THE VERTEBRATE ANIMAL SUBPHYLUM VERTEBRATA 385 



splitting. The splitting of the disaccharide, maltose, will serve as 

 an example of this process: 



(Malt sugar) (Water) (Glucose) 



The two molecules of glucose formed are in a form for ready 

 absorption. 



Gastric Digestion. — The tubular gastric glands located in the 

 mucous layer of the stomach secrete the acid gastric juice which is a 

 solution of 0.2 to 0.5 per cent hydrochloric acid and two important 

 enzymes, pepsin and rennin. The pepsin when present in the acid 

 medium brings about the splitting of complex proteins into inter- 

 mediate proteoses and peptones. Be^inin causes the casein in milk 

 to coagulate. This is the first step in its digestion. It is claimed by 

 some that emulsified fats, such as cream, are partially digested by a 

 gastric lipase. The digesting mass or chyme in the stomach is con- 

 tinually churned and mixed by muscular activity of the walls. "When 

 it becomes saturated (0.4 per cent) with acid and has been reduced 

 to the consistency of soup, it is discharged through the pylorus. 



Intestinal Digestion. — When the chyme is ejected through the 

 pylorus into the duodenum, the hydrochloric acid stimulates cer- 

 tain cells of the intestinal lining, causing them to secrete into the 

 blood a substance of hormone nature, known as secretin. Upon 

 reaching the pancreas this secretin stimulates it to secrete the diges- 

 tive fluid, pancreatic juice, into the small intestine by way of the 

 pancreatic ducts. There is some evidence that secretin also stimu- 

 lates secretion in the liver. 



Pancreatic juice is a clear, watery, alkaline solution containing 

 inorganic salts (carbonates, etc.) and three enzymes; the protease, 

 trypsin, the diastase, am.ijlopsin, and the lipase, steapsin. These act 

 respectively on proteins and peptones, starches and sugars, and fats. 

 This protease is in the form of trypsinogen until it reaches the intes- 

 tine and is activated by an intestinal enzyme, enterokinase. Trypsin 

 completes the work begun by the pepsin in that it converts proteoses 

 and peptones into amino acids, but it also digests complex proteins 

 which have escaped the action of pepsin. It acts more rapidly and 

 efSciently than does pepsin. There are nineteen amino acids that 

 are regarded as hmlding stones of the protein molecule. In a com- 

 plex protein like casein, as many as sixteen of these amino acids will 



