138 THE CIRCULATION. 



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 themselves 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 those peculiar rhythmic move- 

 ments by which the action of the heart is distinguished 

 from that of all the other muscles. By some it has been 

 supposed that the contact of arterial blood with the lining 

 membrane of the left cavities of the heart, and of venous 

 blood with that of the right cavities, furnishes a stimulus, 

 in answer to which the walls of these cavities contract. 

 And they explain the rhythmic order in which these con- 

 tractions ensue, by supposing that the same act, the 

 systole, which expels the stimulating fluid from the ven- 

 tricles, causes the auricles to be filled from the veins ; and 

 that the contraction of the auricles thereupon induced, gives 

 rise, in its turn, to the filling and consequent contraction of 

 the ventricles. 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 atmo- 

 spheric 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 

 exact relation in which the heart stands towards this 



