2fJ0 THE VITAL FUNCTIONS. 



incomparably greater than could have resulted 

 from the action of the auricle alone. Fig. .350 

 represents the heart with its two cavities ; d being 

 the auricle, and e the ventricle ; together with 

 the main trunks of the veins (c, c,) which con- 

 vey 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 necessary to provide additional secu- 

 rities against the retrograde motion of its fluid 

 contents. Valves are accordingly interposed 

 between the auricle and ventricle ; and great 

 refinement of mechanism is displayed in their 

 construction. Fig. 351 represents their appear- 



ance at (v) when the cavities, both of the auricle 

 (d), and the ventricle (e) are laid open : c, c, as 

 before, being the upper and lower venae cavse, 

 and A, the main trunk of the aorta. Tliese 



