THE VASOMOTOR MECHANISM 347 



that the lymph-nodes are able also to arrest, for a time at least, 

 the spread of cancer-cells over the body. The lymph-nodes located 

 on the channels draining the lungs become filled with dust that 

 has worked its way through the pulmonary walls into the lymph, 

 and that is prevented thus from spreading throughout the Body. 



The Movement of the Lymph. This is no doubt somewhat 

 irregular in the commencing vessels, but, on the whole, sets on 

 to the larger trunks and through them to the veins. In many 

 animals (as the frog) at points where the lymphatics communicate 

 with the veins, there are found regularly contractile "lymph- 

 hearts" which beat with a rhythm independent of that of the 

 blood-heart, and pump the lymph into a vein. In the Human 

 Body, however, there are no such hearts, and the flow of the 

 lymph is dependent on less definite arrangements. It seems to 

 be maintained mainly by three things: (1) The pressure on the 

 blood-plasma in the capillaries is greater than that in the great 

 veins of the neck; hence any plasma filtered through the capillary- 

 walls will be under a pressure which will tend to make it flow to 

 the venous termination of the thoracic or the right lymphatic 

 duct. (2) On account of the numerous valves in the lymphatic 

 vessels (which all only allow the lymph to flow past them to 

 larger trunks) any movement compressing a lymph-vessel will 

 cause an onward flow of its contents. The influence thus exerted 

 is very important. If a tube be put in a large lymphatic, say at 

 the top of the leg of an animal, it will be seen that the lymph only 

 flows out very slowly while the animal is quiet; but as soon as it 

 moves the leg the flow is greatly accelerated. (3) During each 

 inspiration the pressure on the thoracic duct is less than that in 

 the lymphatics in parts of the Body outside the thorax (see 

 Chap. XXIII). Accordingly, at that time, lymph is pressed, or, 

 in common phrase, is "sucked," into the thoracic duct. During 

 the succeeding expiration the pressure on the thoracic duct be- 

 comes greater again, and some of its contents are pressed out ; but 

 on account of the valves of the vessels which unite to form the 

 duct, they can only go towards the veins of the neck. 



During digestion, moreover, contractions of the villi and of 

 the intestinal walls press on the lymph or chyle within them and 

 force it on; and in certain parts of the Body gravity, of course, 

 aids the flow, though it will impede it in others. 



