308 THE CIRCULATION 



A second cause of the lymph-flow is the compression of the lymphatics 

 by muscles. The system of the lymphatics ramifies everywhere through- 

 out the body and is subject in very many places to compression by this 

 means. This influence is especially strong probably wherever the vessels 

 are crowded against bones, as, for example, in the arms, where contraction 

 of the biceps, triceps, etc., compresses lymphatics of considerable size 

 against the resistant humerus. In the legs and in the thorax the same 

 conditions ' obtain. The powerful and rapid movements in the small 

 intestine must compress the villi and serve to pump the lymph contained 

 onward toward the lymphatic trunks. The reason for the almost 

 constant succession of strong valves in the lymph-vessels becomes plain 

 when one considers the effect which compression by the skeletal muscles 

 would exert were they not present. The lymph in the vessels would be 

 forced in "that case as strongly backward as onward. By the action 



FIG. 170 



The subcapillary lymphatics of the human conjunctiva. 30 /i- (Toldt.) 



of these valves, however, all the influence exerted is in the right direction, 

 for regurgitation cannot take place even to a small extent so close together 

 are the valves. The muscles exerting this motive-power over the lymph 

 are practically all those of the body, for even where no bones are near, 

 contraction of muscular tissue increases the pressure considerably in 

 the part where it occurs. Muscular activity (like muscle-tissue itself) 

 is much more universal than is often appreciated, for all the muscles 

 are continually in a state of varying tone when not more vigorously 

 contracting. 



A third cause of the lymph's movement is the aspiration of the thorax 

 as it is technically called. This is the suction exerted by the bellows- 

 action of the chest at each respiration on every vessel entering it from 

 without, especially through the diaphragm and from the neck. This 

 matter has been already dwelt upon in considering the causes of the 

 blood's circulation proper (see page 280) and needs no further descrip- 



