189o.] Nervous System on the Cardiac Rhythm, $-c. 473 



and slowing may at times be seen, and strongly suggests the occur- 

 rence of corresponding variations in the activity of the vagus centre 

 as their cause. 



An assumed excitation of the augmentor nerves could not afford 

 a probable explanation of these facts ; and the evidence points very 

 decidedly to changes in the activity of the vagus centre as the 

 primary and paramount cause of both the slowing and the accelera- 

 tion reflexly excited in the way referred to. 



2. The result of stimulation of afferent nerves after section of the vagi 

 ivhile the cardiac augmentor nerves remain intact. — Section of the vagi 

 in the cat leads to a great acceleration of the cardiac rhythm, the 

 pulse rate rising to a maximal rate of 216—250; the maximum 

 varying to some extent in different animals and in different circum- 

 stances. 



In this condition, the augmentor nerves being uninjured, excitation 

 of afferent nerves gives rise to no further acceleration. But this does 

 not constitute any proof as to whether the augmentor nerves are 

 reflexly excited by afferent stimulation or not. For the circumstances 

 are not favourable for augmentor excitation being effective in causing 

 any notable change in the cardiac rhythm. It is, indeed, quite clear, 

 as shown by direct stimulation, that the augmentors can accelerate 

 the heart after section of both vagi, e.g., when the blood pressure is 

 extremely low, and the cardiac rhythm slowed in consequence ; their 

 action is not confined to a counteracting of the controlling influence of 

 the vagi upon the rhythmic mechanism. But when the vagi are cut, 

 and the heart beats at the high maximal rate mentioned above, the 

 blood pressure remaining high and the bodily temperature, the 

 respiration, Ac, being satisfactory, direct stimulation of the aug- 

 mentor nerves is without any appreciable accelerating effect upon the 

 pulse rate. In such circumstances then, when the heart is beating 

 at its high (maximal) rate, the non-occurrence of acceleration in 

 response to excitation of afferent nerves cannot be taken as evidence 

 that the cardiac augmentor centre is not reflexly affected by afferent 

 impulses. 



It is necessary, therefore, to reduce the cardiac rhythm to a lower 

 level, at which it might be expected (judging from the results of 

 direct stimulation of these nerves) that a sudden reflex excitation 

 of the augmentor nerves would be able to manifest itself by an ap- 

 preciable effect on the pulse rate, if the changed pulse rate (accelera- 

 tion) induced by afferent excitation is really due to an excitation of 

 the augmentor nerves. 



The heart may be kept beating at a moderate rate, the normal rate, 

 or somewhat slower or faster than normal, with much regularity for 

 periods of some length by slight continued stimulation of the vagus 

 nerve in the neck, after both these nerves have been cut. When this 



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