294 THE HUMAN BODY 



a great part in the nutritive processes of the Body. The lymph 

 present in any organ gives up things to the cells there and gets 

 things from them ; and thus, although it may have originally been 

 tolerably like the liquid part of the blood, it soon acquires a differ- 

 ent chemical composition. Diffusion or dialysis then commences 

 between the lymph outside and the blood inside the capillaries, and 

 the latter gives up to the lymph new materials in place of those 

 which it has lost and takes from it the waste products it has re- 

 ceived from the tissues. When this blood, altered by exchanges 

 with the lymph, gets again to the neighborhood of the receptive 

 cells, having lost some food-materials it is poorer in these than 

 the richly supplied lymph around those cells, and takes up a 

 supply by dialysis from it. When it reaches the excretory organs 

 it has previously picked up a quantity of waste matters and loses 

 these by dialysis to the lymph there present, which is specially 

 poor in such matters, since the excretory cells constantly deprive 

 it of them. In consequence of the different wants and wastes of 

 various cells, and of the same cells at different times, the lymph 

 must vary considerably in composition in various organs of the 

 Body, and the blood flowing through them will gain or lose dif- 

 ferent things in different places. But renewing during its circuit 

 in one what it loses in another, its average composition is kept 

 pretty constant, and, through interchange with it, the average 

 composition of the lymph also. 



The Lymphatic Vessels. The blood, on the whole, loses more 

 liquid to the lymph through the capillary walls than it receives 

 back the same way. This depends mainly on the fact that the 

 pressure on the blood inside the vessels is greater than that on 

 the lymph outside, and so a certain amount of nitration of liquid 

 from within out occurs through the vascular wall in addition to 

 the dialysis proper. The excess is collected from the various 

 organs of the Body into a set of lymphatic vessels which carry it 

 directly back into some of the larger blood-vessels near where 

 these empty into the heart; in this way the liquid which is forced 

 out of the blood stream in the capillaries gets back into it again. 



The Lacteals. In the walls of the alimentary canal certain 

 food-materials after passing through the receptive cells into the 

 lymph are not transferred locally, like the rest, by dialysis into 

 the blood, but are carried off bodily in the lymph-vessels and 



