136 THE COMMON FROG. [chap. 



and in this way oxygenated blood only is supplied to 

 the head, sense organs, and brain. 



It should be borne in mind that in order to develop 

 this most beautiful and complex apparatus, the co- 

 ordinate development in due proportion of these 

 beneficial obstructions and checks must have been 

 simultaneously effected in order that their purpose 

 should be duly served. In other words, to account 

 for its formation by an indefinite series of minute 

 happy accidents would seem to require such a suc- 

 cessive occurrence of coincidences as to become an 

 improbability so great as to be indistinguishable from 

 impossibility. 



So much for the circulation of the frog in its adult 

 condition. Its larval, or tadpole stage, presents us 

 with a series of changes which, though more familiar, 

 are not less wonderful. 



In the first place, however, it may be well to de- 

 scribe shortly the condition of the circulation in fishes, 

 where the purification of the blood is effected, not 

 by means of the exposure of the blood to the action 

 of air taken into respiratory cavities of the body, but 

 by its subjection in little places of membrane, the 

 gills, to the influence of air mechanically mixed up 

 with and dissolved in the water in which those gills 

 are bathed. 



In fishes, moreover, unlike all air-breathino- animals 

 none of the oxygenated blood is returned to the heart 

 for propulsion, but is collected directly into the great 

 dorsal aorta, whence it is distributed to the whole 

 body, only being returned to the heart after such dis- 

 tribution, so that venous blood alone enters that organ. 



