CAUSE OF THE RHYTHMICAL CONTRACTIONS OF THE HEART. 227 



emptied, by introducing -a few drops of blood into the au- 

 ricle. 1 Our own experiments upon the hearts of alligators 

 and turtles show that when removed from the body and 

 emptied of blood, the pulsations are feeble, rapid, and irreg- 

 ular ; but that when filled with blood, the valves being de- 

 stroyed so as to allow free passage in both directions between 

 the auricles and ventricle, the contractions become powerful 

 and regular. In these experiments, when water was intro- 

 duced instead of blood, the pulsations became more regular, 

 but were more frequent and not as powerful as when blood 

 was used. 2 These experiments show also that the action of 

 the heart may be affected by the character, particularly the 

 density, of the fluid which passes through it, which may ex- 

 plain its rapid and feeble action in anemia. 



It seems well established that the heart, though capable 

 of independent action, is excited to contraction by the blood 

 as it passes through its cavities. A glance at the succession 

 of its movements, particularly in the cold-blooded animals, 

 where they are so slow that the phenomena can be easily ob- 

 served, will show how these contractions are induced. If we 

 look at the organ as it is in action, we see first a disten- 

 tion of the auricle ; this is immediately followed by a con- 

 traction filling the ventricle, which in its turn contracts. 

 Undoubtedly the tension of the fibres, as well as the contact 

 of blood in its interior, acts as a stimulus ; and as all the 

 fibres of each cavity are put on the stretch at the same in- 

 stant, they contract simultaneously. The necessary regular 

 distentionof each cavity thus produces rhythmical and forcible 

 contractions ; and the mere fact that the action of the heart 

 alternately empties and dilates its cavities, insures regular 

 pulsations as long as blood is supplied, and no disturbing in- 

 fluences are in operation. 



The muscular fibres of the heart are endowed with 



1 MILNE-EDWARDS, op. cit, tome iv., p. 126. 



2 Action of the Heart and JRespiration, American Journal of the Medical 

 Sciences, Oct. 1861. 



