THE LYMPH. 157 



that they have a stimulating action on the endothelial cells lining 

 the capillaries and thus produce an actual secretion of lymph. 

 It is more probable, however, that they poison these cells in a 

 way which increases their permeability and thus permits a freer 

 filtration of lymph from the blood plasma. There are other 

 facts nevertheless which support the theory of an actual secreting 

 mechanism within the cells of the capillary walls, but they are 

 too technical to consider here. They suggest that although the 

 physico-chemical laws of diffusion, osmosis, filtration, etc., play 

 the most important role in lymph formation, the cells of the 

 capillary walls may themselves have an active part in the pro- 

 cess. 



Lymph Reabsorption. Within the tissue spaces, and within 

 the cells of the tissues, changes are continually taking place which 

 alter the character of the lymph. Oxygen and food substances 

 are removed from the lymph by the tissue cells, and waste sub- 

 stances, the result of the tissue metabolism, are added to it. In 

 the case of oxygen and carbon dioxide, the exchange is so reg- 

 ulated as to keep constant the supply of these bodies in the 

 lymph. The loss of any substance is quickly compensated for by 

 the addition of new material from the blood. The solid waste 

 matter excreted by the cell can also find its way directly from 

 the cell through the lymph and into the blood plasma. It is 

 probable that during periods of rest or of slight activity the 

 lymphatics are of little importance in the exchange of the lymph. 

 However, when the exudation of lymph becomes increased, as 

 during exercise or following the use of some lymphagogue, or 

 when there are substances in the lymph which the capillaries 

 cannot absorb into the blood, the lymphatics become very im- 

 portant in helping to remove the excess of lymph formed. 



The Movement of the Lymph. The mechanism by which the 

 lymph of the tissues is collected by the capillaries of the lymph- 

 atic system is not understood any better than the mechanism of 

 lymph formation, but no doubt the same laws apply to both pro- 

 cesses. The movement of the lymph along the lymphatic vessels 

 is possible because of the presence of valves along the course of 

 the vessels. 



