86 



OUTLINES OF NATURAL HISTORY. 



The heart is at first like that of Fishes, consisting of 

 only two chambers, and having only the function of driv- 

 ing the blood to the gills. When the lungs are developed, 

 however, the heart undergoes considerable change, be- 

 coming like that of a Eeptile. It now consists of three 

 chambers, and is so constructed that the impure blood 

 mixes in it with the pure blood coming from the lungs, 

 and that the body is supplied with this mixture. As 

 a consequence of this, at any rate in part, the Frog is, 

 like the Fishes and Reptiles, cold- blooded. In other 

 words, its temperature is very little higher than that of 

 the air or water in which it lives. 



The nervous system of the Frog is weU developed, con- 



Pig. 3^ —Skeleton of the common Frog (Rana temporaria) ; d Vertebra 

 of the back, with long side-processes. 



sisting of a brain and spinal marrow, and of the nerves 

 connected with these. There are also two large eyes, fur- 



