THE COURSE AND PHYSICS OF THE CIRCULATION 239 



Some of this fluid is in spaces between the organs and the 

 body walls. Special names like pleural or peritoneal 

 fluid may be used in such cases. Much of the liquid is in 

 microscopic spaces among the cells of the various 

 tissues. Some is in a definite set of vessels, the lymphatics. 

 It has been urged that we should reserve the word lymph 

 for the fluid in the lymphatics, call that in the micro- 

 scopic interstices tissue-fluid, and use local names for the 

 contents of the larger cavities. So far as we know the 

 composition of the liquid in these different places is not 

 far from uniform. 



Lymph may be thought of as fairly represented by 

 the contents of. a blister. The exchanges which take 

 place between the blood-capillaries and the tissues are 

 so free that the lymph must tend to have the same com- 

 position as the plasma. It has been roughly described as 

 " blood minus the red corpuscles/' As its contact 

 with the active cells is so close we should expect it to 

 contain rather less of the substances recognizable as foods 

 and rather more of those known to be wastes than would 

 be found in the blood. This difference is in fact realized. 

 An exception may be noted in the wall of the small intes- 

 tine during digestion for here the lymph may be enriched 

 with foods derived from the canal. 



The Lymphatics. These are channels through which 

 lymph flows slowly from all parts of the body toward two 

 places in the thorax where it is introduced into the 

 systemic veins. We ought not to speak of a lymphatic 

 circulation for there are no vessels, corresponding to 

 arteries, through which lymph is carried outward from 

 the chest. The movement is wholly centripetal and 

 therefore comparable in direction with the flow of blood 

 in the veins. We are to think of the lymph as being 

 formed in the tissues and draining away quite gradually 

 to lose its identity at last in the venous blood. 



There has been much doubt as to how the finest 

 lymphatics, the lymph-capillaries, begin. Some have 

 thought of them as originating in the indefinite micro- 



