DIGESTION IN THE SMALL INTESTINE 401 



It is claimed that the absorption of intestinal juice itself may also play 

 a part in exciting furthei' secretion. 



As regards nervous influence upon secretion, the evidence that 

 such exists is inconclusive. The result which follows division of the 

 nerves supplying an isolated portion of intestine is untrustworthy, 

 since the large amount of fluid then secreted but little resembles the 

 true juice, and is more akin to lymph. It is a result of the vasodila- 

 tation and congestion of the loop. 



The Uses of the Juice. By virtue of its mucus, it is protective 

 and lubricatory. The enterokinase plays an important part in th 

 activation of trypsinogen. Invertase inverts cane-sugar to dextrose 

 and levulose ; maltase converts maltose into two molecules of dextrose ; 

 lactase converts lactose into dextrose and galactose. The action of 

 the lipase and diastase are similar to those of pancreatic juice. As 

 extracts of the intestinal mucous membrane are distinctly more 

 active than the juice itself, it has been suggested that the enzymes 

 are present as such in the mucous membrane, and. may exert most 

 of their action intracellularly during absorpfion. This applies par- 

 ticularly to erepsin, an enzyme which acts upon proteoses and pep- 

 tones, and converts them into amino-acids and ammonia, Since 

 this enzyme occurs in all animal tissues, it may be inferred that 

 its action is mainly intracellular. It occurs in greatest amount in 

 the intestinal mucous membrane, next in the kidney, and then in 

 decreasing amount in the pancreas, spleen, liver, heart muscle, skeletal 

 muscle, and brain. The large amount in the intestinal mucous mem- 

 brane indicates its important function there. Perhaps, by the reversible 

 action of which enzymes are capable, it brings about the synthesis of 

 the products of protein digestion during their absorption through the 

 intestinal mucous membrane (see later, p. 424). 



Functions o! the Small Intestine. The functions of the small 

 intestine may be briefly reviewed as follows : In the upper part of the 

 intestine, into which the digestive fluids are poured, the function is 

 essentially digestive; in the middle and lower parts, the function is 

 in the main absorptive. By the time the contents of the small pass 

 into the large intestine, practically all the foodstuffs have been 

 absorbed that are going to be absorbed. Little or no absorption of 

 foodstuffs takes place in the large intestine. 



A certain amount of fermentation of carbohydrate, as the result 

 of bacterial activity, may occur under normal conditions in the small 

 intestine, but as a rule there is no putrefaction of proteins. 



26 



