268 OF FOOD ANtf THE DIGESTIVE PROCESS. 



that form of it obtained from the mucous membrane of the stomach of 

 the Pig, which bears a close resemblance to that of Man. When this 

 membrane is digested in a large quantity of warm water, it is purified 

 from the various soluble substances it may contain ; but the pepsine is 

 not taken up, as it is not soluble in warm water. By continuing the 

 digestion in cold water, the pepsine is then extracted nearly pure. 

 When this solution is evaporated to dryness there remains a brown, 

 grayish, viscid mass, having the appearance of an extract, and the 

 odour of glue. A similar substance may be obtained by adding strong 

 alcohol to a fresh solution of pepsine ; for the latter is then precipitated 

 in white flocks, which may be collected on a filter, and which produce a 

 gray compact mass when dried. Pepsine enters into chemical combina- 

 tion with many acids ; forming compounds which still redden litmus- 

 paper ; and this appears to be its condition in the gastric juice. 



471. The muriate and acetate of pepsine possess a very remarkable 

 solvent power for albuminous substances. A liquid which contains only 

 17 ten-thousandths of acetate of pepsine, and 6 drops of muriatic acid 

 per ounce, possesses solvent power enough to dissolve a thin slice of 

 coagulated albumen, in the course of six or eight hours' digestion. With 

 12 drops of muriatic acid per ounce, the same quantity of white of egg 

 is dissolved in two hours. A liquid which contains only half a grain of 

 acetate of pepsine, and to which the muriatic acid and white of egg are 

 alternately added, so long as the latter is dissolved, is capable of taking 

 up 210 grains of coagulated white of egg, at a temperature between 95 

 and 104. The same acid with pepsine dissolves blood, fibrine, meat, 

 and cheese; whilst the acid without the pepsine requires a very long 

 time to do so at ordinary temperatures. Very dilute muriatic acid, 

 however, at the boiling point, dissolves these albuminous substances ; 

 and the solution has the same characters, as that which is made by the 

 agency of pepsine. The horny tissues, such as the epidermis, horn, 

 hair, &c., and the yellow fibrous tissue, are not affected by the acid 

 solution of pepsine. It appears from these experiments, that the acid 

 is the real solvent ; and that the action of the pepsine is limited to dis- 

 posing the albuminous matter for solution, producing in it a change 

 analogous to that which maybe effected by heat. Hence it maybe 

 considered, like ptyaline, as a sort of ferment ; its office being to pro- 

 duce a tendency to change, on the substances on which it acts, without 

 itself entering into new combinations with any of their elements. 



472. These experiments appear to afford an explanation of the pro- 

 perties of the gastric fluid, as ascertained by direct experiment upon it. 

 When drawn direct from the human stomach, it is found to possess the 

 power of dissolving various kinds of alimentary substances, whilst these 

 are submitted to its action at a constant temperature of 100 (which is 

 about that of the stomach), and are frequently agitated. The solution 

 appears to be in all respects as perfect as that, which naturally takes 

 place in the stomach ; but a longer time is required to make it. This 

 is easily accounted for by the difference of the conditions ; for no ordi- 

 nary agitation can produce the same effect with the curious movements 

 of the stomach ( 458) ; fresh gastric fluid is poured out, as it is wanted, 

 during the natural process of digestion; and the continual removal of 



