238 



THE VITAL FUNCTIONS. 



resistances to overcome, a considerable impulsive 

 force is required ; implying a sudden as well as 

 powerfid 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 re- 

 ceived from the veins with a sudden impulse ; and 

 thus the ventricle, being rapidly distended, is ex- 

 cited 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 auricle alone. Fig. 350 represents the heart 



350 



with its two cavities ; d being the auricle, and e the 

 ventricle; together with the maintrunksof the veins 

 (c, c,) which 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 the heart being thus increased, it becomes ne- 

 cessary to provide additional securities against the 

 retrograde motion of its fluid contents. Valves are 



