128 THE COMMON FROG. [chap. 



CHAPTER IX. 



THE CIRCULATION OF THE FROG, 



Not only every animal, but every living being, 

 requires, in order to carry on the functions of life, 

 to interchange some of the gaseous elements of its 

 body with gases of the medium (air or water) in 

 which it ha,ppens to live. 



Another function of extreme generality is that of 

 conveying to all the parts and organs of the body 

 nutritious matter for their growth or for the repair 

 of those destructive effects which the processes of 

 life inevitably produce in them. 



. In all members of the highest sub-kingdom of 

 animals {i.e. in all Vertebrata) these processes of 

 gaseous interchange and nutrition are effected by 

 means of closed vessels, along which the stream of 

 nutritious fluid (the blood) is continually carried in 

 a definite and constant course. During some or 

 other part of that course the blood becomes exposed 

 to conditions specially favourable to the gaseous 

 interchange, the blood parting with carbonic acid 

 gas and obtaining in its place an increased supply 

 uf oxygen. 



