328 GASTRIC DIGESTION. [BOOK :i. 



quantity of fibrin, we are led to take, as a measure of the activity 

 of a specimen of gastric juice, not the quantity of fibrin which it 

 will ultimately dissolve, but the rapidity with which it dissolves 

 a given quantity. 



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 neutialised juice be 

 properly acidified, 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 hydro- 

 chloric 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 effec- 

 tual, and the degree of acidity most useful varies with the dif- 

 ferent acids. The presence of neutral salts, such as sodium 

 chloride, in excess is injurious. The action of mammalian gastric 

 juice is most rapid at 35 40 C. ; at the ordinary temperature it 

 is much slower, and at about C. ceases altogether. The juice 

 may be kept however at C. 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 pla.ced 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. 



183. 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 



