272 Introduction [Dec. 



the thin walls of blood capillaries, and also into lymph Channels, 

 and through the thin walls of lymph capillaries, thus becoming 

 directly, and by way of the lymphatic trunks, a part of the cir- 

 culating blood. In a normal tissue there is practical equality be- 

 tween the total income of plasma and the total outgo of lymph. 

 In an edematous tissue this equilibrium is modified — a balance 

 accumulates in favor of income (and retention) of lymph (water). 



If these Statements are correct, and if edema is simply patho- 

 logical lymph accumulation, we should expect to find that edema 

 in any tissue results from inadequate removal of lymph from the 

 tissue Spaces to the circulation, or from inordinate transudation of 

 plasma from the blood to the tissue Spaces, or from both conditions. 



Physiologists are not in agreement in their views on the exact 

 nature, and the relative importance, of all the influences and factors 

 which induce lymph formation. The following summary indicates 

 the powers, processes, and properties which have long been regarded 

 as the dominant influences in the passage of blood plasma to the 

 tissue Spaces and in its compensatory return as lymph to the 

 circulation : 



I. Factors which directly or indirectly favor the continu- 



OUS PASSAGE OF BLOOD PLASMA ( WATER AND DISSOLVED SUB- 

 STANCES) TO THE TISSUE SPACES. 



A. Conditions which directly or indirectly facilitate filtration, 



difftcsion, or osmosis, or all of these processes. 



(a) Permeability of the walls of the blood capillaries. 



(b) High pressures in the arterial blood capillaries (as 

 compared with those in the tissue spaces). 



(c) Low pressures in the tissue Spaces (as compared with 

 those in the blood capillaries). 



(d) Renewal (from supplies in the alimentary tract) of 

 water and plasma constituents in the blood. 



(e) Outward movement of lymph from the tissue Spaces 

 into lymph Channels and blood capillaries. 



B. Special cellular influences in lymph production. 



(a) Secretory activity of the endothelium of the blood 



capillaries. 

 (5) Active intracellular utilization of, and demand for, 



plasma constituents (water and nutrients). 



