cords and the cortical 

 nodules and break up 

 into rich capillary net- 

 works for the supply 

 of the denser lym- 

 phoid tissue. 



Both meduUated 

 and non-meduUated 

 nerves enter the 

 node at the hilum in 

 company with the 

 blood-vessels. They 

 are chiefly sympa- 

 thetic fibres destined 

 for the involuntary 

 muscle of the vessels 

 and of the capsule. 

 The distribution of the 

 meduUated fibres is 



uncertain. According to Tonkof?, tibrillae are traceable into the lymphatic tissue of 

 the medulla. 



Development. — The origin of the first lymph-cells, the lymphocytes, is uncer- 

 tain, these elements appearing outside the vessel's as derivatives from the mesoblast 

 (page 688). After the establishment of the lymphoid tissue new cells are continually 

 being formed within the various lymph-nodes and nodules. 



The development of the lymphatic vessels has generally been believed to 

 proceed from the veins by a process of budding (Ranvier), similar to that followed 

 in the extension of the blood-vessels; and certain recent investigators, — Sabin,' 

 who studied the development of the lymphatics in pig embryos, and F. T.- Lewis,' 

 who worked with rabbit embryos, — while differing as to details of the development 

 of the definitive lymphatic stems, agree as regards their origin in this manner. 



Sabin, by employing a method of injection, found that the first traces of a 

 lymphatic system appear in pig embryos, 14.5 mm. in length, as two small out- 

 growths, which develop, one on each side, at the junction of the subclavian and 

 jugular veins ; from these, by a process of endothelial budding, vessels gradually 

 grow towards the skin, radiating and anastomosing in all directions to form a 

 subcutaneous net-work, which gradually extends throughout the anterior half of the 

 body. Later two additional outgrowths develop at the junction of the femoral and 

 post-cardinal veins, and give rise to a subcutaneous net-work throughout the posterior 

 half of the body, the two sets of net-works thus formed eventually uniting. 



Lewis's studies of serial sections of rabbit embryos gave somewhat different 

 results and indicated that Sabin' s method of study did not suffice to reveal the actual 

 origin of the lymphatics. He found the first of these vessels along the course of the 

 internal jugular vein as a series of spaces, each of which he supposed to represent an 

 independent outgrowth from the vein. These spaces eventuallv fused to form a 

 single lymph-channel accompanying each vein, and other channels were found to 

 arise in a similar manner in connection with the subcardinal, mesenteric, and azygos 



' Amer. Jour, of Anatomy, vol. i., 1902. 

 ^ Amer. Jour, of Anatomy, vol. v., 1905. 



