CHEMISTRY OF DIGESTION AND NUTRITION. 247 



started by a stimulus of some kind. According to Rohrnann, 1 it varies in 

 quantity in different parts of the small intestine, being very scanty in the upper 

 part and more abundant in the lower. The intestinal secretion is a yellowish 

 liquid with a strong alkaline reaction. The reaction is due to the presence of 

 sodium carbonate, the quantity of which is about 0.25 to 0.50 per cent. The 

 chemical composition of the secretion has not been satisfactorily determined, 

 but its digestive action has been investigated with success. Upon proteids and 

 fats it is said to have no specific action that is, it contains neither a proteolytic 

 nor a fat-splitting enzyme. The possible value of its sodium carbonate in aiding 

 the emulsification of fats has been referred to in the preceding paragraph. 

 Upon carbohydrates the secretion has an important action. In the first place, 

 it has been shown that it contains an amylolytic enzyme which is more abun- 

 dant in the upper than in the lower part of the intestine. This enzyme doubt- 

 less aids the amylopsin of the pancreatic secretion in converting starches to 

 sugar (maltose) or sugar and dextrin. What is still more important, however, 

 is the presence of inverting enzymes capable of converting cane-sugar (saccha- 

 rose) into dextrose and levulose, and of a similar enzyme capable of changing 

 maltose (or dextrin) to dextrose. Both of these effects are examples of the 

 conversion of di-saccharides to mouo-saccharides. 



The di-saccharides of importance in digestion are cane-sugar, milk-sugar, 

 and maltose. The first of these forms a common constituent of our daily diet ; 

 the second occurs always in milk ; and the third, as we have seen, is the main 

 end-product of the digestion of starches. These substances are all readily 

 soluble, and we might expect that they would be absorbed directly into the 

 blood without undergoing further change. As a matter of fact, however, it 

 seems that they are first dissociated under the influence of the inverting enzymes 

 into simpler mono-saccharide compounds, although in the case of lactose this 

 statement is perhaps not entirely justified, our knowledge of the fate of this 

 sugar during absorption being as yet quite incomplete. According to some 

 authors, lactose is absorbed unchanged (see Chemical section). The general 

 nature of this change is expressed in the three following reactions : 



C 12 H 22 U + H 2 = C 6 H 12 6 + C 6 H 12 6 . 



Maltose. Dextrose. Dextrose. 



0,^0,, + H 2 = C 6 H 12 6 + C 6 H 12 6 . 



Cane-sugar. Dextrose. Levulose. 



C 12 H 22 O n + H 2 = C 6 H 12 6 + C 6 H 12 6 . 



Lactose. Dextrose. Galactose. 



For the reactions by means of which these different isomeric forms of sugar are 

 distinguished reference must be made to the Chemical section. The final stage 

 in the artificial digestion of starches is the formation of maltose or of a mixture 

 of maltose and dextrins. In the intestines, however, the process is carried 

 a step farther by the aid of the inverting enzymes, and the maltose, and appar- 

 ently the dextrins also, are converted into dextrose. According to this descrip- 

 tion, all of the starch is finally absorbed into the blood in the form of dextrose ; 

 1 Pfliiger's Archivfiir die gesammte Physiologic, 1887, vol. 41, p. 411. 



