THE NUTRITION OF THE TISSUES 617 



fore, be attracted by osmosis towards the cell. Presumably 

 katabolic products will be at greater concentration within the 

 cell than in the tissue fluid, and in the tissue fluid than in the 

 blood. Consequently water would be attracted into the cell at 

 a rate determined by the magnitude of the osmotic pressure 

 maintained within the cell by its own metabolism, and from the 

 blood into the tissue fluid by the rate at which the metabolic 

 products pass from the cell into the tissue fluid. It is not, 

 therefore, difficult to imagine a possible way in which the cell 

 might regulate the passage of water and all diffusible substances 

 from the blood. 



Before we can hope to understand how this regulation is 

 actually carried out we must have information on two other 

 factors, namely, the exact chemical changes that go on in different 

 cells, and the permeability of cells in both directions for various 

 substances. It is possible that it is variations in the metabolism 

 and permeability of cells, more than differences in the permea- 

 bility of the capillary wall, which determine the variations in the 

 composition and flow of lymph found in different areas, and that 

 the objections which appear to exist to the view of Moore and 

 Parker, that proteids are diffusible in the body, do not hold 

 good. 



If blood proteids are non-diffusible, they can only pass 

 through the capillary wall by the force of filtration. By analogy 

 it would seem likely that cells would have the power of regulating 

 this passage from the blood, and if so, they must be able to 

 alter the main factor in filtration, the capillary blood pressure. 

 It is usual for arterial dilatation to accompany tissue activity, and 

 it is probable that arterial vaso-dilatation cannot cause increased 

 tissue activity, therefore tissue activity must cause the vaso-dilata- 

 tion, or both must be due to a common cause. The latter 

 appears to be the case in the salivary glands ; but in muscle and 

 other important tissues no such nervous arrangement is known. 

 In the case of muscle it has been suggested that products of 

 its own metabolism other than acid or C0 2 may cause the vaso- 

 dilatation, and more recently Bayliss and Starling have found 

 evidence that the products of tissue metabolism exert a specific 

 local vaso-dilator action. 



This filtration of proteids entails the passage of large quan- 



