CIRCULATION IN THE VERTEBKATA. 



1S7 



structure. The supply of blood to the heart, beins; in a con- 

 stant stream, produces a gradual dilatation of the cavity which 

 receives it; and the muscular fibres of that cavity are not ex- 

 cited to contraction, until tlicy are stretched to a certain 

 point. But in order effectually to drive the blood into eve- 

 ry part of the arterial system, where it has great resistances 

 to overcome, a considerable impulsive force is required, im- 

 plying 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, be- 

 ing rapidly distended, is excited to a much more quick and 

 forcible contraction than the auricle, and propels the blood 

 it contains into the artery, with an impetus incomparably 

 greater than could have resulted from the action of the au- 

 ricle alone. Fig. 350 represents the heart with its two ca- 



vities; D being the auricle, and e the ventricle; together'witli 

 the main trunks of the veins (c, c,) wliich convey the blood 

 into the auricle; and those of the arteries (a,) which receive 

 it from the ventricle for distribution over the whole system. 

 The force of contraction in the principal cavity of i the 

 heart being tlius increased, it becomes necessary to provide 



