522 DIGESTION IN THE INTESTINES [CH. XXXIV. 



substances secreted by such glands as the thyroid and suprarenal 

 must be included under this term, and the part played by carbonic 

 acid in the regulation of breathing (see p. 380) also comes into the 

 same category. In our study of gastric digestion, we have seen the 

 powerful peptogenic action of dextrin, a substance formed during 

 the salivary digestion of starch ; Edkins has given the name gastrin 

 to the special hormone which is the result of the action of the 

 salivary products on the gastric mucous membrane. Another 

 example of a hormone is furnished by the material formed in the 

 ovary, and which, passing into the maternal blood-stream, stimulates 

 the mammary gland to action. 



Adaptation of the Pancreas. To a certain degree it cannot be doubted that 

 the pancreas adapts its secretion to the work it has to do. Thus, whereas gastric 

 juice has a maximal flow soon after the ingestion of food, the pancreatic flow 

 does not attain its full force until some time later, that is, when it is wanted. The 

 view that this is due to the hormone named secretin, which is not formed until 

 the gastric contents enter the intestine, fully explains the reason for the delay. 



But Pawlow went further than this, and stated that the proportion of the 

 various enzymes of the juice was adapted to the proportions of proteins, carbo- 

 hydrates, and fats in the food taken. Considerable doubt has been cast on these 

 results, because of the failure to confirm one of the most remarkable instances of 

 such adaptation ; this is the power of the pancreas to secrete lactase (an enzyme 

 capable of hydrolysing lactose into glucoses). Normal pancreatic juice contains 

 no lactase, but certain observers stated that by feeding an animal on milk, the 

 pancreas could be educated to secrete it. Careful experiments by Plimmer have 

 recently shown this is not really so, and so much more stringent experimental 

 conditions will have to be imposed before the other cases of adaptation can be 

 considered proven. 



Internal Secretion of the Pancreas See Diabetes, next chapter. 



The Succus Bntericus. 



Succus entericus has been obtained free from other secretions by 

 means of a fistula. Thiry's method is to cut the intestine across in 

 two places; the loop so cut out is still supplied with blood and 

 nerves, as its mesentery is intact; this loop is emptied, one end is 

 sewn up, and the other stitched to the abdominal wound, and so a 

 cul-de-sac from which the secretion can be collected is made. The 

 continuity of the remainder of the intestine is restored by fastening 

 together the upper and lower portions of the bowel from which the 

 loop has been removed. Vella's method resembles Thiry's, except that 

 both ends of the loop are sutured to the wound in the abdomen. Fig. 

 360 illustrates the two methods. 



The succus entericus possesses the power of converting disaccha- 

 rides into monosaccharides. This power it owes to three enzymes. 

 Invertase or sucrase is the enzyme which inverts cane sugar that is, it 

 converts cane sugar into dextrose and laevulose. The original use of the 

 term " inversion " has been explained on p. 410. It may be extended to 

 include the similar hydrolysis of other disaccharides, although there 



