DIGESTION. 687 



The residue left on the filter consists of bilirubin. Purify it by washing 

 with water, drying, and heating the mass with chloroform. On filtering and 

 evaporating the solution in a watch-glass rhombic plates or prisms of bilirubin 

 are left, which examine microscopically, and to which apply the tests men- 

 tioned above. 



Succus entericus. The small intestine secretes several important 

 enzymes : erepsin, which acts mainly upon the proteoses and peptones, 

 splitting them into peptides and amino-bodies; inverting enzymes act- 

 ing upon the disaccharides (invertase, maltase, lactase) ; and entero- 

 kinase, which is necessary for tryptic digestion. In addition the 

 small intestine forms the prosecretin, from which the stimulating 

 hormone secretin is derived and carried to the pancreas. The succus 

 intestinalis is alkaline, and aids in neutralizing the acid from the 

 stomach. 



Fermentative and putrefactive changes. In addition to the 

 alterations brought about by the digestive enzymes, the food also 

 undergoes fermentative and putrefactive changes, due to the action 

 of bacteria, always present in the intestine. Some of these bacteria 

 convert carbohydrates into acetic, butyric, lactic, and succinic acids, 

 while carbon dioxide, methane, and hydrogen are also liberated. 

 Certain fats probably form neurine and similar toxic substances. 

 By putrefaction of proteins are formed : phenol, several aromatic 

 derivatives, notably indole and skatole, volatile fatty acids, carbon 

 dioxide, methyl-mercaptan, and hydrogen sulphide. 



As intermediate products, the bacteria convert to some extent the 

 food material into the same substances which are formed by the 

 action of pancreatic juice ; these products, however, are not useful 

 to the organism, but are only intermediate stages of far-reaching 

 decompositions. The end-products of bacterial action pass out of 

 the intestine in the feces and as flatus, or are absorbed and carried 

 to the liver, where most of the aromatic compounds are conjugated 

 with potassium acid sulphate, and in this form are secreted in the 

 urine. Thus the quantity of aromatic sulphates in the urine is a 

 measure of putrefaction in the intestine. 



Absorption in the small intestine. It seems advisable under 

 this heading to follow in outline the foodstuffs from their ingestion 

 to their delivery into the circulation. 



The carbohydrates are first acted upon by the saliva, ptyalin split- 

 ting the starches into maltose, and maltase splitting the maltose into 

 dextrose. While this action continues for some time in the cardiac 

 end of the stomach, it is, in all, not very extensive. There is no 



