CHAP, i.] DIGESTION. 295 



been found fairly constant. From this it may fairly be inferred 

 that the peptone is absorbed in proportion as it is formed. 



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 fermen- 

 tative 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. 



In the small intestine, the semi-digested acid food, or chyme, 

 as it passes over the biliary orifice, causes gushes of bile, and at the 

 same time, as we have seen (p. 265), the pancreatic juice, which 

 flowed freely into the intestine at the taking of the meal, is 

 secreted again with renewed vigour, when the gastric digestion is 

 completed. These two alkaline fluids 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 

 reached. Even in the lower part of the ileum the chyme may be 

 acid ; possibly however in such cases it has been reacidified in con- 

 sequence of acid fermentations taking place in the intestinal 

 contents. The reaction of these contents appears to vary in fact 

 according to the nature of the food, the changes which it under- 

 goes, and other circumstances. Moreover it is probably not the 

 same in all animals. In a dog fed on starch and fat, the contents 

 of the intestine may remain acid throughout. 



The conversion of starch into sugar, which as we have seen is 

 probably arrested in the stomach, is resumed with great activity 

 and indeed completed by the pancreatic juice, possibly assisted by 

 the succus entericus ; portions however of still undigested starch 

 may be found in the large intestine and even at times in the faeces. 



The pancreatic juice, as we have seen, emulsifies fats, and also 

 splits them into their respective fatty acids and glycerine. Tlio 

 fatty acids thus set free become converted by means of the alkalino 

 contents of the intestine into soaps ; but to what extent saponifica- 

 tion thus takes place is not exactly known. Undoubtedly soaps 

 have to a small extent been found both in portal blood and in the 



