METABOLISM 89 



The flow of half-digested food into the intestine 

 mixed with hydrochloric acid (chyme) that is poured 

 stimulates the cells of the lining membrane of the 

 latter to produce a substance called secretin, which, 

 passing into the blood, is carried to the pancreas. 

 Here it stimulates the excretion of pancreatic juice, 

 which flows out into the intestine, apparently in 

 direct response to the inflow of food, but really in 

 response to another sort of stimulus. A chemical 

 excitant like secretin, or thyroiodin, or the other 

 products just described, has been called a hormone, 

 and it is likely that the number and importance of 

 such substances will be greatly increased by further 

 investigation. They are the products of proto- 

 plasmic activity, like the zymogens, but, unlike 

 them, they are not destroyed by boiling, even in 

 hydrochloric acid. 



