174 blood and lymph 



The Lymph. 



In the pages preceding we have attempted to make it 

 clear that the materials required by the tissues are brought 

 to them by the blood, and that the wastes (carbon dioxide, 

 urea, etc.) given off by the tissues are finally conveyed by 

 the blood to organs which remove them from the body. 

 In the lower animals where the blood fluid is not inclosed 

 in tubes, and bathes aU the tissues, this exchange between 

 tissue cells and blood fluid is a comparatively simple 

 matter. In man and the higher animals the fact that the 

 blood is contained always in closed tubes makes it less easy 

 to see how this exchange takes place. The explanation of 

 this exchange involves a study of another important liquid 

 of the body known as the lymph. All the tissue elements 

 of the body are bathed in this liquid. Furthermore, the 

 lymph spaces are connected by a set of tubes known as 

 the Ijmiphatics, which permit the flow of the lymph from 

 one part of the body to another. 



■Origin and structure of the lymph. — Examination of 

 the composition of the lymph contained in the lymph tubes 

 and spaces shows that it is practically identical in struc- 

 ture with the plasma of the blood, and in fact it is sup- 

 posed to be plasma which has in some way passed out 

 of the blood vessels and become a tissue bathing fluid. 

 It was once thought that the l3Tnph in the tubes and 

 the lymph in the spaces were in direct communication. 

 Recent investigation has shown that the lymph tubes are 

 closed as completely as the blood vessels, but the lymph 

 contained in them is identical in structure and origin with 

 that in the spaces. How has the plasma that is contained 

 in the blood vessels been able to escape through the walls 



