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 

 the 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 /ate. 



4c. Absorption. Absorption is principally an osmotic phenomenon. 

 Osmosis may be briefly defined as the passage of water and dissolved 

 substances through a permeable membrane. If the membrane separates 

 two liquids of unequal density, the greater flow is toward the liquid of 

 greater density. Osmosis may be illustrated by the following demon- 

 stration experiment : Tie a wet piece of parchment paper or animal bladder 

 over the end of a thistle tube. Fill the tube with a concentrated solu- 

 tion of copper sulphate and support it in a beaker of water so that the 

 level of the water and the copper sulphate is the same. Examine at the 

 end of the laboratory period and also at succeeding laboratory periods. 

 Observe the results and explain. 



The intestine and blood vessels are lined with permeable-membranes 

 through which osmosis takes place in a similar manner. 



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

 2. 



