266 



DR JAMES A. GUNN ON 



canal in the former animal.* In the cat the constriction of the arterioles is insufficient 

 to counterbalance the slowing of the heart, and the blood pressure falls. 



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Fig. 15. 



With these experiments as examples of the effects of small and large doses, an 

 account may now be given of further experiments made to define more accurately 

 the nature of the production of these effects. 



The Cause of the Sloiving of the Heart. 



Experiment 34. — In this experiment, doses were given corresponding to those given 

 in Experiment 32, with this difference, that sufficient quantities of atropine sulphate 

 were administered during the experiment to ensure that, before each injection of 

 harmaline, the terminations of the vagus nerves in the heart were paralysed. The 

 conditions of the experiment are indicated in Table IX. 



In this experiment the first injection of O'OOl grm. per kilo reduced the pulse rate 

 from 36 to 31 per ten seconds as compared with a reduction of from 41 to 29 per ten 

 seconds by the same dose in Experiment 32, where no atropine was given. It seems 

 probable that, in case of the first injection of a small dose of harmaline, part of the 

 slowing of the heart is due to reflex stimulation of the vagus by the rise of blood 

 pressure. 



On the other hand, doses of 0'002 grm. and 0'004 grm. per kilo produce an amount of 

 slowing of the heart (when the terminations of the vagus are paralysed by atropine) 

 quite commensurate with that produced by the same doses in Experiment 32, where the 



* Laudeh Brunton, Textbook of Pharmacology and Therapeutics, 1887, p. 288. 



