128 ON THE DIGESTIVE FUNCTION, &c. 



From this brief survey 'of the process of digestion it is obvious that the 

 stomach itself performs by far the principal part ; in some animals, indeed, 

 it appears to perform the whole ; and it is hence necessary that we examine 

 the general structure and powers of this organ with a little more minuteness. 



In man the stomach is situated on the left side of the midriff; in its figure 

 it resembles the pouch of a bag-pipe ; its left end is most capacious ; its 

 upper side is concave, its lower convex ; and the two orifices for receiving 

 and discharging the food are both situated in the upper part. In its substance 

 it consists of three distinct coats or layers, the external and internal of which 

 are membranous, and the middle muscular. The internal coat, moreover, is 

 lined with a villous or downy apparatus, and is extremely convoluted or 

 wrinkled ; the wrinkles increasing in size as the diameter of the stomach 

 contracts. 



From what I have already observed, it must appear that the process of 

 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 ; chymifica- 

 tion, or its reduction into pulp, which is the office of the stomach ; and chy- 

 lification, 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 cer- 

 tain 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 orifices 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 appears 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 supported by Grew 

 and Santarelli, ascribed the effect to concoction, produced, like the ripening 

 and softening of fruits beneath a summer sun, by the high temperature of the 

 stomach from causes just pointed out. Pringle and Macbride advocated the 

 doctrine of fermentation, thus uniting the two causes of heat and putrefac- 

 tion assigned by the Greek writers ; while Borelli, 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 ob- 

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

 explain the fact for which they were invented. 



Boerhaave endeavoured to give them force by interunion, and hence com- 

 bined the mechanical theory of pressure with the chemical theory of concoc- 

 tion; while Haller contended for the process of maceration. But still a 

 something else was found wanting, and continued to be so till Cheselden 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, Reau- 

 mur, Spallanzani, and other celebrated physiologists. And though Chesel- 

 den 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 afterward clearly detected, and its power incontrover- 

 tibly established. 



* See Series i. Lecture x 



