TRANSFERENCE OF FOOD MATERIALS 119 



tion of hydrogen ions by the gastric glands. The high con- 

 centration of hydrogen ions in gastric juice can be produced 

 only by the transformation of energy and cannot be due to 

 diffusion, osmosis, nitration or surface tension. 



Root pressure seems a similar condition. The pressure 

 produced in the stem of a plant* cannot be explained on known 

 physical processes. It would be interesting to know whether 

 there is increased oxidation in the root cells during the pro- 

 duction of the flow of liquid. 



It is well to bear these points in mind, because we can 

 concentrate our attention on the behaviour of the cell and not 

 spend our time trying to explain what is not yet clearly 

 understood. 



Digestion in the Stomach 



The next stage of digestion occurs in the stomach. The 

 gastric juice contains acid and this acid destroys the ptyalin. 

 As each portion of food passes from the oesophagus into the 

 stomach it is forced into the centre of the preceding masses of 

 food. The result is that the food forms a series of concentric 

 layers, the outside being the earliest swallowed and the inside 

 the last swallowed.! The acid must diffuse through these 

 various layers, hence the central portions do not become acid 

 for some time. The action of ptyalin is therefore not confined 

 to the short time that the food is in the mouth, but it may 

 continue for from twenty to forty-five minutes, depending 

 on the mass of food which must be rendered acid. 



The chief digestive action brought about by the gastric 

 juice is due to the enzyme pepsin. This acts in conjunction 

 with the hydrochloric acid which causes the acidity of the 

 secretion. The action of pepsin is to convert proteins into 

 peptones according to the scheme shown on the next page. 



In gastric digestion the metaprotein is that which exists in 

 acid solution. The terms acid and alkali metaprotein are 

 misleading. When the protein forms a salt with acid the 

 protein acts as a base and acquires a positive charge, hence 

 it is wrong to speak of the metaprotein in an acid solution as 

 acid metaprotein. Similarly the metaprotein in alkaline 

 solution should not be called alkali metaprotein. We ought to 

 say metaprotein in acid solution and metaprotein in alkaline 

 solution respectively. 



The presence of acid is necessary for the action of pepsin 

 and the acid may be regarded as a co-enzyme. The activity 



* S. Hales, Statical Essays, 1731, vol. i, London. 



| P. Griitzner, Arch. f. d. ges. Physiol., 1905, vol. 106, p. 463. 



