CIRCULATION IN THE VERTEBRATA. 



259 



finements into its structure. The supply of 

 blood to the heart, being in a constant stream, 

 produces a gradual dilatation of the cavity which 

 receives it ; and the muscular fibres of that cavity 

 are not excited to contraction, until they are 

 stretched beyond a certain point. But in order 

 effectually to drive the blood into every part of 

 the arterial system, where it has great resistances 

 to overcome, a considerable impulsive force is 

 required ; implying a sudden as well as powerful 

 muscular action. This object is attained, in all 

 vertebrated animals, by providing a second 

 muscular cavity, termed a ventricle, into which 

 the first cavity, or auricle^ throws the blood it has 

 received from the veins, with a sudden impulse ; 

 and thus the ventricle, being rapidly distended, 

 is excited to a much more quick and forcible 



350 



contraction than the auricle, and propels the 

 blood it contains into the artery, with an impetus 



