FACTORS CONCERNED IN LYMPH FORMATION 461 



as regards their composition and mode of formation scarcely more 

 closely allied to lymph than sweat is. They are almost free from 

 protein, and are secreted by special structures the choroid plexus 

 and the uveal epithelium quite different from any that can bf 

 concerned in the formation of ordinary lymph. 



It is very true these liquids are not blood, but that is scarcely a 

 sufficient reason for calling them lymph, else we might classify 

 sweat or even milk as lymph also. If a term is desirable to indicate 

 that they have certain relations with lymph, they might perhaps 

 be spoken of as lymphoid secretions. It may be that the essential 

 difference in the chemical composition of these lymphoid secretions 

 and lymph the practical absence of protein is related to the 

 difference in the manner of their formation. The uveal and choroidal 

 epithelial cells, interposing the depth of their columns or cubes 

 between the blood and the free surface at which the liquid escapes, 

 may well be suited to hinder the passage of the protein molecules 

 which find their way with greater ease through the thin endothelium 

 of the capillary wall into the tissue spaces, and from these into the 

 lumen of the closed lymphatics (see p. 433). Nevertheless, we shall 

 recognize later on in the glomeruli of the kidney an instance of 

 blood capillaries but little pervious to proteins, and there are several 

 other facts which show that the capillaries may differ considerably 

 in different organs in the readiness with which they permit the 

 various constituents of the plasma to pass through their walls. 

 Further, in discussing the mechanism by which lymph is formed, 

 we shall see reason to doubt whether mechanical filtration, due to 

 differences of hydrostatic pressure on the two surfaces of the 

 capillary endothelium, has much, if anything, to do with the 

 passage either of protein or of the other constituents of the lymph 

 from the lumen of the capillaries into the tissue spaces. At first 

 glance, indeed, such a process would seem to be admirably fitted to 

 explain the fact that, while lymph differs but little from blood- 

 plasma in the proportions of its other constituents, it is at most no 

 more than half as rich in protein. For there are many filters which 

 allow substances in ordinary solution and their solvent to pass 

 through without alteration in their relative proportions, while 

 substances like proteins in colloid solution are kept back to a 

 greater or less extent. 



Factors concerned in Lymph Formation. The teaching of Ludwig, 

 that lymph is formed by the filtration, and in a minor degree by the 

 diffusion, of the constituents of blood-plasma through the walls of the 

 capillaries into the tissue spaces, was based on such facts as the 

 increase in the tissue liquid of a limb or organ which occurs when 

 the exit of blood from it by the veins is hindered, or when the 

 quantity of the circulating liquid is increased by the injection of 

 blood or salt solution. It was first seriously called in question by 



