SALIVA AND GASTRIC JUICE. 253 



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 pro- 

 teid 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 re- 

 spects 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 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 mus- 

 cular coat, finely minced, rubbed in a mortar with pounded glass and extracted 

 with water. The aqueous extract filtered and acidulated (it is in itself somewhat 

 acid), until it has a free acidity corresponding to 0.2 per cent, of hydrochloric 

 acid, contains but little of the products of digestion, such as peptone, but is fairly 

 potent. 



2. The mucous membrane similarly prepared and minced is allowed to digest 

 at 35 C. in a large quantity of hydrochloric acid diluted to 0.2 per cent. The 

 greater part of the membrane disappears, shreds only being left, and the some- 

 what opalescent liquid can be decanted and filtered. The filtrate has powerful 

 digestive (peptic) properties, but contains a considerable amount of the products 

 of digestion (peptone, etc.) arising from the digestion of the mucous membrane 

 itself. 1 



3. The mucous membrane, similarly prepared and minced, is thrown into a com- 

 paratively large quantity of concentrated glycerin, and allowed to stand. The mem- 

 brane may be previously dehydrated by being allowed to stand under alcohol, but 

 this is not necessary, and a too prolonged action of the alcohol injures or even de- 

 stroys the activity of the product. The decanted clear glycerin, in which a com- 

 paratively small quantity of the ordinary proteids of the mucous membrane are 

 dissolved, if added to hydrochloric acid of 0.2 per cent, (about 1 c.c. of the glycerin 

 to 100 c.c. of the dilute acid are sufficient), makes an artificial juice tolerably free 

 from ordinary proteids and peptone, and of remarkable potency, the presence of 

 the glycerin not interfering with the results. 



Before proceeding to study the action of gastric juice on proteids, it will 

 be useful to review very briefly the chief characters of the more important 

 members of the group. 



The more important proteids which we have thus far studied are : 1. 

 Fibrin, insoluble in water and not really soluble (i. e., without change) in 

 saline solutions. 2. Myosin, insoluble in water, but soluble in saline solu- 

 tions, provided these are not too dilute or too concentrated. 3. Globulin 

 (including paraglobulin, fibrinogen, etc.), insoluble in water, but readily 

 soluble in even very dilute saline solutions. 4. Albumin, serum-albumin, 

 soluble in water in the absence of all salts. 5. Acid-albumin, into which 

 globulins and myosin are rapidly converted by the action of dilute acids, 



1 These, however, may be removed by concentration at 40 C. and subsequent dialysis. 



