128 THE CIRCULATION. 



influx of blood, and save the force which, would otherwise 

 be required to dilate them. 



The capacity of the two ventricles is probably exactly the 

 same. It is difficult to determine with certainty how much 

 this may be ; but, taking the mean of various estimates, 

 it may be inferred that each ventricle is able to contain on 

 an average, about three ounces of blood, the whole of which 

 is impelled into their respective arteries at each contraction. 

 The capacity of the auricles is rather less than that of the 

 ventricles : the thickness of their walls is considerably less. 

 The latter condition is adapted to the small amount of 

 force which the auricles require in order to empty them- 

 selves into their adjoining ventricles; the former to the 

 circumstance of the ventricles being partly filled with blood 

 before the auricles contract. 



Cause of the Rhythmic Action of the Heart. 



It has been attempted in various ways to account for the 

 existence and continuance of the rhythmic movements of 

 the heart. By some it has been supposed that the contact 

 of blood with the lining membrane of the cavities of the 

 heart, furnishes a stimulus, in answer to which the walls 

 of these cavities contract. But the fact that the heart, 

 especially in Amphibia and fishes, will continue to contract 

 and dilate regularly and in rhythmic order after it is 

 removed from the body, completely emptied of blood, and 

 even placed in a vacuum where it cannot receive the 

 stimulus of the atmospheric air, is a proof that even if the 

 contact of blood be the ordinary stimulus to the heart's 

 contraction, it cannot alone be an explanation of its 

 rhythmic motion. 



The influence of the mind, and of some affections of the 

 brain and spinal cord upon the action of the heart, proves 

 that it is not altogether, or at all times, independent of 

 the cerebro-spinal nervous system. Yet the numerous 

 experiments instituted for the purpose of determining the 



