886 APPENDIX. 



blood, while sodium carbonate, urea, sugar, and other soluble crystalloidal 

 substances will pass from the blood into the salt solution in the peritoneal 

 cavity. Through the action of this process of diffusion we can understand 

 how certain constituents of the blood may pass to the tissues of various glands 

 in amounts greater than can be explained if we supposed that the lymph 

 of these tissues is derived solely by filtration from the blood-plasma. An- 

 other important conception in this connection is the possibility that the 

 capillary walls may be permeable in different degrees to the various soluble 

 constituents of the blood, and furthermore the possibility that the permea- 

 bility of the capillary walls may vary in different organs. With regard to 

 the first possibility it has been shown that the blood capillaries are more 

 permeable to the urea molecules than to sugar or NaCl. With the aid of 

 these facts it is possible to explain in large measure the transportation of 

 material from the blood to the tissues, and vice versa. For example, to follow 

 a line of reasoning used by Roth, we may suppose that the functional activity 

 of the tissue elements is attended by a consumption of material which in 

 turn is made good by the dissolved molecules in the tissue lymph. The 

 concentration of the latter is thereby lowered, and in consequence a diffu- 

 sion stream of these substances is set up with the more concentrated blood. 

 In this way, by diffusion, a constant supply of dissolved material is kept 

 in motion from the blood to the tissue elements. On the other hand, the 

 functional activity of the tissue elements is accompanied by a breaking down 

 of the complex proteid molecule, with the formation of simpler, more stable 

 molecules of crystalloid character, such as the sulphates, phosphates, and 

 urea or some precursor of urea. As these bodies pass into the tissue lymph 

 they tend to increase its concentration, and thus by the greater osmotic 

 pressure which they exert serve to attract water from the blood to the 

 lymph, forming one efficient factor in the production of lymph. On the 

 other hand, as these substances accumulate in the lymph to a concentration 

 greater than that possessed by the same substances in the blood, they will 

 diffuse toward the blood. By this means the waste products of activity 

 are drawn off to the blood, from which, in turn, they are removed by the 

 action of the excretory organs. 



Diffusion of Proteids. This simple explanation on purely physical 

 grounds of the flow of material between the blood and the tissues can only 

 be applied, however, at present to the diffusible crystalloids, such as the 

 salts, urea, and sugar. The proteids of the blood, which are supposed to 

 be so important for the nutrition of the tissues, are practically indiffusible, 

 so far as we know. It is difficult to explain their passage from the blood 

 through the capillary walls into the lymph. Provisionally it may be assumed 

 that this passage is due to filtration. The blood-plasma in the capillaries is 

 under a slightly higher pressure than the lymph of the tissues, and this 

 higher pressure tends to squeeze the blood constituents, including the 

 proteid, through the capillary walls. This explanation, however, can not be 

 said to be satisfactory; and in this respect the purely physical theory of 

 lymph formation waits upon a clearer knowledge of the nature of the nutri- 

 tive proteids and their relations to the capillary wall (see Lymph, p. 427). 



