1893.] Nervous System on the Cardiac Rhythm, §c. 469 



not due simply to a fall of blood pressure; it occurs while the blood 

 pressure is still high, and shows no constant relation to alterations in 

 the pressure, sometimes occurring when the blood pressure is rising, 

 and sometimes when it is falling, and at other times when there is no 

 important alteration of level. 



The cause of the alteration of phases of slowing and acceleration is 

 to be sought in an alternately increased and diminished activity of 

 the vagus centre. Further evidence regarding the occurrence of such 

 variations in the conditions of the centre will be adduced later in 

 this paper. 



In those cases alluded to — when the heart continues to beat at a 

 rapid rate even during profound anesthesia — the cause appears to lie 

 mainly in a continued suspension of the activity of the vagus centre, 

 a great diminution or a removal of its controlling influence on the 

 heart. In this condition chloroform may still slow the heart, though 

 only to a relatively slight extent, by its direct influence on its 

 intrinsic rhythmic mechanism of the organ. 



II. On the Relation of the Heart's "Rhythm to Changes in Blood 



Pressure. 



It is well known, since the work of Ludwig and Thiry upon the 

 subject, that changes in blood pressure may affect the rate of the 

 heart's beat otherwise than through the mediation of the cardiac 

 regulating nerves. 



Many investigators have dealt with this relation, with wide varia- 

 tions in the results arrived at ; and such variation is not surprising 

 when one considers the different conditions that may be present in 

 such experiments, differences as to the animals used, the drugs 

 (curare, chloral, morphia, &c.) administered, the various levels from 

 which the change of blood pressure may start, the duration of such 

 pre-existing level, the character and duration of the change itself, the 

 conditions as regards intra-cardiac tension (in one or more parts of 

 the organ) attendant on changes in the arterial pressure, &c. 



In view of such circumstances, I have found it necessary to per- 

 form a considerable number of experiments to demonstrate how 

 far the cardiac rhythm was affected by blood pressure changes in 

 such conditions of experiment as those commonly present in the 

 present investigation (simple chloroform anaesthesia, &c), especially 

 as regards the competency of more or less sudden blood pressure 

 changes of short duration to account for important changes in the 

 rate of the heart's beat. 



Such experiments I have performed in the following conditions: — 



A. When the heart ivas released from all direct influence which may 

 be exercised through the cardiac regulating nerves. — By section of all 



