206 THE BODY AT WORK 



cannot make a model of a living cell. In the case of the 

 salivary gland, as we have already seen, living cells take water 

 from lymph, and discharge it as saliva in apparent opposition 

 to osmotic force. They reverse the direction of the flow which 

 would occur were lymph and saliva separated by a membrane. 

 But a cell is not a membrane. It is an extremely complicated 

 structure with an elaborate architecture of its own. As well 

 might we compare the distribution of water by a County 

 Council water-cart and its passage through a brewery. Accord- 

 ing to all the laws of hydrostatics, the water which flows into a 

 brewery should leave it through its drains. Its exit in barrels 

 on drays is antiphysical. When the physiologist can explore 

 the living cell, he will discover that the imbibition and extrusion 

 of water, the selection, retention, and discharge of salts, are 

 phenomena as strictly physical as their passage through a 

 dialyser in his laboratory. In the meantime he can but con- 

 template the cell with a certain degree of awe. His best 

 devised model of a urinary tubule may lead him into error, for 

 the simple reason that he cannot line it with living cells. A 

 living cell has a power which upsets all calculations, falsifies all 

 experimental findings. Its protoplasm can isolate and place 

 out of action any of the substances which enter it. If observa- 

 tions eventually prove to us that water passes from the urinary 

 tubules into the blood, " in the face of osmotic force," we shall 

 be constrained to explain this antiphysical phenomenon as due 

 to the action of living cells. The cells, we shall say, take up- 

 fluid from the urinary tubules, fix its urea and other salts in 

 their protoplasm, discharge its water into the venous blood, 

 return the urea and other salts to the urine. Given this 

 property of protoplasm, such a process is strictly in accordance 

 with physical laws. 



Enough has been said regarding the theory, or want of theory, 

 of the action of the kidney. Turning now to matters of obser- 

 vation, it can easily be shown that the epithelium of the 

 tubules has the power of excreting into the urine highly complex 

 materials which diffuse with difficulty. If a substance soluble 

 in blood, but insoluble in urine, an alkaline salt of indigo, for 

 example, be injected into the vascular system, it is rapidly 

 excreted by the kidney. The indigo is precipitated even before 



