PULSE EATE AND ARTERIAL PRESSURE. 145 



pulse is due to a direct specific influence of the drug on the 

 roots of the vagus as supposed by one of us,* or to the stimula- 

 tion of these roots by the increased pressure of blood in the 

 cranium produced by the contraction of the arterioles, as sup- 

 posed by the other.f In order to do this we diminished the 

 blood-pressure by the inhalation of nitrite of amyl after it had 

 become high and the pulse slow from the injection of digitalin. 

 If the slowing of the pulse were due to a specific action of the 

 digitalin on the vagus roots, it ought to continue althougli the 

 pressure falls, but if due to stimulation of these roots by the 

 high blood-pressure, it should disappear whenever the pressure 

 is reduced. Our experiments showed that whenever the pres- 

 sure fell after the inhalation of the nitrite of amyl the pulse 

 became quick. It might thus appear that the slowing is due in 

 part at least to the high pressure, and not altogether to a direct 

 influence of the digitalin on the vagus ; but this must be decided 

 by farther experiment. 



Lastly, we tried to discover whether digitalis causes contrac- 

 tion of the vessels by acting directly on their walls or on the 

 vaso-motor centre. This we sought to do by observing whether 

 the injection of digitalin into the circulation caused any altera- 

 tion in the calibre of the vessels of the rabbit's ear after the 

 sympathetic nerve of the same side as well as both vagi had 

 been divided in the neck. The vagi were divided in order to 

 prevent the digitalin from slowing the heart, and thus disturbing 

 the circulation, and the sympathetic to prevent any influence 

 beino: transmitted to the vessels of the ear from the vaso-motor 

 centre. The results of these experiments were not constant,, 

 and we are unable to draw any definite conclusions from them ; 

 but the fact that the vessels of the ears were occasionally seen 

 to empty themselves more quickly after the injection of digitalin 

 than before, seems to us to indicate an action upon the walls- 

 of the vessels themselves. 



The conclusions to which we have arrived are shortly, 1st, 

 that digitalin causes contraction of the arterioles. This is 

 proved by the small height of the pulse-wave, and by its descent 



* Brunton, Op. cit. {vide antea, pp. 71 and 72). 

 t Meyer, Op, cit. 



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