AN INTRODUCTION TO ANIMAL BIOLOGY 53 



sary that digested food be transported to cells which are 

 far away from the digestive organs. The distribution of 

 oxygen is accomplished by the blood, which absorbs oxygen 

 from the air or water external to the body and then carries 

 it to the internal cells. When the frog is under water, oxygen 

 (a small quantity of O is always mixed with or dissolved 

 in water exposed to the air) is absorbed by the blood flowing 

 through the blood-capillaries in the skin; but when the 

 animal is on land, oxygen is absorbed directly from the air, 

 partly by the blood flowing in the capillaries in the skin and 

 also by the blood flowing in the capillaries of the lungs. In 

 our own bodies the blood absorbs most of the necessary 

 oxygen in the lungs. In fishes the blood flowing through 

 the capillaries in the delicate membranes of the gills absorbs 

 oxygen from the water just as the frog's skin absorbs some 

 oxygen when the animal is under water. 



(D) Examine specimens of frogs' lungs with injected blood- 

 vessels, frogs' skin with injected blood-vessels, gills of a fish injected 

 to show blood-vessels. (These may be obtained by injecting with a 

 hypodermic syringe or sharp-pointed pipette some colored mixture, 

 such as starch and carmine in water, into the large arteries of a 

 frog killed with anaesthetics.) 



49. Excretions. It has been stated that the cell-substance 

 (including foods absorbed by cells) is continually being oxi- 

 dized in every living cell of the frog's body. This chemical 

 change produces a number of substances which are of no 

 further use in the cells of the body ; in fact, they would be 

 poisonous if allowed to accumulate. These waste substances 

 are called excretions. 



We have already noted 12-16 that animal substance may 

 be analyzed into water, gas, carbon, and mineral matters; 

 and also that the carbon may be burned. Something similar 

 takes place when cell-substance is oxidized in the living 

 body, and the chief excretions produced are of four kinds ; 

 water, carbon dioxide (a gas formed by burning the carbon of 



