MOVEMENT OF THE LYMPH. 207 



probably arise from these combined sources, viz., escape from the 

 bloodvessels and proliferation of the tissue cells. 



The lymph cells, therefore, whether they have their origin in 

 a lymph gland, the spleen, or in connective tissue, perform a kind 

 of circuit, going with the lymph into the blood, and are distrib- 

 uted with the latter to the tissues, whence they may be once 

 more carried into the lymph stream. 



MOVEMENT OF THE LYMPH. 



In some of the lower animals small muscular sacs occur in the 

 course of the main lymph channels which pump the lymph into 

 the great veins by contracting rhythmically, much in the same 

 way as the heart. 



In man and the higher animals, no such lymph hearts have 

 been found ; the onward movement of the fluid depends chiefly 

 on the pressure with which the irrigation stream leaves the blood- 

 vessels. The fluid in the bloodvessels, as we shall presently see, 

 is under considerable pressure, which causes the plasma to leave 

 the capillaries. Hence, if a lymphatic trunk be tied, there is 

 intense filling of all its tributaries until a great pressure (8-10 

 mm., soda solution) is developed in the vessels. 



While the pressure exerted on the small tributaries of the 

 lymph channels is considerable, that in the thoracic duct is ex- 

 tremely small, for the following reasons : The blood in the large 

 veins, into which the duct opens, is under less pressure than in 

 any other part of the vascular system, owing to the thoracic suc- 

 tion, or absence of pressure in the thorax, caused by the elastic 

 traction of the lungs. In fact, the pressure in the large veins, 

 e.g., brachial, etc., varies from to 4 mm. Hg, and that in the 

 vense cavse is always negative, except in forced expiration, and 

 varies, according to the period of the respiratory rhythm, from 

 5 mm., in inspiration, to 2 mm. in expiration. 



The fact that the lymph at the origin of the small channels is 

 at a pressure of 8 to 10 mm. of water, whilst at the entrance to 

 the vein it is nil, would be sufficient to explain the movement, 

 even if there were no other force aiding its movement. 



It must be remembered that every lymph vessel is furnished 



