1893.] Nervous System on the Cardiac Rhythm, Sfc. 465 



The main results of these experiments I shall now state briefly : — 



1. 'Experiments in which the Entire Cardiac Regulating Mechanism 



was Intact. 



When chloroform is administered by inhalation in the usual way, 

 two well-marked stages are usually evident in its action on the 

 cardiac rhythm : — ■ 



A. A stage of acceleration far beyond the ordinary pulse rate of 

 the animal. From the normal rate of 120 — 130 (cat), the pulse may 

 rise as high as 240 — 250. The acceleration occurs during the general 

 excitement induced by the anaesthetic. 



After a time, as the administration of chloroform progresses, the 

 acceleration diminishes and gives place to the second stage. 



B. A stage of moderate or slow pulse rates. In this stage the 

 cardiac rhythm falls towards the normal rate ; it may come to be 

 closely similar to the normal rate (120 — 130 per minute), or it may 

 stand at a higher or a lower level, though within such limits as to 

 render the pulse frequency a comparatively moderate one (90 — 150 

 per minate). This stage usually continues during deep anaesthesia, 

 and even when the narcosis becomes very profound and dangerous. 



These stages, as seen in the cat, closely resemble, in their general 

 character, the similar phases usually recognisable during the admin- 

 istration of chloroform in man. 



It should be mentioned that in some cases the reduction of pulse 

 rate which follows the primary acceleration is comparatively slight, 

 the frequency of the beat remaining high above the normal standard 

 even during profound anaesthesia. The conditions determining such 

 a difference from the usual course of events cannot be discussed here ; 

 they are considered in my detailed paper on the subject. In some 

 other other animals {e.g., the hare) the heart commonly beats with 

 great rapidity (260, &c.) even during deep anaesthesia, and in this 

 respect such animals differ notably from the cat and from man. 



During a certain period of chloroform anaesthesia the cardiac 

 rhythm may readily be altered to a very important degree — either in 

 the direction of slowing or acceleration — by excitation of afferent 

 nerves or by changes in the blood pressure, &c. The period alluded 

 to comprises the latter part of the first stage or stage of acceleration 

 and the earlier part of the second stage or stage of moderate or 

 slow pulse rate. I do not wish to imply that at no other phase of 

 chloroform administration can changes in the heart's rhythm be 

 brought about by similar causes, but only that the period referred to 

 is the one most favourable to their ready manifestation, and the one 

 in which their action is most constant. In the latter part of the 

 second stage the cardiac rhythm passes beyond the influence of afferent 

 impulses. 



