CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 389 



The reaction is distinctly acid, and the acidity is normally due 

 to free hydrochloric acid. This is shewn by various proofs, among 

 which we may mention the conclusive fact that the amount of 

 chlorine present in gastric juice is more than would suffice to 

 form chlorides with all the bases present, and that the excess if 

 regarded as existing in the form of hydrochloric acid corresponds 

 exactly to the quantity of free acid present. Lactic and butyric 

 and other acids when present are secondary products, arismg- 

 either by their respective fermentations from articles of food, 

 or from the decomposition of their alkaline or other salts. In 

 man the amount of free hydrochloric acid in healthy juice may 

 be stated to be about '2 per cent., but in some animals it is 

 probably higher. 



202. On starch gastric juice has no amylolytic action ; on 

 the contrary when saliva is mixed with gastric juice any amylo- 

 lytic ferment which may be present in the former is at once 

 prevented from acting by the acidity of the mixture. Moreover 

 in a very short time, especially at the temperature of the body, 

 the amylolytic ferment is destroyed by the acid so that even on 

 neutralisation the mixture is unable to convert starch into sugar. 



On dextrose healthy gastric juice has no effect. And its power 

 of inverting cane-sugar seems to be less than that of hydrochloric 

 acid diluted to the same degree of acidity as itself. In an un- 

 healthy stomach however containing much mucus, the gastric 

 juice is very active in converting cane-sugar into dextrose. This 

 power seems to be due to the presence in the mucus of a special 

 ferment, analogous to, but quite distinct from, the ptyalin of 

 saliva. An excessive quantity of cane-sugar introduced into the 

 stomach causes a secretion of mucus, and hence provides for its 

 own conversion. 



On fats gastric juice has at most a limited action. When 

 adipose tissue is eaten, the chief change which takes place in the 

 stomach is that the proteid and gelatiniferous envelopes of the 

 fat-cells are dissolved, and the fats set free. Though there is 

 experimental evidence that emulsion of fats to a certain extent 

 does take place in the stomach, the great mass of the fat of a meal 

 is not so changed. 



Such minerals as are soluble in free hydrochloric acid are for 

 the most part dissolved ; though there is a difference in this and 

 in some other respects between gastric juice and simple free 

 hydrochloric acid diluted with water to the same degree of acidity 

 as the juice, the presence either of the pepsin or of other bodies 

 apparently modifying the solvent action of the acid. 



The essential property of gastric juice is the power of dissolving 

 proteid matters, and of converting them into a substance called 

 peptone. 



Action of gastric juice on proteids. The results are essentially 

 the same whether natural juice obtained by means of a fistula or 



