CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 321 



corresponds exactly to the quantity of free acid present. Lactic 

 and butyric and other acids when present are secondary products, 

 arising 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. 



180. 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 hydrochloriQ 

 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 dis- 

 solving 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 

 artificial juice, i.e. an acid infusion of the mucous membrane of 

 the stomach, be used. 



Artificial gastric juice may be prepared in any of the following ways. 



1. The mucous membrane of a pig's or dog's stomach is removed 



from the muscular coat, finely minced, rubbed in a mortar with 



21 v 



