vi INTESTINE AS AN ORGAN OF EXCRETION 365 



from the digesting part of the intestine into the large bowel, 

 which acts as a sort of reservoir, while the valves hinder any 

 reflux that might disturb the digestive processes still going ou 

 throughout the small intestine. However acceptable this theory 

 may be from the mechanical point of view, it is less valid for the 

 chemist. The digestive processes effected by the juices with 

 which the faeces are saturated can go on in the large intestine 

 also. In fact, the fermentative and putrefactive processes set up 

 by the intestinal bacteria are continued, and even reach their 

 maximal development, in the large bowel. For while the con- 

 tents of the ileum are slightly alkaline, neutral, or faintly acid, 

 the reaction of the large intestine in carnivora, and in man, is 

 always decidedly acid. This depends not on any acid property of 

 the secretion of the mucous membrane of the large bowel, but 

 upon the acid fermentation developed in its contents, as proved 

 by the development of methane and hydrogen which, as we have 

 seen, is conspicuous in the large intestine. We know, on the 

 other hand, that although the large intestine is provided along its 

 entire length, from the vermiform appendix to the anus, with 

 innumerable crypts (see Fig. 44, p. 125) its secretion is alkaline, 

 and rich in mucin, with no constituent capable of acting chemi- 

 cally upon the three main groups of food-stuffs. All the chemical 

 changes in the large intestine (with the exception of such as may 

 be due to the enzymes from the small intestine, before the acidity 

 of the medium arrests their activity) must therefore be caused by 

 the intestinal bacteria. 



In all probability the chemical changes that take place in the 

 large intestine are unimportant in the carnivora and in man, but 

 they have an enormous importance in the herbivorous rodents, 

 and for solipeds, in which the caecum is much larger than the 

 stomach. The contents of the caecum in the horse and in 

 ruminants, unlike those of the carnivora and of man, are always 

 very abundant ; not acid, as stated by Tiedemann, Gmelin, and 

 others, but always decidedly alkaline, even (according to numerous 

 observations of Colin) more strongly alkaline than the contents of 

 the different parts of the small intestine and the entire colon, 

 where the reaction becomes gradually acid owing to the fermenta- 

 tion which gives rise to formation of lactic, butyric, and other 

 fatty acids. According to Colin, the alkaline fluid which saturates 

 the contents of the caecum is derived principally from the mixture 

 of digestive secretions poured out into the small intestine, and 

 only to a minimal extent from the mucous secretion of the crypts 

 of the caecum. Since the incompletely digested food-stuffs remain 

 for a long time in the caecum, it is evident that the digestive 

 processes initiated in the small intestine must be continued there. 



Colin's logical conclusion was experimentally confirmed by 

 Paladino (1875), who demonstrated in a series of researches into 



