60 THE CIRCULATION OF THE BLOOD 



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 removing waste 

 products from them. In thus coming in contact with the tissues 

 the plasma finds itself in the extravascular lymph spaces and its 

 name is simply changed to lymph. It 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 different 

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

 is in progress and chyle is introduced into the lymphatic circula- 

 tion. 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 osmotic power. The 

 inorganic salts are in about the same proportion in both fluids. 

 It is significant that the amount of urea and related excrementi- 

 tious products is more abundant 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, filtration and osmosis 

 when existing conditions of pressure, etc., are taken into consid- 

 eration. Others hold that these laws are insufficient in them- 



