The Frog — An Amphibious Vertebrate Animal 429 



best in a definite acid or alkaline environment. If too much acid or too 

 much alkali is present, a specific enzyme may not function, while it might 

 do so if the acid-alkaline reaction were changed to its specific optimum. 

 They are powerful chemical substances because a small amount may pro- 

 duce a large reaction. Their chemical composition is unknown, but they 

 are probably of protein makeup. They cause chemical changes in other 

 substances without, or with very slight, destruction of their own substance. 

 Many, if not all, enzymes may be stored in an inactive state in cells until 

 they are needed later. 



Changes Which Foods Undergo: In the mouth there are no mastica- 

 tion, no digestion, and no enzymes (no glands). In the esophagus cer- 

 tain glands produce an alkaline mucous secretion which becomes active 

 when mixed with the acid gastric juice secreted by the glands in the walls 

 of the stomach. The cardiac end of the stomach has long, tubular, 

 branched, deeply set glands for the secretion of mucus. The pyloric end 

 of the stomach has short, tubular, shallow glands for secreting gastric 

 juice which contains the enzyme pepsin and about 0.4 per cent hydro- 

 chloric acid (HCl). In other words, the reaction is as follows: 



Pepsin + HCl + Proteins —> Soluble Peptones 



After the partially digested foods pass the pyloric valve from the stom- 

 ach into the duodenum, they are mixed with the alkaline pancreatic juice 

 which is secreted by the pancreas and brought to the duodenum by the 

 pancreatic ducts. The alkalinity of the pancreatic juice is due to sodium 

 carbonate (Na2C03). The three specific enzymes of the pancreatic 

 juice are (1) amylopsin, (2) trypsin, and (3) steapsin. Their specific 

 actions are shown: 



Amylopsin + Starch -^ Maltose (double sugar) 

 Trypsin + Proteins or Peptones (in an alkaline 



reaction) -^ Amino Acids 

 Steapsin + Fats — > Glycerin + Fatty Acids 



The hepatic cells of the tubular glands of the liver secrete a green- 

 ish bile which is stored in the gall bladder until needed. The bile is 

 mixed with the pancreatic juice in the common bile duct before they 

 enter the duodenum. Certain bile enzymes convert fats, when in an 

 alkaline environment, into a soapy emulsion capable of osmosing through 

 the intestinal walls into the blood and lymph systems. The liver also 

 stores glycogen or animal starch (CeHioOs)!!. This is changed by cer- 

 tain liver enzymes into usable sugar when needed. Wastes are also elimi- 

 nated with the bile from the liver. 



