40 THE BODY AT WORK 



water to traverse it. The solution passes through un- 

 changed in composition. Only solid particles are kept back. 

 The rapidity with which fluid passes through a filter varies 

 as the difference between the pressure on the one side and 

 the pressure on the other. A membrane does not allow 

 of filtration. Water and things dissolved in water pass 

 through it by osmosis. Some things it will not allow to pass ; 

 such, for example, as gum, mucin, white of egg. To others 

 it offers resistance in varying degrees. Most of the things 

 that can diffuse through a membrane are capable of crystalliza- 

 tion ; but the membrane exercises some control over the 

 passage of even crystallizable substances when in solution. 

 If a membranous tube containing water in which proteins, 

 sugar, and various salts are dissolved is hung in a basin of 

 pure water, the proteins remain in the tube ; the sugar and the 

 salts pass through its wall into the surrounding water. But 

 they pass at different rates. Those of small molecular weight 

 pass more quickly than those whose molecule is heavy. After 

 a time a condition of equilibrium is established. No more 

 salts pass out of the tube. If now the contents of the tube 

 and the contents of the basin are analysed, it will be found 

 that the tube contains all the proteins, some of the sugar, 

 and some of each of the salts, although not in the proportions 

 in which they were present at the commencement of the 

 experiment. The water in the basin contains some sugar and 

 some of each of the salts, but not in the same proportions in 

 which they are found in the tube. As a matter of fact, the same 

 number of molecules would be present, per unit volume, on 

 each side of the membrane in the tube and in the basin. 

 In this respect the percentage composition of the two solu- 

 tions would be the same. But some of the molecules being 

 heavy, others light, the weight of salts which unit volume of 

 the solution in the tube would contain would not be the 

 same as the weight of salts in unit volume of the solution in 

 the basin. A membrane exerts a discriminating action on 

 the substances which pass through it. Secretion is osmosis 

 in disguise. It may be even filtration in disguise. A gland- 

 cell (like an amoeba) takes things up and passes them out 

 without regard to their osmotic equivalent. It seems to 

 exercise a choice. It seems to act in disregard of the laws 



