THE LYMPH AND ITS MOVEMENT 69 



needs of the cells. But the lymph and the blood, every- 

 thing considered, can hardly be looked upon as two sepa- 

 rate and distinct liquids. Not only do they supplement 

 each other in their work and possess striking similarities, 

 but each is made in its movements to pass into the vessels 

 occupied by the other, so that they are constantly mixing 

 and mingling. For these and other reasons, they are more 

 properly regarded as two divisions of a single liquid one 

 which, by adapting itself to different purposes, 1 supplies 

 all the conditions of a nutrient fluid for the cells. 



Movements of the Lymph. As compared with the 

 blood, the lymph must be classed as a quiet liquid. But, as 

 already suggested, it has certain movements which are nec- 

 essary to the purposes which it serves. A careful study 

 shows it to have three well-defined movements as follows: 



1. A movement from the capillaries toward the cells. 



2. A movement from the cells toward the capillaries. 



3. A movement of the entire body of lymph from the 

 lymph spaces into the lymphatics and along these chan- 

 nels to the ducts through which it enters the blood. 



By the first movement the cells receive their nourish- 

 ment. By the second and third movements the lymph, 

 more or less laden with impurities, is returned to the 

 blood stream. (See Figs. 28 and 30.) 



Causes of the Lymph Movements. Let us consider first 

 the movement through the lymph tubes. No pump, like 

 the heart, is known to be connected with these tubes and 



1 In certain small animals of the lowest types a single liquid, serving as a 

 medium of exchange between the cells and the body surface, supplies all the needs 

 of the organism. In larger animals, however, where materials have to be moved 

 from one part of the cell group to another, a portion of the nutrient fluid is used 

 for purposes of transportation. This is confined in channels where it is set in 

 motion by suitable organs. The portion which remains outside of the channels 

 then transfers material between the cells, on the one hand, and the moving liquid, 

 on the other. 



