CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 499 



certain extent undergo in the stomach changes leading to emulsion, 

 similar to those which, as we shall see, are carried out in the small 

 intestine. 



But such cases as these cannot be regarded as typical cases of 

 gastric digestion, and in man, at all events, living on a mixed diet 

 the work of the stomach appears to be to a large extent preparatory 

 only to the subsequent labours of the intestine. It is true that 

 our information on this matter is imperfect, being chiefly drawn 

 from the study of cases of gastric or duodenal fistula, in which 

 probably the order of things is not normal, or being in large 

 measure deductions from experiments on animals, whose economy 

 in this respect must be largely different from our own ; but we are 

 probably safe in concluding that, in ourselves, the chief effect of 

 gastric digestion is by means of the disintegration spoken of above 

 to reduce the lumps of food to the more uniform chyme and so 

 to facilitate the changes which take place in the small intestine. 

 During that disintegration some of the proteid in the meal is con- 

 verted into peptone ; and some at least of the peptone so formed 

 is probably absorbed at once ; but much proteid remains unchanged 

 or at least is not converted into peptone, and the fats and starches 

 undergo in themselves very little change indeed. 



In the act of swallowing, no inconsiderable quantity of air is 

 carried down into the stomach, entangled in the saliva, or in the 

 food. This is sometimes returned in eructations. When the gas of 

 eructation or that obtained directly from the stomach is examined, 

 it is found to consist chiefly of nitrogen and carbonic acid, the 

 oxygen of the atmospheric air having been largely absorbed. In 

 most cases the carbonic acid is derived by simple diffusion from the 

 blood, or from the tissues of the stomach, which similarly take up 

 the oxygen. In many cases of flatulency, however, it may arise 

 from a fermentative decomposition of the sugar which has been 

 taken as such in food or which has been produced from the starch, 

 the gas being either formed in the stomach or passing upwards 

 from the intestine through the pylorus. 



The enormous quantity of gas which is discharged through the 

 mouth in cases of hysterical flatulency, even on a perfectly empty 

 stomach, and which seems to consist largely of carbonic acid, 

 presents difficulties in the way of explanation ; it is possible that 

 it may be simply diffused from the blood, but it is also possible 

 that in many cases it is derived from air which the patient has 

 hysterically swallowed, the oxygen having been removed, in the 

 stomach, by absorption and replaced by carbonic acid. 



In the Small Intestine. 



280. The semi-digested acid food, or chyme, as it passes 

 over the biliary orifice, causes as we have seen ( 253) gushes of 

 bile, and at the same time the pancreatic juice flows into the 



