128 THE BODY AT WORK 



but an active extract is obtained by treating the crushed cells 

 with HC1. It changes some substance which they contain (pro- 

 visionally termed " prosecretin ") into the efficient hormone. 



In the lower portion of the small intestine any maltose 

 that -remains is converted into diffusible dextrose. A very 

 large amount of water has been poured into the canal in the 

 various digestive juices. This, together with water drunk, 

 is absorbed in the large intestine. At the lower end of the 

 alimentary canal nothing remains but indigestible substances 

 taken with food, chiefly cellulose, and the pigments and other 

 bodies which, as already said, are eliminated in bile. 



The absorption of water is checked by the ingestion of 

 extremely soluble salts, such as sulphate of magnesia, the 

 heavy molecule of which diffuses with difficulty. We attribute 

 the fact that sulphate of magnesia remains in the intestine, 

 and prevents water from diffusing out of it, to its slowness in 

 passing through a membrane, because this is what would 

 happen in dialysis ;* but we must remember that the living wall 



* Notice the distinction between nitration and dialysis. If water containing 

 soluble and insoluble substances is placed in a porous jar, the water and the 

 soluble substances pass through the pores of the jar. The rate of flow varies 

 as the pressure. If water containing soluble substances is placed in a bladder, 

 and the bladder is suspended in a vessel of water, some of the substances 

 which it may contain white of egg, for example are non-diffusible ; others will 

 pass from the water inside the bladder to the water which surrounds it. But 

 every diffusible substance has its own osmotic value. Some pass through the 

 membrane rapidly, soon establishing a condition of equilibrium in the two 

 fluids ; others take a long time. Further, if the water on one side of the 

 membrane contains a certain percentage weight of a salt, the molecules of 

 which are large say sulphate of magnesia and the water on the other side 

 the same percentage weight of a salt of smaller molecule say chloride of 

 sodium water containing the salt of smaller molecule will pass into the water 

 containing the salt of heavier molecule with a certain force. If, to start with, 

 the two solutions are at the same level, the level of the solution containing 

 the less diffusible salt, sulphate of magnesia, will at the commencement 

 of the experiment rise. It is therefore said to exert a greater osmotic 

 pressure than the more diffusible salt chloride of sodium. Equilibrium will 

 not be established until the fluid on one side of the membrane contains the 

 same number of molecules per unit volume as the fluid on the other side. If 

 the molecules of magnesic sulphate are pictured as oranges, and the molecules 

 of sodic chloride as nuts, it will be understood that equilibrium is not 

 established until the oranges and nuts to the pint on one side equal in number 

 the oranges and nuts to the pint on the other. When these principles are 

 applied to the passage of water containing products of digestion through the 

 wall of the alimentary canal, it is evident that, if we understand all the condi- 

 tions, the process cannot be explained as merely an exhibition of osmosis. Take 

 the simplest illustration. When blood-serum is placed in the intestine it is 

 absorbed. If it were in a dialyser, there would be equilibrium between the 



