DIGESTION AND THE DIGESTIVE AGENTS. 349 



practical purity. The best success in obtaining it, is made 

 by an operation called a Thiry fistula, which consists in 

 cutting out from the course of the intestine a loop, and then 

 sewing the cut ends together again. In that way the food 

 is again enabled to pass uninterruptedly along the intestine, 

 save that the intestine has been shortened by the loop re- 

 moved. The two cut ends of this loop are then sewed into 

 the abdominal wall so as to connect with the exterior. The 

 nerves and blood vessels of this cut loop are left untouched, 

 and so when food passes along the intestine, secretion is 

 also induced in this separated piece. This secretion is then 

 collected and studied. 



Such a fluid is light yellow and strongly alkaline. It 

 possesses traces of albumin and is a little heavier than 

 water. The alkalinity of this solution is due to the pres- 

 ence of a relatively large amount of sodium carbonate, but 

 the general chemical constitution further than that is not 

 known. In spite of statements by some physiologists to 

 the contrary, there is no satisfactory evidence that the in- 

 testinal juice exerts any action whatever upon proteids or 

 fats. Upon starches it has a slight effect. It contains a 

 ferment much like the amylopsin of the pancreas which 

 changes starch into sugar. However, on account of the 

 scantiness of the intestinal juice this figures very little in 

 ordinary digestion. The important use of the intestinal 

 juice seems, however, to be its action upon the sugars. It 

 contains a ferment capable of converting cane sugar, mal- 

 tose and lactose into ordinary glucose or grape sugar. The 

 sugars which occur regularly in our diet are ordinary cane 

 sugar (the sugar of commerce) , lactose (the sugar in sweet 

 milk), and maltose, the sugar into which the starches 

 eaten have been changed by the ptyalin and the amylopsin. 

 None of these sugars are, however, found in the blood. 

 The sugar in the blood is glucose or grape sugar, and it 

 was long a question just how the sugars of the diet were 

 changed into the sugar of the blood. This change occurs 

 in the wall of the intestine under the action of the intestinal 



