EXCRETION 201 



nature, accompany the water until he had tried the experiment 

 of separating blood from water containing the inorganic salts 

 of urine by a permeable membrane the blood being at such a 

 pressure as the physiologist told him he might expect it to 

 have in renal arterioles, the water at such a pressure as he might 

 expect it to have at the upper end of a urinary tubule. He 

 would find that urea, and still more uric acid, is very reluctant 

 to pass through the membrane. Again, when asked whether 

 water, in which urea and other things were dissolved, would 

 leave the tubule say from the loop of Henle to pass back 

 into the blood, he would repeat his experiment with a mem- 

 brane. This time he would allow the urine and the blood to 

 be at the same pressure (or, possibly, would assign a higher 

 pressure to the former), and he would dilute the urine to make 

 the conditions agree with those which Ludwig supposed to 

 exist ; but his experiment would prove to him that, unless the 

 urine were very dilute indeed, water would still tend to pass 

 into it from the blood, and not vice versa. And here it may be 

 remarked that the results of these experiments might have 

 been predicted by calculation. When Ludwig advanced his 

 theory, osmosis was a mysterious phenomenon. Its laws 

 have since been accurately ascertained. Given the molecular 

 weights of bodies in solution and their degree of concentra- 

 tion, the direction in which they will pass through a mem- 

 brane can be predicted. The force with which water will 

 tend to pass from one solution to another can be calculated. 

 Urine as secreted contains far more urea, sodic chloride, and 

 other salts than blood. It has a much higher degree of con- 

 centration. The concentration of blood is 0-55; that of urine, 

 1-85. Water passes from a less concentrated to a more con- 

 centrated solution, not vice versa. As a solution of a problem 

 in hydrostatics Ludwig's hypothesis is untenable. 



Osmosis. Cells of all kinds, both vegetable and animal, are 

 limited, or surrounded by a layer of cell-substance which is 

 firmer than, and probably different in constitution from, the 

 substance in the interior of the cell. This outer layer is a 

 living membrane. The nutrition and growth of the cell are 

 dependent upon the capacity of its limiting membrane for 

 regulating the ingress and egress of water and of substances 

 dissolved in water. The phenomena of osmosis that is to 



