CHAP. XII.] THE PROCESSES OCCURRING IN THE COLON. 451 



1 per cent. The sugar exhibits much greater variations than the 

 albumin, varying between 0'3 and 4*75 per cent. 



Macfadyen, Nencki and Sieber estimate that from 30 42 per 

 cent, of the solid matters of the intestinal contents entering the colon 

 are composed of albuminous matters (including native albumins, 

 albumoses and peptones, insoluble albuminous and albuminoid 

 matters), about 8'5 per cent, of fats, about 45 per cent, of carbo- 

 hydrates and of substances soluble in alcohol, and about 8'5 per cent, 

 of mineral matters. They found no indol and neither leucine nor 

 tyrosine in the intestinal contents which had passed through the 

 whole length of the small intestines. 



The facts which we have passed under review bring very clearly 

 before us that as the intestinal contents pass from the ileum into the 

 colon, they yet contain considerable quantities of alimentary con- 

 stituents which are not only capable of absorption but which, as 

 analyses of the faeces teach us, are actually absorbed in the colon. 



Although, as we have seen, processes of fermentation are ripe in 

 the small intestine, they are almost entirely confined to the carbo- 

 hydrates, and scarcely affect the albuminous substances. The acids, 

 especially lactic acid, developed by the action of the various micro- 

 organisms on the sugars are unfavourable to the action of putrefactive 

 bacteria on the proteids an action which is essential to the changes 

 which shall convert the unabsorbed intestinal residue into the f wees. 



SECT. 3. THE FINAL DIGESTIVE PROCESSES IN THE LARGE IN- 

 TESTINE. ITS POWERS OF ABSORPTION. THE PROCESSES WHICH 



CONVERT THE CONTENTS OF THE COLON INTO FAECES. 



Thesecretion We have seen that the intestinal juice poured out 



of Lieberkiihn's by the glands of the small intestine is characterised by 

 glands in the a highly alkaline reaction, which depends upon the 

 presence of sodium carbonate, and by the presence in it 

 of enzymes capable of acting only on certain of the carbohydrates 

 (sugars). In spite, however, of this alkaline reaction of the juice 

 poured out in the small intestine, the contents possess a marked 

 acid reaction, due to the products of fermentation engendered by the 

 action of micro-organisms on the sugars. 



The mucous membrane of the colon possesses, according to 

 Nencki, Macfadyen and Sieber, a more powerfully alkaline reaction 

 than that of the small intestine. Presumably its secretion consists of 

 a mucin-containing liquid rich in sodium carbonate. All evidence, 

 as we shall point out, seems to shew conclusively that it contains no 

 enzymes. 



The processes of the large intestine which are of importance in 

 so far as the nutrition of the organism is concerned, are chiefly 

 processes of absorption ; though, perhaps to a less extent than in the 



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