PRINCIPLES OF ANIMAL BIOLOGY 17 



4. Metabolism. 



4a. Ingestion takes place in higher animals through the mouth. 



46. Secretion and Digestion. Digestive juices and enzymes are se- 

 creted by specialized cells which often unite to form glands. Examine 

 a cross-section of the stomach of a frog. Note the layer of cells, the 

 mucosa, lining the interior. Note the elongated nucleus near the base of 

 each cell. At frequent intervals the mucosa dips down into the under- 

 lying tissue in the form of slender tube-like pits. These pits are the 

 glands. Find a gland which is cut throughout its whole length. At 

 some depth in it note a group of clear vacuolated cells. If the section is 

 cut from the anterior (cardiac) end of the stomach, the gland will extend 

 much deeper than the group of clear cells. The gland is everywhere 

 composed of a single layer of cells around a slender open canal. Draw 

 (without stippling) a stomach gland. 



The glands of the stomach secrete hydrochloric acid and pepsin. 

 Test the action of these as follows: Place a small piece (half as large as 

 a pea) of hard-boiled white of egg into each of three test-tubes or dishes, 

 taking care to make the pieces of equal size. To one tube add 10 cc. 

 of a 0.2 per cent, solution of hydrochloric acid (2 cc. of the acid to a liter 

 of water); to another 10 cc. of a solution of pepsin in water (1 gram of 

 pepsin to a liter of water); to the third 10 cc. of a solution of pepsin in 

 0.2 per cent, hydrochloric acid. Put all the tubes into a water bath or 

 incubator, and keep at a temperature of 40C. Observe the three 

 tubes at the end of the laboratory period, and daily thereafter. What 

 conclusion do you draw from the experiment? 



Among other digestive glands found in higher animals may be men- 

 tioned the salivary glands opening into the mouth, the pancreas opening 

 into the intestine, and the intestinal glands in the walls of the intestine. 

 The enzymes secreted by these glands digest the various kinds of foods, 

 namely, proteins, carbohydrates, and fats. 



4c. Absorption. Osmotic phenomena probably play a part in the 

 absorption of digested food. Osmosis occurs whenever two solutions 

 of equal molecular and ionic concentration are separated by a mem- 

 brane which allows the liquid solvent but not the dissolved substances 

 to pass through, resulting in a flow of liquid toward the solution 

 of greater concentration. Osmosis may be illustrated by the following 

 experiment. Tie a piece of frog skin over the large end of a thistle 

 tube containing a concentrated sugar solution, and support the tube 

 (large end down) in a beaker of water. Mark the level of the sugar 

 solution in the tube. Examine at intervals and measure the rise of the 

 column of liquid in the thistle tube. 



The walls of the intestine, which allow water to pass but retard 

 the passage of many dissolved substances, exhibit osmotic phenomena. 



4d. Circulation. This is accomplished in higher animals by the 

 2 



