374 



healthy state of the nervous system, very gentle irritation of the 

 vagus increases the heart's action, while a slightly stronger applica- 

 tion diminishes the frequency and force of its contractions. This 

 conclusion is in harmony with an observation which I made inci- 

 dentally upwards of a year ago, that irritation of the posterior part 

 of the brain of a frog with a fine needle was repeatedly followed by 

 improvement in the circulation, whereas it was by the application of 

 a stronger stimulus, that of galvanism, to the same part of the 

 cerebro-spinal axis that Weber first induced an inhibitory action 

 on the heart. 



It is said, on apparently good authority*, that division of the 

 vagus in mammalia is invariably followed by increase of the action 

 of the heart ; this, if true, would be a strong ground for believing 

 in an inhibiting influence constantly operating upon it through 

 this nerve. But it is also stated that the same thing does not 

 occur in frogs ; and this circumstance appeared to me to throw 

 much doubt upon the evidence regarding mammalia. I therefore 

 made careful experiments on the effects of cutting both vagi, once 

 upon a calf and four times upon rabbits ; taking the number of the 

 heart's beats immediately before and immediately after section of 

 each nerve by the momentary stroke of a sharp pair of scissors. 

 In no case was the rate increased at all by the operation, and the verv 

 gradual diminution in frequency that commonly took place appeared 

 to depend on general exhaustion from other circumstances attending 

 the experiment. In one rabbit, in which I had removed the skin and 

 pectoralis major from the prsecordial region, so as to see the move- 

 ments of the heart distinctly through the transparent pericardium and 

 intercostal muscles, I noticed particularly that the strength of the 

 contractions, as well as their frequency, remained quite unaffected by 

 the division of the vagi. From these facts I feel warranted in con- 

 cluding that, whatever may occur under exceptional circumstances, 

 there is certainly no constant control exercised over the heart's 

 action through those nerves. 



The influence of the spinal system upon the heart is, however, 

 very apparent after a struggle, which almost invariably increases the 

 frequency and force of the beats ; and I found that this continued 



* Pfluger, op. cit. 



