STRUCTURE AND FUNCTIONS OF BLOOD AND LYMPH. 259 



osmotic pressure and is said to be exerted by any solution of 

 higher concentration toward any of lower concentration. 



DIALYSIS. A membrane which is permeable to water but not 

 to any particles which may be dissolved in it is known as a semi- 

 permeable membrane; one which allows dissolved substances as 

 well as water to pass is a permeable membrane. When two so- 

 lutions of different concentration are separated by a membrane 

 of this latter class we have in addition to the movement of water 

 under the driving force of osmotic pressure a movement of dis- 

 solved particles through the membrane. This is a special case of 

 the general phenomenon of diffusion. Diffusion may be defined 

 as the tendency of substances in solution to distribute themselves 

 evenly throughout the solvent; Where this distribution necessi- 

 tates the passage of particles through permeable membranes the 

 phenomenon is called dialysis. The effect of both osmosis arid 

 dialysis is to equalize the concentrations of the solutions on the 

 two sides of the membrane, but it must be remembered that 

 they are entirely distinct phenomena. To illustrate: suppose we 

 have on the two sides of a permeable membrane solutions re- 

 spectively of sugar and salt of the same concentration, that is, 

 having the same number of particles in solution; there would then 

 be no flow of water in either direction since the osmotic pressure 

 of both solutions is the same, but since neither the sugar nor the 

 salt is evenly distributed throughout the solvent there will be 

 dialysis of both substances until an even distribution is obtained. 



Again, both osmosis and dialysis bring about changes in the 

 concentration of the solutions affected by them whereas filtration 

 does not. In considering the influence of the membranes of 

 the Body upon its liquid contents these facts must be borne in 

 mind. 



The Renewal of the Lymph. Osmotic phenomena play a great 

 part in the nutritive processes of the Body. The lymph present 

 in any organ gives up things to the cells there and gets things 

 from them; and thus, although it may have originally been tol- 

 erably like the liquid part of the blood, it soon acquires a different 

 chemical composition. Diffusion or dialysis then commences be- 

 tween the lymph outside and the blood inside the capillaries, and 

 the latter gives up to the lymph new materials in place of those 

 which it has lost and takes from it the waste products it has re- 



