Chap. XV] DIGESTIVE PROCESSES 297 



of food starts the gastric secretion, so the presence of acid chyme in 

 the intestine starts the pancreatic secretion. This effect is due to 

 a special substance called secretin which is formed by the action 

 of the acid upon some substance present in the mucous membrane 

 of the intestine. This secretin is absorbed by the blood and carried 

 to the pancreas, which it stimulates to activity. Like the secretin 

 of the gastric juice, this is not an enzyme but a hormone. 



Pancreatic juice. — Healthy pancreatic juice is a clear, some- 

 what viscid fluid, with a very decided alkaline reaction. The 

 amount secreted in twenty-four hours is about 15 to 25 ounces 

 (7.1 to 11 litres). It contains few solids and is dependent for its 

 remarkable power on three enzymes : (1) trypsin, (2) diastase 

 (amylopsin), and (3) lipase (steapsin). 



Action of pancreatic juice upon food. — Pancreatic juice has 

 the power of acting on all the food-stuffs, proteins, carbohydrates, 

 and fats. This action is due to its enzymes. 



(1) Trypsin. — Tr^^Dsin, like pepsin, has the power to decompose 

 proteins, but the action is more rapid and more powerful, and the 

 protein molecule is broken up into simpler substances than pep- 

 tones, depending on the amount of trypsin and the time that it acts. 

 If complete hydrolysis takes place, the end products consist 

 chiefly of amino-acids. Unlike pepsin, trypsin requires a neutral or 

 alkaline medium. The preliminary action of pepsin, on a protein 

 molecule, hastens the action of trypsin, and renders it more com- 

 plete than if the trypsin acted alone. 



Diastase {Amylopsin). — The action of diastase is similar to 

 that of ptyalin. It causes hydrolysis of starch with the produc- 

 tion of maltose. The starchy food that escapes digestion in the 

 mouth and stomach becomes mixed with this enzyme and con- 

 tinues under its action until the ileo-caecal valve is reached. 



Lipase (Steapsin). — Lipase is an enzyme capable of decom- 

 posing fats. This action is twofold : 



(a) It emulsifies them. 



(6) It splits them up into fatty acids and glycerine. 



(a) If we shake up olive oil with water, the two cannot be got 

 to mix : as soon as the shaking ceases, the oil floats to the top ; 

 but if we shake up olive oil with pancreatic juice, the oil remains 

 evenly suspended in it. The reason of this is that the oil has been 

 minutely divided into tiny droplets, and each droplet surrounded 



