248 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



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 inverting 

 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 the inverting 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 prseter- 

 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 some 

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

 the large intestine ; but the changes here of most interest are the absorption 

 which takes place and the bacterial decompositions. The latter are described 

 briefly below. 



Bacterial Decompositions in the Intestines. Bacteria of different 

 kinds have been found throughout the alimentary canal from the mouth to 

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

 acid reaction prevents the action of those putrefactive bacteria which decompose 

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

 fermentation in the carbohydrates. Under certain abnormal conditions 

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

 carbohydrates may be pronounced, but this must be -considered as pathological. 



In the small intestine the secretions are all alkaline, and it was formerly 

 taken for granted that the intestinal contents are normally alkaline. If this were 

 so the bacteria would find a favorable environment. It was supposed that putre- 

 faction of the proteids must certainly occur, especially during the act of tryptic 

 digestion, and this supposition was borne out by the extraordinary readiness of ar- 

 tificial pancreatic digestions to undergo putrefaction when not protected in some 

 way. Two recent cases 1 of fistula of the ileum at its junction with the colon in 

 human beings have given opportunity for exact study of the contents of the 

 small intestine. The results are interesting, and to a certain extent are opposed 

 to the preconceived notions as to reaction and proteid putrefaction which have 

 just been stated. They show that the contents of the intestine at the point 

 where they are about to pass into the large intestine are acid, provided a mixed 



1 Macfadyen, Nencki, and Sieber: Archiv fur expermentelle Pathologic u. Pharmakologie, 1891, 

 vol. 28, p. 311 ; Jakowski : Archives des Sciences biologiques, St. Petersburg, 1892, vol. 1. 



