"ON THE DIGESmE FUNCTION, ETC. 



131 



digestion in man consists of three distinct acts : mastication, which is the 

 office of the mouth, and by which the food is first broken down ; chymifi- 

 cation, or its reduction into pulp, which is the office of the stomach ; and 

 chyhfication, or its dilution into a fluid state, which is the office of that 

 part of the intestinal canal which immediately communicates with the 

 stomach. The whole of this process is completed in about three hours, 

 and under certain states of the stomach, to which I shall advert presently, 

 almost as quickly as the food is swallowed. The most important of these 

 three actions is that of chymification ; and, while it takes place, both ori- 

 fices of the stomach are closed, and a degree of chilliness is often produced 

 in the system generally, from the demand which the stomach makes upon 

 it for an auxiliary supply of heat, without an augmentation of which it ap- 

 pears incapable of performing this important function. 



Considering the comparatively slender texture of the chief digesting 

 organ, and the toughness and the solidity of the substances it digests, it 

 cannot appear surprising that mankind should have run into a variety of 

 mistaken theories in accounting for its mode of action. Empedocles and 

 Hippocrates supposed the food to be softened by a kind of putrefaction, 

 Galen, whose doctrine descended to recent times, and was zealously sup- 

 ported by Grew and Santareil^ ascribed the eflect to concoction, pro- 

 duced, like the ripening and softening of fruits beneath a summer sun, by the 

 high temperature of the stomach frr.rrt causes just pointed out. Pringle 

 and Macbride advocated the doctrine of fermentation, thus unitipg the 

 two causes of heat and putrefaction assigned by the Greek writers ; while 

 BorelH, Keil, and Pitcairn resolved the entire process into mechanical 

 action, or trituration ; thus making the muscular coating of the stomach 

 an enormous mill-stone, which Dr. Pitcairn was extravagant enough to 

 conceive ground down the food with a pressure equal to a weight of not 

 less than a hundred and seventeen thousand and eighty pounds, assisted at 

 the same time, in its gigantic labour, by an equal pressure derived from the 

 surrounding muscles * 



Each of these hypotheses, however, was encumbered with insuperable 

 objections; and it is difficult to say which of them was most incompetent 

 to explain the fact for which they werf' invented. 



Boerhaave endeavoured to give them force by interunion, and hence 

 combined the mechanical theory of pressure with the chemical theory of 

 concoction ; while Haller contended for the process of maceration. But 

 still a something else was found wanting, and continued to be so till Chesel- 

 den in lucky hour threw out the hint, for at first it was nothing more than 

 a hint, of a menstruum secreted into some part of the digestive system ; a 

 hint which was soon eagerly laid hold of, and successfully followed up by 

 Haller, Reaumur, Spallanzani, and other celebrated physiologists. And 

 though Cheselden was mistaken in the peculiar fluid to which he ascribed 

 the solvent energy, namely the saliva, still he led forward to the important 

 fact, and the gastric juice was soon afterwards clearly detected, and its 

 power incontrovertibly established. 



This wonderful menstruum, the most active we are acquainted with in 

 nature, is secreted by a distinct set of vessels, that exist in the texture of 

 the stomach, and empty themselves into its cavity by innumerable orifices 

 invisible to the naked eye ; and it is hence called gastric juice, from yccTTvip^ 

 which is the Greek for stomach. Mr. Cruickshank supposes about a 



/ * See Ser. I. Lect. X. 



