616 THE RELATION OF LYMPH TO 



formation and absorption of tissue fluid. Being living cells, 

 possess a wholly unknown permeability, which might be constant 

 or variable. 



THE RELATION OF LYMPH TO THE NUTRITION OF THE 



TISSUES 



Interchange of material between tissue fluid and blood would 

 be meaningless except as a means of making good an interchange 

 which was taking or had taken place between the cell and the 

 fluid which bathes it. The only exception to this would appear 

 to be the case in which interchange took place, in order to keep 

 the volume and composition of the blood uniform. 



Living cells appear to determine themselves the rate at which 

 they take up nourishment ; it would therefore seem likely that 

 they initiate the passage of nutriment from the blood to the 

 tissue fluid, just as much as from the tissue fluid to themselves. 

 And, if this passage is brought about by diffusion, osmosis, and 

 filtration, some change in the cell would have to start these 

 processes at work. 



Internal respiration is considered to take place by diffusion. 

 Oxygen passes from a point of high pressure in the blood to a 

 point of low pressure in the cell across the tissue fluid, and at a 

 rate determined by the difference in pressure between these two 

 points. And since the oxygen pressure in arterial blood is approxi- 

 mately constant, the rate of diffusion is actually determined by 

 the consumption of oxygen by the cell. C0 2 passes in an 

 opposite direction, according to its rate of manufacture by the 

 tissue. In this process no alteration in the volume of the tissue 

 fluid is necessary. In an exactly analogous way it is possible for 

 all diffusible substances to pass to and from the cell and blood 

 according as the cell needs or manufactures them, and without 

 altering the quantity of tissue fluid. We know that in the case 

 of the limbs such an interchange does go on without any overflow 

 from the tissue spaces into the lymphatics. 



We have seen that the osmotic pressure of the total output 

 of a cell must be higher than that of its total income, and 

 especially during the breaking down of proteid ; water will, there- 



