90 NUTRITIONAL PHYSIOLOGY 



juice is produced by all parts of the extensive lining, but 

 more abundantly in the upper than in the lower segments. 

 The three secretions have some characters in common. 

 They are all alkaline in reaction, owing to the presence in 

 them of sodium carbonate. This confers upon them in a 

 considerable degree the power to neutralize acids. As 

 the acid chyme from the stomach meets the alkaline secre- 

 tions in the duodenum there must be more or less carbonic 

 acid gas evolved. This may be helpful to the digestive 

 process, since the tendency will be to lighten the texture 

 of the food particles, much as dough is lightened by the 

 agency of yeast. 



It is not merely the acid from the stomach which may be 

 combated by the alkali of the juices below; there are two 

 other sources of acid to be taken into account. One of 

 these is found in the bacterial fermentation, chiefly of 

 sugars, which goes on in the intestine. The second is the 

 entirely normal formation of free acids occurring in the 

 course of fat digestion. So far as the first class of acids are 

 neutralized the products are mainly lactates and buty- 

 rates; the fatty acids may be converted into soaps. There 

 is no guarantee of an exact proportionality between the 

 acids and the alkali, and it is impossible to say which will 

 be in excess in a particular part of the canal. Generally, 

 however, the resulting reaction of the mixture is not far 

 from the neutral point. 



The united volume of the three secretions is held to be 

 very large, but any estimate tends to mislead, since 

 throughout the length of the intestine we have the with- 

 drawal of water keeping pace approximately with its in- 

 flow. In a certain section selected for observation the 

 bulk of the contents may show little change during a long 

 period, and yet there may have been profuse secretion 

 entirely disguised by counterbalancing absorption. In 

 this consideration we see indicated an important service 

 shared by all the digestive secretions, that of supplying 

 liberal quantities of water to act as the solvent and carrier 

 of food-stuffs destined for absorption. If absorption were 



