GENERAL PHYSIOLOGY OF THE FROG 17 



tion, the carbohydrates, fats, and proteins are too complex to be absorbed and 

 used by the animal, and must be broken down into simpler substances before 

 they can be utilized. The splitting of these organic foods into simpler, utilizable 

 substances is the process of digestion, and the performance of this process is the 

 function of the alimentary tract and its glands. Digestion is brought about 

 by means of certain substances, called enzymes, which are manufactured in the 

 walls of the alimentary tract and in the digestive glands, chiefly the pancreas. 

 By means of these enzymes, organisms are able to produce chemical changes in 

 foods which cannot be imitated in the laboratory at all or which can be imitated 

 only by the use of boiling temperatures and reagents which would be fatal to 

 life. Neither the chemical nature of enzymes nor the mode of their action is 

 known, but it is probable that they attach themselves either physically or 

 chemically to the molecules of the substance upon which they act, thus upsetting 

 the equilibrium within the molecule, and causing it to fall into fragments, 

 whereupon the enzyme is set free again unchanged. In general each enzyme 

 is capable of acting upon only one substance or class of substances. Thus, 

 enzymes which split up proteins are called proteases; those which split starches 

 and sugars are diastases; and those which split fats are Upases (see Holmes, 

 chap, vii, pp. i34~3 8 > J 42, i5 6 > l6 3)- 



In the following experiments, the action of each of these three general kinds 

 of enzymes is demonstrated. As it is rather impractical to obtain enzymes from 

 the frog, human and pig enzymes having the same action are used instead. 



1. Action of a protease pepsin. 



a) The gastric juice : This digestive fluid is secreted by glands located in the 

 wall of the stomach. It contains about 0.4 per cent hydrochloric acid, an enzyme 

 called pepsin, and a number of salts. An "artificial" gastric juice is readily 

 made by adding 0.4 per cent hydrochloric acid to commercial dried pepsin, gen- 

 erally obtained from the hog's stomach. 



b} Action of pepsin: Into a test tube put 5-10 c.c. of artificial gastric juice, 

 and into another 5-10 c.c. of 0.4 per cent hydrochloric acid. Add to each a small 

 quantity of boiled white of egg, cut into very small pieces. Place in a water bath 

 or incubator kept at 37 C. (why?) for at least two hours. What becomes of the 

 protein? Is the acid alone without the pepsin capable of producing this effect? 

 Pepsin from the frog's stoma v ch has the same action (Holmes, p. 142). 



2. Action of a lipase pancreatic lipase. 



a) Reaction of milk : To 10 c.c. of milk in a test tube add a few drops of neutral 

 litmus solution, or test with red and blue litmus paper. Litmus is a vegetable 

 dye, which is pink in acid solution, blue in alkaline solution, and purplish in 

 neutral solution. Is milk acid, alkaline, or neutral? 



V) Action of pancreatic lipase: A solution of this enzyme is obtained by 

 dissolving dried pancreas, sold commercially as pancreatin, in a slightly alkaline 

 solution. Add a few cubic centimeters of this solution to the litmus-containing 



