214 MANUAL OF PHYSIOLOGY. 



on the pressure with which the irrigation stream leaves the blood 

 vessels. The fluid in the blood vessels, 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 

 venae 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, while 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 

 with closely set valves, which prevent the fluid it contains from 

 being forced backward, so that any accidental local pressure ex- 

 ercised on the exterior of a lymph channel helps the fluid onward 

 to the veins. Along their entire extent these vessels are subject 

 to certain forces which must materially aid the flow of the lymph 

 stream. The first of these is the pressure exerted on the small 

 vessels by the movement of the muscles in the neighborhood. 

 The second is the unequal distribution of atmospheric pressure, 

 which has full force on the peripheral channels, but is- kept off 

 the thoracic duct and its termination, as already mentioned, by 

 the rigidity of the thoracic wall, which, together with the tendency 

 of the elastic lungs to shrink, causes a permanent negative pres- 

 sure in the thoracic cavity through which the duct passes. And, 

 lastly, the thin-walled lymphatics are everywhere surrounded 

 with very elastic textures, inclosed in an elastic skin, which exert 



