THE FORMATION OF LYMPH. 



A S the blood circulates through the capillary blood-ves- 

 A~\ sels, some of its liquid constituents exude through the 

 thin walls of these vessels, carrying nutriment to the tissue 

 elements. This exudation is called lymph ; it receives 

 from the tissues the products of their activity, and is col- 

 lected by the lymph channels, which converge to the 

 thoracic duct — the main lymphatic vessel — and thus the 

 lymph once more re-enters the blood stream near to the 

 entrance of the large systemic veins into the right auricle. 



Lymph is a fluid which comes into much more intimate 

 relationship with metabolic processes in the tissues than the 

 blood, and the composition and mode of formation of lymph 

 are exceedingly important and interesting physiological 

 questions. 



Like most physiological problems it has undergone many 

 vicissitudes. After the disappearance of a belief in a special 

 vital force, Ludwig and his pupils taught that the formation 

 of lymph was a physical process in which filtration played 

 the most important part. There have always been diffi- 

 culties in the way of a full acceptance of this view, and 

 within recent years Heidenhain has urged that in the 

 normal formation of lymph, filtration plays little or no part ; 

 but that lymph is really formed by the selective or secretory 

 activity of the endothelial cells which make up the walls 

 of the capillary blood-vesselsi A dictum of this kind sup- 

 ported by exhaustive and careful experiments, coming from 

 a master of his science, was accepted with readiness by a 

 considerable number of physiologists. 



It came at a time when with the swing of the pendulum 

 physiologists were beginning to question whether physics 

 and chemistry would, after all, explain all vital problems, 

 and when in consequence a new school of "vitalists" was 

 rising up. Physiological absorption is something else than 

 mere filtration and absorption, and so the view that in 

 lymph formation there is also the vital action of cells to be 



