CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 473 



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 

 converted into peptone ; and 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 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 

 intestine freely. These two alkaline fluids, especially the more 

 strongly and constantly alkaline pancreatic j nice, tend to neutralize 

 the acidity of the chyme, but the contents of the duodenum do not 

 become distinctly alkaline until some distance from the pylorus is 



