CHEMISTRY OF DIGESTION AND NUTRITION. 309 



C 12 T!„O u + H a O = C 6 H 12 6 + C f> H 12 6 . 



Maltose. Dextrose. Dextrose. 



C 12 H 22 O u + H 2 = C fi H,A + C 6 H 12 6 . 



Cane-sugar. Dextrose.. Levulose. 



C 12 H 22 O n + H 2 = C 6 H 12 6 + 6 H 12 ( ),, 



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 sugar-splitting enzymes, and the maltose, and ap- 

 parently 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 ; 

 and this conclusion falls in with the fact that the sugar found normally in 

 the blood exists always in the form of dextrose. With reference to the 

 sugar-splitting enzymes found in the small intestine, it should be added that 

 they occur more abundantly in the mucous membrane than in the secretion 

 itself. Indeed, the secretion is normally so scanty, especially in the upper 

 part of the intestine, that it cannot be supposed to do more than moisten the 

 free surface, and it is probable that the action of these enzymes takes place 

 upon or in the mucous membrane, as the last step in the series of digestive 

 changes of the carbohydrates immediately preceding their absorption. 



Digestion in the Large Intestine. — Observations upon the secretions of 

 the large intestine have been made upon human beings in cases of anus praeter- 

 naturalis in which the lower portion of the intestine (rectum) was practically 

 isolated. These observations, together with those made upon lower animals, 

 unite in showing that the secretion of the large intestine is mainly composed 

 of mucus, as the histology of the mucous membrane would indicate, and that 

 it is very alkaline, and probably contains no digestive enzymes of its own. 

 When the contents of the small intestine pass through the ileo-csecal valve into 

 the colon they still contain a quantity of incompletely digested material mixed 

 with the enzymes of the small intestine. It is likely, therefore, that seme 

 at least of the digestive processes described above may keep on for a time in 

 the large intestine; but the changes hereof most interest are the absorption 

 that takes place and the bacterial decompositions. The latter arc described 

 briefly below. 



Bacterial Decompositions in the Intestines. — Bacteria of different 

 kinds have been found throughout the alimentary canal from the mouth t<> 

 the rectum. In the stomach, however, under normal conditions, the strong 

 acid reaction prevents the action of those putrefactive bacteria that decom- 

 pose proteids, and prevents or greatly retards the action of those thai set up 

 fermentation in the carbohydrates. Under certain abnormal conditions 

 known to ns under the general term of dyspepsia, bacterial fermentation of 

 the carbohydrates may be pronounced, l>nt this must be considered as path- 

 ological. 



