1893.] Nervous System on zhe Cardiac Rhythm, cj-c. 467 



The rate of action rises far above the normal, provided the blood 

 pressure remain good, and the temperature and general condition of 

 the animal continue satisfactory ; the pulse rate rises from a normal 

 rhythm of 120 — 130 to one varying from 216 to as high a figure of 

 240 — 250, according to the depth of the anaesthesia and other cir- 

 cumstances. 



When the heart is beating in this very rapid . fashion the further 

 administration of chloroform deepening the anaesthesia, induces, as a 

 rule, a very distinct reduction in the pulse rate, while at the same 

 time the blood pressure falls markedly. The reduction in the pulse 

 rate is not at all comparable in extent to that which usually follows a 

 similar dose of chloroform while the vagi are intact : the diminution 

 in rapidity in the present case (when the vagi or all the cardiac nerves 

 are cut) is comparatively a slight one, e.g., a reduction from the rate 

 of 240 to one of 200, &c. 



But, though the slowing of the rhythm is a much less extensive one 

 than occurs while the vagi are intact, it is at the same time a very 

 appreciable and constant one ; and, moreover, it is important, de- 

 pending as it evidently does on some cause affecting the heart itself, 

 and not exerted through the influence of the regulating (extra- 

 cardiac) nerves. 



Now, as to the mode in which this slowing effect is produced in the 

 action of the cardiac mechanism, the possible influence of the 

 markedly-lowered blood pressure has to be taken into account. We 

 know that in certain circumstances a great lowering of the blood 

 pressure may lead to a very pronounced slowing of the pulse rate, 

 and the question arises as to whether such a cause is sufficient to 

 explain the reduction in rate caused by chloroform in the conditions 

 now under consideration. I conclude that it is not, for these 

 reasons : — 



1. The diminution in the rate of heart beat may begin early when 

 the fall of pressure is only slightly developed, and 



2. A similar, or indeed a much greater fall of blood pressure 

 lasting for similar periods, when induced by mechanical causes {e.g., 

 compression of portal vein, &c), entirely fails to cause any diminu- 

 tion of rate like that which occurs when the pressure is lowered by 

 chloroform. Experiments in which reduction of blood pressure of 

 equal amount and duration were alternately induced by {a) chloro- 

 form and (b) mechanical causes have clearly shown this. 



There is good reason to believe that chloroform has a special 

 influence on the rhythmic mechanism of the heart, an influence not 

 dependent on the concomitant lowering of the blood pressure caused 

 by the drug. 



Again, as to the nature of the influence exerted by chloroform in 

 this way, it can be shown that the slowing is not dependent on a 



