308 THE TISSUES AND MECHANISMS OF DIGESTION. 



stomach, very little work apparently being left for the intestines ; that is to 

 say, the larger part of the meal is reduced in the stomach to actual solution 

 and a considerable quantity is probably absorbed directly from the stomach. 

 In such cases the amount of peptone found in the stomach during the diges- 

 tion of the meal is found to be fairly constant, from which it may be in- 

 ferred that the peptone is absorbed as soon as it is formed. There is also 

 evidence that fat may to a 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 

 labors 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 duodena) 

 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 the 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 may 

 be 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 upward 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. 



241. The semi-digested acid food, or chyme, as it passes over the 

 biliary orifice, causes, as we have seen ( 223), 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 pan- 

 creatic juice, tend to neutralize the acidity of the chyme, but the con- 

 tents of the duodenum do not become distinctly alkaline until some distance 

 from the pylorus is reached. The rapidity with which the change in the 

 reaction is completed is not the same in all animals, and in the same animal 



