RHYTHM OF THE HEART. Ill 



but that during its dilatation, the heart remained flat. And 

 the same was shown by Dr. Clendinning, who, applying the 

 points of a pair of spring callipers to the heart of a live ass, 

 found that their points were separated as often as the heart 

 swelled up in the contraction of the ventricles, but approached 

 each other by the force of the spring when the ventricles di- 

 lated. Seeing how slight the force exerted in the dilatation of 

 the ventricles is, it has been supposed that they are only di- 

 lated by the pressure of the blood impelled from the auricles ; 

 but that both ventricles and auricles dilate spontaneously is 

 proved by their continuing their successive contractions and 

 dilatations when the heart is removed, or even when they are 

 separated from one another, and when therefore no such force 

 as the pressure of blood can be exercised to dilate them. By 

 such spontaneous dilatation they at least offer no resistance to 

 the 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 aver- 

 age, about three ounces of blood, the whole of which is im- 

 pelled into their respective arteries at each contraction. The 

 capacity of the auricles is rather less than that of the ven- 

 tricles : 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 ad- 

 joining ventricles ; the former to the circumstance of the ven- 

 tricles being partly filled with blood before the auricles con- 

 tract. 



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 con- 

 tract. 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 can- 

 not 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 



