506 OF FOOD, AND THE DIGESTIVE PROCESS. 



retards it. If even moderate exercise be taken immediately after a full meal, 

 however, it is probably rather injurious than beneficial ; but if an hour be per- 

 mitted to elapse, or if the quantity of food taken have been small, it is of de- 

 cided benefit. The influence of temperature on the process of solution, is 

 remarkably shown in some of Dr. B.'s experiments. He found that the gas- 

 tric juice had scarcely any influence on the food submitted to it, when the 

 bottle was exposed to the cold air, instead of being kept at a temperature of 

 100. He observed on one occasion, that the injection of a single gill of 

 water at 50 into the stomach, sufficed to lower its temperature upwards of 

 30 ; and that its natural heat was not restored for more than half an hour. 

 Hence the practice of eating 'ice after dinner, or even of drinking largely of 

 cold fluids, is very prejudicial to digestion. 



667. From the foregoing statements we may conclude, that the process by 

 which the food is dissolved in the Gastric fluid is of a purely Chemical na- 

 ture, since it takes place out of the living body as well as in it, allowance 

 being made for the difference in its physical condition. That the natural pro- 

 cess of digestion is imitated, when the food is submitted to the action of the 

 gastric juice in a vial, not only in regard to the disintegration of its particles, 

 but as to the change of character which they are made to undergo, is proved 

 by the fact, that the artificial chyme thus formed exhibits the same changes as 

 the real chyme, when submitted to the action of the bile ( 658). The pro- 

 cess of digestion, however, may be freely conceded to be vital, in so far as it 

 is dependent upon the agency of a secreted product, which vitality alone (so 

 far at least as we at present know) can elaborate ; and all for which it is here 

 contended is, that, when this product is once formed, it has an agency upon 

 the alimentary matter, which, though not yet fully understood, is conformable, 

 . in all that is known of its operation, to the ordinary laws of chemistry. Thus, 

 Digestion is conformable to Chemical solution, -Jirst, in the assistance which 

 both derive from the minute division of the solids submitted to it; secondly, 

 in the assistance which both derive from the successive addition of small por- 

 tions of the comminuted solid to the solvent fluid, and from the thorough in- 

 termixture of the two by continual agitation ; thirdly, in the limitation of the 

 quantity of food on which a given amount of gastric juice can operate, which 

 is precisely the case with chemical solvents ; -fourthly, in the assistance which 

 both derive from an elevation of temperature, the beneficial influence of heat 

 being only limited, in the case of digestion, by its tendency to produce decom- 

 position of the gastric fluid ; -fifthly, in the different action of the same solvent 

 upon the various solids submitted to it. 



668. It may be considered a well-established fact, that diluted acids alone 

 have no power of chymifying alimentary substances, although capable of 

 partially dissolving some of them ; but that their presence in the gastric fluid 

 is essential to its effectual action. The active agent in the process appears to 

 be an Organic compound, to which the name of pepsin has been given. The 

 properties of this have been investigated by Wasmann, who first succeeded in 

 obtaining it in an isolated state ; his observations were made upon the mucous 

 membrane of the stomach of the Pig, which greatly resembles that of Man. 



a. When this membrane is digested in a large quantity of water at from 85 to 05, many 

 other matters are removed from it besides pepsin; but if this water l>e removed, and the 

 digestion be continued with fresli water in the eold, very little but pepsin is then taken up. 

 Pepsin appears in be but sparingly soluble in water; when its Dilution is evaporated to dry- 

 tit-ss, there remains a brown, grayish, -\iscid mass, with the odour of glue, and having the 

 appearanee of an extract. The solution of this in water is turbid, and still possesses a por- 

 tion of the characteristic power of pep>in, but greatly reduced. When strong alcohol is added 

 to a fresh solution of pepsin, the latter is precipitated in white flocks, which maybe collected 

 on'a filter, and produce a grey compact mass when dried. Pepsin enters into chemical com- 



