CHAPTER XIV 

 LYMPH FORMATION AND CIRCULATION 



GENERAL CONSIDERATIONS 



Lymphatics are modified veins. They grow from the veins in embry- 

 onic life as buds of endothelium, which are first visible in the human 

 embryo in the sixth week of development. The earliest outgrowth oc- 

 curs from the internal jugular vein, and the endothelial buds soon be- 

 come hollow and join together, forming first a plexus and subsequently 

 a sac, from which again lymphatic vessels made of endothelium grow 

 out to invade the skin of the head, neck, thorax and arm, and partly 

 the deep structures of the head. The sac is ultimately transformed into 

 groups of lymph glands. At a later stage similar nodes develop from 

 certain of the abdominal veins, forming a retroperitoneal sac, from which 

 grow out the lymphatics of the abdominal and, to a certain extent, of 

 the thoracic viscera. A similar pair of sacs also develops from the iliac 

 veins supplying the lymphatics for the skin of the legs and abdominal 

 walls. The retroperitoneal and iliac sacs then become connected with 

 the jugular sac by means of the thoracic duct. In the embryo there are 

 no valves in the lymphatic vessels, so that the whole system can be in- 

 jected either from the thoracic duct or from the skin, showing clearly 

 that the superficial and deep lymphatics are parts of one closed system 

 of vessels. 



Anatomists have succeeded in tracing the course of the lymphatics in 

 many parts of the body. This knowledge is of great importance in 

 connection with the spread of infections, etc. Lymphatics are abun- 

 dant in the skin, the intestine, and connective tissues, but are absent 

 from the muscle bundles, from the hepatic lobules (though present in 

 the connective tissue between them), from the substance of the spleen, 

 and from the central nervous system. 



The lymphatics have the same functions as blood capillaries, namely, 

 to absorb substances from the tissue spaces. There is some evidence to 

 show that this absorption may be selective. When injections are made 

 into the peritoneal cavity, the pathway of absorption may be either the 

 blood vessels or the lymphatics, according to the nature of the sub- 

 stance injected. True solutions are absorbed by the blood, but granules 



115 



