47 () Dr. J. A. MacWiiliam, Influence of the Central [May 18, 



It is obvious that, whatever effects a continuation of altered intra- 

 cardiac tension or muscular action may have upon the peripheral 

 efficiency of the vagi, those causes are not in any way capable of 

 accounting for such sudden and extensive reflex acceleration as is 

 frequently seen when an afferent nerve is stimulated. 



The primary and essential cause of the altered cardiac rhythm is 

 clearly to be sought elsewhere, and it is obviously to be found in an 

 altered activity of the central vagus mechanism. The acceleration 

 depends on a diminution in activity of the vagus or cardio-inhibitory 

 centre; just as the cardiac slowing is due to an increased activity of 

 the same centre.* 



Influence of Muscular Exertion upon the Cardiac Rhythm. 



The striking association of cardiac acceleration with motor effort 

 following excitation of afferent nerves distinctly suggests the 

 existence of a somewhat close connexion between the motor me- 

 chanism and the cardiac regulating centres in the medulla. As has 

 already been stated, the essential connexion is with the vagus centre ; 

 a diminished activity of that centre is associated with motor effort 

 and leads to an accelerated heart beat. 



And there is much to be said in favour of the hypothesis that a 

 similar lessening of the controlling influence exercised by the vagus 

 centre on the heart occurs during muscular exertion and constitutes 

 at least one of the causes of the characteristic quickening of the 

 pulse. f 



It may be noted that in certain conditions of impaired health the 

 heart becomes accelerated with extreme readiness from exertions, a 

 very brief interval elapsing between the commencement of muscular 

 actions and the change in cardiac rhythm. In some such instances 

 it is difficult to find a feasible explanation either in an assumed in- 

 fluence of the products of contraction or in a supposed excitation of 

 the augmentor nerves. 



It is interesting to note that animals endowed with great staying 

 powers (dog, horse, &c.) have, in many cases at least, a comparatively 

 slow pulse rate in the quiescent condition ; and that the heart is 

 capable of an immense relative increase in the rate of its action as 

 seen during muscular exertion or on section of the vagi. The con- 



* Some recent observers (Roy and Adami, 1 Phil. Trans.,' 1892) have assumed 

 that the cardiac acceleration resulting fro in stimulation of afferent nerves may he 

 taken as an indication of the action of the augmentor nerves on the heart. The 

 untrust worthiness of this assumption — in the light of the results stated above — 

 need hardly be commented on. 



f It is hardly necessary to remark that various influences are probably concerned 

 in determining the pulse rate during ordinary muscular exerH^'e. 



