THE FROG, A TYPICAL VERTEBRATE 



251 



In an average human being the lungs can hold about 330 cubic 

 inches of air, but at each inspiration only about 30 cubic inches 

 of this is renewed. The fresh air drawn in differs from the ex- 

 pired air as follows : 



The inspired air therefore loses oxygen, which is nearly re- 

 placed in expired air by carbon dioxide. This change is due to 

 the passage of oxygen from the haemoglobin in the red blood cor- 

 puscles, and the transference of carbon dioxide from the blood 

 corpuscles into the lung cavities. The oxygen obtained by the 

 blood in this way is carried to the tissues and delivered to the 

 cells. 



We may explain why the cells must have oxygen as follows : 

 Every action of an animal uses up part of the protoplasm in the 

 body, and just as coal furnishes the power for the work done by 

 a steam engine so food supplies the protoplasm for doing the 

 work of the animal. The breaking down of the protoplasm is 

 caused by the union of oxygen with it, producing a kind of slow 

 combustion or oxidation. The burning of substances like wood 

 in air is an example of rapid combustion, and the rusting of iron 

 an example of slow combustion. In every case oxygen unites 

 chemically with the substances. 



Oxidation within the cells results in the performance of work, 

 and the breaking down of the protoplasm is accompanied by the 

 production of carbon dioxide and nitrogenous waste substances. 

 The carbon dioxide is carried to the lungs by the blood and there 

 excreted. The nitrogenous substances are carried chiefly to 

 the kidneys. The processes included in the taking up of oxygen 

 by the protoplasm of the cells and the giving off of carbon dioxide 

 constitute what is known as internal respiration. 



