70 THE INDIVIDUAL ORGANISM 



The principal substances secreted by the pancreas upon stimulation 

 by secretin are water and the inorganic constituents of the pancreatic 

 juice. The pancreas is also under control of the vagus nerve, which plays a 

 large part in many visceral reflexes, particularly in glandular control. 

 Stimulation of the vagus nerve by the entry of chyme into the duodenum 

 reflexly stimulates the cells of the pancreas, and this nervous stimulus is 

 chiefly responsible for the secretion of the enzymes in the pancreatic juice. 



The secretion from the pancreas (pancreatic juice) contains three 

 important enzymes: trypsin, 1 which digests proteins into polypeptids or 

 even into amino acids; steapsin (also called pancreatic lipase), which 

 digests fats into fatty acids and glycerol; and amylopsin, which changes 

 starches (and glycogen) into disaccharides. Steapsin has only a weak 

 reaction until it has been activated by the bile salts in the intestine. 

 Other enzymes are present in the intestinal juice that is secreted by the 

 mucosa of the intestine itself. These include erepsin, which acts upon the 

 peptones, proteoses, and polypeptids to form amino acids; and the invert- 

 ing enzymes — maltase, lactase, and invertase — which split the disaccharides 

 maltose, lactose, and cane sugar, respectively, into monosaccharides. 



Bile is secreted continuously by the liver and stored in the gall bladder ; 

 secretin merely increases the rate of its production. Discharge of bile 

 into the intestine is caused by contraction of the walls of the gall bladder 

 in response to stimuli produced by entry of chyme into the intestine. As 

 in the pancreas, these stimuli are in part nervous, in part chemical. The 

 contact of fat or of acid with the walls of the duodenum apparently 

 liberates another hormone, related to but not the same as secretin; this 

 hormone causes contraction of the gall bladder, forcing bile into the 

 intestine. 



Although the bile contains no enzymes, it plays an important part both 

 in digestion 2 and in the absorption of digested fats. In both these proc- 

 esses the bile salts are the effective agents. Besides activating steapsin, 

 they emulsify the fats in the intestine, breaking them up into minute 

 particles and enormously increasing the surface exposed to enzyme action. 

 Indirectly they aid also in the digestion of proteins and carbohydrates, by 

 removing the fat from the surface of particles of these substances and thus 

 exposing them to more efficient enzyme action. The role of bile in absorp- 

 tion is mentioned below. 



The enzymes of the intestine all work best in a slightly alkaline to 

 neutral medium; but chyme, as we have seen, is quite strongly acid. 



1 This is secreted by the pancreas in the form of an inactive precursor, trypsinogen, 

 which is changed to active trypsin by an activator, enterokinase, that is secreted by 

 the intestine. 



2 Bile is also in part an excretion, the bile pigments representing waste products 

 from the breakdown of hemoglobin. 



