IX 



CARDIAC MUSCLE AND NERVES 



323 



effect that the vagi were motor nerves, easily exhausted by 

 electrical stimuli ; and, lastly, that of Brown-Sequard, who held 

 the vagus to be a vasomotor nerve of the coronary system. 

 Budge's view was soon found to be imtenable, since even the 

 weakest currents retard or arrest the movements of the heart. 

 Still easier was it to overthrow Brown-Sequard's hypothesis, seeing 

 that ligatiou of the coronary arteries does not arrest the heart, 

 and in the frog's heart, which has no cardiac vessels, the vagus 

 still produces the same effect. Only the Webers' theory, therefore, 

 remains, and the object of later researches has merely been to 

 determine the mode and 

 mechanism by which the 

 retardation or inhibition 

 of the beats is effected in 

 vagus stimulation. The 

 most important results of 

 these observations are 

 briefly stated as follows: 



In warm-blooded ani- 

 mals the standstill brought 

 about by the vagus never 

 lasts more than a minute. 

 If the curve of arterial 

 pressure is registered dur- 

 ing vagus excitation by 

 Ludwig's kymograph, a 

 more or less rapid depres- 

 sion may be observed, 

 according as arrest (Fig. 

 145); or merely slowing 

 of the beats (Fig. 146), is 

 obtained. The inhibitory 

 action is more pronounced 

 in mammals than in birds, 

 where as a rule there is only delay (Claude Bernardj, or arrest 

 of a few seconds (Wagner, Meyer). In the poikilothermic verte- 

 brates, on the other hand, the standstill is more pronounced than 

 in mammals. 



Cardiac arrest by vagus stimulation has repeatedly been 

 determined on man. Henle obtained it in 1852 on a decapitated 

 criminal, whose right auricle was still beating (ultima moriens) : 

 Czermak, Thanhoffer, Concato, Malerba, Wasilewsky, Cardarelli 

 obtained it by compression or friction of the neck along the course 

 of the vagus and the carotids. This is an experimenturn peri- 

 culosum, since it may produce disquieting systems of syncope 

 (Thanhoffer). 



The latent period of vagus excitation is comparatively long, 



FIG. 145. Depressor effect of strong excitation of vagus 

 in dog. (Morat.) The period of stimulation is marked 

 at B on the abscissa. The carotid is connected with 

 Lud wig's kymograph. 



