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Dr. C. Bolton. Physiological Action of a [Mar. 20, 



These results show that the vagus is certainly exerting its tonic effect upon 

 the heart during the early stages of the poisoning and that, therefore, the first 

 rise of blood pressure is not due to vagus paralysis but is probably of 

 peripheral origin. Later on the vagus has lost control over the heart and its 

 section produces no effect. 



The fall of blood pressure which occurs in the later stage is, therefore, due 

 to gradually increasing weakness of the heart muscle and not to vagus 

 irritation. 



Excitation of the Vagus. 

 Two effects are to be noticed : — 



1. In the early stages of the rise of blood pressure excitation produces a fall 

 of blood pressure with slowing of the heart, but the effect is usually much less 

 than in the case of the normal animal. Before the blood pressure reaches its 

 maximum, excitation of the vagus produces little or no effect whatever upon 

 the blood pressure and during the final fall excitation produces no effect. 



2. Instead of the pressure falling at once as it does in the normal animal 

 on stimulation, there may be a latent period of a greater a less length which 

 becomes longer towards the end as the vagus ceases to produce any effect. 

 Finally prolonged stimulation produces no effect at all. 



These results of excitation of the vagus point to the same conclusion as 

 was deduced from the results of section of that nerve after injection of the 

 poison — namely, that the vagus is exercising its tonic influence upon the 

 heart, though to a less extent than normal, in the early periods, but that it 

 gradually ceases to exert this tonic influence owing to increased irritability of 

 the heart muscle, the blood pressure now rising to its maximum, and that 

 subsequently it has lost all control over the heart. 



It is more likely that the vagus is unable to hold the heart in check, owing 

 to increased irritability of the heart muscle than that the vagus is paralysed, 

 because prolonged stimulation of the nerve may sometimes produce an effect ; 

 if the vagus was paralysed this would not happen. 



In addition to this, stimulation of the vagus may produce an effect upon the 

 respiration, although the blood pressure is unaffected, showing that the nerve 

 is not totally disabled, and it is unlikely that the nerve ends in the heart 

 would be paralysed whilst those in the walls of the bronchi were unaffected. 



Excitation of the Depressor Nerve. 



Eabbits are used for these experiments as the depressor nerve can be 

 isolated in that animal. Excitation of the central end of the depressor nerve 

 at all stages of the poisoning brings about a fall of blood pressure. As 



