ix CAKDIAC MUSCLE AND NERVES 305 



studies of the frog's heart there was a great tendency to deny any 

 kind of automaticity, and to regard cardiac rhythm as a simple 

 reflex phenomenon dependent on rhythmic excitation determined 

 by external stimuli. Goltz assumed that systole, by removing the 

 cardiac stimulus (due to the blood and its gases), induced diastole. 

 But this hypothesis disregards the elementary consideration that 

 the excised heart, in which rhythmical filling and emptying is no 

 longer possible, continues to beat vigorously. 



Others ascribe a kind of elastic resistance to the heart, alternat- 

 ing with the different phases of its activity. The stimuli, as 

 continuous agents, must develop a certain tension in order to 

 surmount this elastic resistance, before they can produce their 

 effect. When systole is over, resistance rises, and a new discharge 

 can only take place after the latent excitation has overcome the 

 corresponding tension. This schematic representation implies a 

 tacit recognition that the immediate cause of rhythmical activity 

 is a condition intrinsic to the organ. But the naivete of this 

 hypothesis, which predicated a kind of elastic resistance, was shown 

 on the discovery of periodic rhythm, which, in its multiple 

 manifestations, its varied course, and its crisis, shows how change- 

 able the internal conditions which determine the activity of the 

 heart may be, when the external conditions remain constant and 

 almost unaltered. " The rhythm of the heart-beats " (as we concluded 

 in 1873) " is the external expression of a corresponding rhythm of 

 the nutritive process which is accomplished within the organ." 



Henceforward, no one contested the theory of the automaticity 

 of cardiac action, since it was impossible to invent a hypothesis 

 which explained the many and complex forms of its rhythm by 

 any external stimulus. 



The automatic excitability of the heart does not of course 

 exclude its reflex excitability, and the doctrine of Bidder and 

 Eckhard, who assumed the existence of distinct automatic and 

 reflex mechanisms, must be modified to the effect that the different 

 parts of the heart exhibit different degrees of excitability, whether 

 automatic or reflex. 



It can be asserted on the ground of weighty arguments that 

 automatic excitability is most pronounced in the venae cavae and 

 sinus, less in the auricles, and least in the ventricle. The facts 

 already cited showing that (i.) the duration of cardiac arrest, after 

 applying ligatures or sections to the heart by the methods of Stannius 

 and Eckhard, becomes increasingly greater, the lower these are 

 placed on the auricles between the orifice of the sinus and the 

 auriculo- ventricular groove ; (ii.) the periodic groups become con- 

 stantly smaller and the pauses longer with physiological separation, 

 as in our method, combined with the action of serum and of 

 pressure ; (iii.) the arrest of cardiac activity by exhaustion due to 

 whatever means, which takes place in different parts of the organ 



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