94 INTRODUCTION TO ZOOLOGY. 



Additional Facts About the Frog. 



The frog breathes by lungs, but the lung capacity 

 is small in consequence of the small number and lai"ge 

 size of the air cells. Blood is plentifully supplied to 

 the skin by a branch of the pulmo-cutaneous artery. 

 The skin is naked, and when moist, an exchange of 

 carbon dioxide for oxygen can take place through it 

 quite readily. This supplements the lung breathing. 

 This accounts in part for the fact that frogs are usually 

 found near a pond or stream of water. When the 

 ground or grass is wet, however, frogs often go long 

 distances from a pond. A frog confined in a dry place, 

 without water, will soon die. 



Small frogs and toads are sometimes seen hopping 

 over the ground in great numbers, usually after a 

 rain, and this has led to a belief that frogs and toads 

 rain down. The true explanation is that a great num- 

 ber of tadpoles have been developed in a near-by pond 

 or some other body of water, and are just ready to 

 move out from the pond. They seize upon the time 

 of a rain, when the ground and grass and atmosphere 

 are very moist, to leave the place of their develop- 

 ment for some place where food can be more easily 

 secured. 



The heart has tliree chambers : two auricles and 

 one ventricle. Only impure, or carbonated blood, is 

 poured into the right auricle, and oxygenated blood, 

 which comes from the lungs, into the left auricle. 

 The two kinds mix in the ventricle, but by a curious 

 arrangement of valves in the beginning of the aorta, 

 the greater portion of the blood that goes to the lungs 

 is that which comes from the right auricle, and the 

 greater part that goes to the system is that which 

 comes from the left auricle. Tliis kind of circulation, 

 in which pure and impure blood mix in the heart, is 

 called a double, incomplete circulation. 



