60 THE CIRCULATION OF THE BLOOD 



coagulates when drawn, since the fibrin factors are present ; 

 but the process is less prompt and the clot is less firm than 

 in the case of blood. 



In order to form an idea as to the constituents of lymph 

 it is only necessary 'to say that its ultimate origin is the blood 

 plasma, except in so far as its composition is changed during 

 digestion. The plasma makes its way through the capillary 

 walls out to the tissues bringing nourishment to them and re- 

 moving waste products from them. In thus coming in con- 

 tact with the tissues the plasma finds itself in the extravas- 

 cular lymph spaces and its name is simply changed to lymph. 

 lit thus appears that lymph may enter the extravascular 

 spaces by the direct passage of plasma out of the vessels or 

 by being excreted, as it were, from the tissue cells. 



In any case the constituents of lymph are not very differ- 

 ent from those of plasma, except, of course, when intestinal 

 digestion is in progress and chyle is introduced into the lym- 

 phatic circulation. It contains the three plasma proteids, 

 urea, fat, lecithin, cholesterin, sugar and inorganic salts. 

 The proteids are less abundant than in plasma, as might be 

 supposed when it is remembered that they possess little os- 

 motic power. The inorganic saks are in about the same 

 proportion in both fluids. It is significant that the amount 

 of urea and related excrementitious products is more abun- 

 dant in lymph than in plasma ; their source is the destructive 

 metabolism going on in the cells to which the plasma has been 

 supplied, this plasma finding its way back as lymph. It is by 

 no means certain, however, that all the plasma escaping from 

 the capillaries is carried away by the lymphatic system. 

 Some may reenter the blood-vessels. 



There is no unanimity of opinion as to the exact method 

 of passage of plasma through the capillary walls into the 

 lymph spaces. Some maintain that the phenomena can be 

 explained by the ordinary physical laws of diffusion, filtra- 

 tion and osmosis when existing conditions of pressure, etc., 

 are taken into consideration. Others hold that these laws are 



