472 Dr. J. A. Mac William. Influence of the Central [May 18, 



Mechanism through which Changes in the Cardiac Rhythm are induced 

 by Afferent Impulses. 



A. Reflex Slowing. — The essential mechanism of reflex slowing is 

 clear enough, the change being due, as is commonly believed, to an 

 increased activity of the vagus centre; it does not depend on altera- 

 tions in the blood pressure, or asphyxial conditions, or on any 

 inflnence of the augmentor nerves, for it may be readily excited in 

 the usual way after these nerves have been cut. 



B. Reflex Acceleration. — It is evident that this change might be 

 due to an alteration in the regulating influence exercised by the 

 central nervous system on the heart through the cardiac nerves (vagi 

 and augmentors), or to changed conditions induced in the heart itself, 

 or to a combination of such causes. 



Influence of the Cardiac Augmentor Nerves and the Vagi. 



I have come to the conclusion that the augmentor nerves do not 

 play the essential part, i.e., that the reflex acceleration is not essen- 

 tially dependent upon excitation of the augmentor nerves. 



The chief evidence on this point may be briefly stated as follows : — 



1. The latency and character of the acceleration which often results 

 from excitation of afferent nerves may be entirely different from what 

 has been shown by many observers to be characteristic of the action of the 

 augmentor nerves. — Stimulation of the latter is followed, as is well 

 known, by a long latent period, and then a quickening or rhythm 

 begins, and gradually increases to a maximum (5, 10, or 15 seconds 

 after the beginning of the stimulation, usually) ; later, the accelera- 

 tion declines and gradually disappears. But when an afferent nerve 

 is excited, the resulting cardiac acceleration not unfrequently presents 

 itself after an exceedingly short and hardly appreciable interval, oc- 

 curring with remarkable suddenness, and at once, or almost at once, 

 attaining a very high value, or, it may be, its maximum. Such cases, 

 though by no means the most common ones as regards the results of 

 afferent excitation, are sufficiently numerous and unmistakable to be 

 highly significant in the present connexion, showing, as they do, 

 features in the change of rhythm which differ most strikingly from 

 those characteristic of the action of the angmentors, and which, on 

 the other hand, correspond most closely with what occurs when the 

 controlling influence of the vagi is diminished or removed. 



In some cases the extensive acceleration which may follow stimu- 

 lation of afferent nerves sometimes, after lasting for a variable 

 period, suddenly cives place to a phase of slow heart beat, the reflex 

 acceleration vanishing as abruptly as it had begun. A more or less 

 rhythmical alternation of suddenly recurring phases of acceleration 



