524 PHYSIOLOGY CHAP. 



theory. Cohnheim had already on several occasions expressed the 

 idea that the vessel walls must be something more than a simple 

 passive filter. Following out this idea, Heidenhain affirmed that 

 the lymphagogic effects of the double series of substances above 

 indicated should be considered as proving that the epithelioid 

 cells which constitute the walls of the blood capillaries are to be 

 considered as secretory cells analogous to gland cells, capable, i.e., 

 of separating certain substances from the blood, and of pouring 

 them into the system of lymph spaces with a brisk displacement 

 of water, to provide for the various and specific nutritive needs of 

 the different tissues and organs. Heidenhain alleges that certain 

 secretory organs, such as the udder of the milch cow, are capable 

 of yielding 25 litres of milkier diem, containing 42 '5 grms. of lime. 

 Since the lymph poured into the thoracic duct does not contain 

 more than O1S per thousand grams, 236 litres of lymph would 

 be required to provide the gland cells with all the lime needed for 

 the production of the milk, on the hypothesis that they derive all 

 the materials required for their function from the lymph as such. 

 If, on the other hand, we assume secretory activity on the part of 

 the cells forming the capillary walls, it is easy to explain how, with 

 slight translocation of water, they are able to supply the gland with 

 all the material required. Seeing that each organ or tissue must 

 obtain its specific nutritive materials from the lymph, it is assumed 

 that they pour out specific products into the lymph, which excite 

 the secretory activity of the capillary walls, and thus provoke 

 secretion of those substances which the organ requires. 



It cannot be denied that this theory of Heidenhain is a very 

 bold one. Not because (as one of our younger physiologists main- 

 tains) it diverges from the principle of the mechanical interpretation 

 of functional processes the admission of one secretion within the 

 body more or less could not sensibly modify the general trend 

 of science ; but because Heidenhain, prior to formulating his 

 secretory theory, did not examine fundamentally to what point 

 the process of lymph formation could be interpreted by the aid of 

 the physical laws at present known to us. 



To the secretory theory, W. Cohnstein, in a series of interest- 

 ing papers (1893-1896), opposes what he terms the transudation 

 theory, according to which the formation of lymph is due to two 

 well-determined physical processes : filtration, which depends on 

 the difference of pressure between the two liquids separated by a 

 permeable membrane, represented by the capillary walls ; and 

 diffusion, due to the different chemical constitution of the two 

 fluids. The lymph contained in the extra - capillary lymph 

 spaces is during life the subject of continuous changes, pro- 

 duced by the metabolic activity of the parenchymal cells, which 

 draw from it the substances required for their nutrition, and 

 pour out the progressive and retrogressive products of their 



