396 , GASTRIC DIGESTION. [BOOK n. 



The greater the surface presented to the action of the juice, the 

 more rapid the solution ; hence minute division and constant move- 

 ment favour digestion. And this is probably, in part at least, the 

 reason why a fragment of spongy filamentous fibrin is more readily 

 dissolved than a solid clump of boiled white of egg of the same size. 

 Neutralisation of the juice wholly arrests digestion; fibrin may be 

 submitted for an almost indefinite time to the action of neutralised 

 gastric juice without being digested. If the neutralised juice be 

 properly acidifie'd, it may again become active ; when gastric juice 

 however has been made alkaline, and kept for some time at a 

 temperature of 35, its solvent powers are not only suspended but 

 actually destroyed. Digestion is most rapid with dilute hydrochloric 

 acid of '2 p.c. (the acidity of natural gastric juice). If the juice 

 contains much more or much less free acid than this, its activity is 

 distinctly impaired. Other acids, lactic, phosphoric, &c. may be 

 substituted for hydrochloric,; but they.are not so effectual, and the 

 degree of acidity most useful varies with the different acids. The 

 presence of neutral salts, such as sodium chloride, in excess is 

 injurious. The action of mammalian gastric juice is most rapid at 

 350 40 C. ; at the ordinary temperature it is much slower, and at 

 about C. ceases altogether. The juice may be kept however at 

 0C. for an indefinite period without injury to its powers. The 

 gastric juice of cold-blooded vertebrates is relatively more active 

 at low temperatures than that of warm-blooded mammals or 

 birds. 



At temperatures much above 40 or 45 the action of the juice 

 is impaired. By boiling for a few minutes the activity of the most 

 powerful juice is irrevocably destroyed. The presence in a concen- 

 trated form of the products of digestion hinders the process of solu- 

 tion. If a large quantity of fibrin be placed in a small quantity of 

 juice, digestion is soon arrested ; on dilution with the normal hy- 

 drochloric acid ( - 2 p.c.), or if the mixture be submitted to dialysis 

 to remove the peptones formed, and its acidity be kept up to the 

 normal, the action recommences. By removing the products of 

 digestion as fast as they are formed, and by keeping the acidity up 

 to the normal, a given amount of gastric juice may be made to 

 digest a very large quantity of proteid material. Whether the 

 quantity is really unlimited is disputed ; but in any case the 

 energies of the juice are not rapidly exhausted by the act of 

 digestion. 



205. Nature of the action. All these facts go to shew that 

 the digestive action of gastric juice on proteids, like that of saliva 

 on starch, is a ferment-action; in other words, that the solvent 

 action of gastric juice is essentially due to the presence in it of a 

 ferment-body. To this ferment-body, which as yet has been only 

 approximately isolated, the name of pepsin has been given. It is 

 present not only in gastric juice but also in the glands of the 

 gastric mucous membrane, especially in certain parts and under 



