206 



Prof. J. A. Mo William. 



Play 17, 



IV. "On the Rhythm of the Mammalian Heart." By J. A. 

 Mc William, M.D., Professor of the Institutes of Medicine 

 in the University of Aberdeen. Communicated by Pro- 

 fessor M. Foster, Sec. U.S. Received April 26, 1888. 



The following are some of the general conclusions arrived at from a 

 prolonged investigation- of the rhythm of the mammalian heart. The 

 experiments were conducted on the cat, dog, rabbit, rat, hedgehog, and 

 gninea-pig, the cat being the animal most commonly used. The 

 animals were anaesthetized, artificial respiration was kept up, the 

 thorax was laid open, and the action of the heart was recorded by 

 various adaptations of the graphic method — 



1. Minimal stimulation of the quiescent cardiac muscle is at the 

 same time maximal ; a stimulus which is strong enough to excite con- 

 traction at all excites a maximal contraction. The strength of an 

 artificially excited beat does not depend on the strength of the 

 stimulus ; it is equally strong with maximal and minimal excitation. 

 I have tested this point in various ways ' — 



(1.) On the excised heart which has ceased contracting spon- 

 taneously, but is still quite capable of being artificially excited to 

 beat. 



(2.) On the intact heart reduced to a state of quiescence by vagus 

 stimulation. 



(3.) On intact hearts which beat slowly in consequence of cooling 

 and other circumstances ; the stimulations were applied during the 

 quiescent period intervening between two spontaneous contractions. 



2. The condition of fibrillar contraction or heart- delirium induced 

 in the ventricles of excitable hearts by the application of interrupted 

 currents and other means can be recovered from even after long 

 periods (three-quarters of an hour, &c.) under the combined influence 

 of artificial respiration, rhythmical compression of the ventricles, and 

 the administration of pilocarpin. 



When the excitability of the cardiacmusele has been much depressed 

 (by pilocarpin, certain phases of exhaustion, &c), the application of 

 interrupted currents does not induce fibrillar contraction, but merely a 

 series of rhythmic beats in the case of a quiescent organ, or an accelera- 

 tion of the rhythm already present in a heart which is beating spon- 

 taneously. 



3. The spontaneous rhythmic power possessed by the terminal parts 

 of the great veins, the auricles, and the ventricles, seems, in some con- 

 ditions at least, to be myogenic. 



4. In the intact heart the auricles and ventricles do not beat in 

 virtue of their own independent rhythmic power, but in obedience to 



