-LYMPH ABSORPTION 615 



The composition of tissue fluid is unknown. Since it is 

 impossible to say in point of time when a specimen of lymph 

 was drained off from the tissue spaces, it is impossible to compare 

 the osmotic pressures of blood and tissue fluid more than 

 approximately. 



That being the state of our knowledge about these physical 

 factors, it is obvious that there is great difficulty in interpreting 

 the results of experiments in which more than one of these factors 

 has probably been altered. We can, however, say that experi- 

 ments have shown that the importance of osmosis and diffusion 

 is relatively greater, and that of filtration relatively less, than was 

 thought to be the case. 



It is impossible at present to prove or deny a mechanical 

 formation and absorption of lymph. As a working hypothesis 

 the mechanical view is the better. It deals with processes of 

 which we understand something, and, even if they are ultimately 

 proved to be insufficient, there can be little doubt that they do 

 play a large part in the history of lymph. To explain lymph 

 formation and absorption as a secretion is at present no ex- 

 planation ; it is merely to invoke a force or process of which we 

 can now form no conception. It implies, further, that we already 

 know the exact limitations of the various physical factors in the 

 body, and that experiments exist which conclusively demonstrate 

 their insufficiency, neither of which is true. 



We may briefly recapitulate the conclusions at which we 

 have arrived regarding the formation of lymph in the normal 

 body. 



1. The formation of tissue fluid is determined by the meta- 

 bolism of the tissue cells. The only exception to this is the fact 

 that in order to keep the volume of the circulating blood constant 

 fluid can be taken up from or given out to the tissue spaces. 



2. Tissue fluid is not secreted by tissue cells, but is poured out 

 from the capillaries in obedience to osmosis, diffusion, and filtration, 

 which in turn are controlled and determined by the degree of 

 activity of the tissues. 



3. Tissue fluid is absorbed into the capillaries by a combination 

 of osmosis and diffusion, but it is uncertain how these forces are 

 brought into play. 



4. The cells of the capillary wall do not secrete tissue fluid. 

 There is no evidence that they play any but a passive part in the 



