GALVANIZATION OF THE PNEUMOGASTRICS. 235 



the observations on alligators, it shows that the motor nerves 

 are not all affected at the same time, and that the pneumo- 

 gastrics resist the action of this peculiar poison after the 

 motor nerves generally are paralyzed. This shows a conser- 

 vative provision of Nature which guards particularly the im- 

 portant influence exerted by these nerves upon the heart. 1 



Our knowledge of the inherent properties of the muscular 

 fibres of the heart, and the effects of the passage of blood 

 through its cavities, which together are competent to keep 

 up for a time regular pulsations without the intervention of 

 the nervous system, taken in connection with the facts just 

 stated, concerning the influence of section or galvanization 

 of the pneumogastric nerves, enables us to comprehend pretty 

 well the influence of these nerves on the heart. They un- 

 doubtedly perform the important function of regulating the 

 force and frequency of its pulsations. 



Hardly any reflection is necessary to convince us of the 

 importance of such a function, and how it must of necessity 

 be accomplished through the pneumogastrics. It is impor- 

 tant, of course, that the heart should act at all times with 

 nearly the same force and frequency. We have seen that 

 the inherent properties of its fibres are competent to make it 

 contract, and the necessary intermittent dilation of its cavi- 

 ties makes these contractions assume a certain regularity ; 

 but the quantity and density of the blood are subject to very 

 considerable variations within the limits of health, which, 

 without some regulating influence, would undoubtedly causo 

 variations in the heart's action, so considerable as to be inju- 

 rious. This is shown by the comparatively inefficient and 

 palpitating action of the heart when the pneumogastrics are 

 divided. These nerves convey to the heart a constant influ- 

 ence, which we may compare to the insensible tonicity im- 

 parted to voluntary muscles by the general motor system. 



1 For details of these experiments the reader is referred to an article by the 

 author, on the Action of the Heart and Respiration, in TJie American Journal of 

 Medical Sciences, Oct., 1861. 



