UPON THE VASCULAR SYSTEM. Ill 



do not, however, seem to me to conclusively show that cardiac motor fibres 

 are absent from this nerve. Schiff's statement is that when the vagi are 

 divided in the necks of rabbits, and the lower end of one or both nerves 

 very gently stimulated, the heart's action is quickened. He has further said 

 that it is difficult to hit upon the precise amount of stimulation which will effect 

 this. Although other observers of undoubted reputation for skilful experi- 

 mentation have failed to obtain this result, the above statement is nevertheless 

 positive evidence which cannot be discarded unless the negative evidence be 

 very strong. It seemed to me that it was possible to investigate this matter in 

 a manner more exact and reliable than that adopted by previous experimenters ; 

 accordingly, I performed a number of experiments in 1866-67, in the following- 

 manner : — In frogs and rabbits I exposed the vagi in the neck, and then opened 

 the trachea and larynx anteriorly, — in order that asphyxia and consequent ex- 

 citement might be prevented, and also to enable me to see movements of the 

 arytenoid cartilages. I then divided the vagi on a level with the thyroid 

 cartilage. I always stimulated the nerve by induced currents obtained from 

 Du Bois Reymond's induction machine. The electrodes consisted of clean 

 copper wire ; the battery of one of Daniell's cells. On stimulating the lower 

 end of the vagus, I always watched the corresponding arytenoid cartilage as 

 well as the heart. The movement of the former served as a strict test for the 

 proper application of the electrodes, in short — for the proper stimulation of the 

 nerve. The observations were begun by ascertaining the strength of current 

 necessary to affect the recurrent laryngeal filaments in the vagus — so that 

 movement of the arytenoid cartilage ensued. Having ascertained this, I made 

 the current still weaker, and then began the observations on the heart. As is 

 well known, the strength of the induced currents obtained from Du Bois Rey- 

 mond's machine depends on the distance between the primary and secondary 

 coils. The strength is inversely as the distance. Seeing that I began with 

 very weak currents — that is, with the secondary far removed from the primary 

 coil — and, being anxious to test the effect of all currents intermediate between 

 the very weak ones at the commencement and those strong enough to retard 

 the pulse, I always increased the strength of the current while the nerve was 

 being stimulated, and the effect upon the heart observed. As long as the 

 stimulus was not strong enough to slow the heart, the nerve was usually 

 stimulated for about half a minute. Whenever the animal struggled, the obser- 

 vation was at once abandoned, and repeated when all excitement had subsided, 

 — the effect of struggling being to increase the cardiac movement. In the case 

 of rabbits, the cardiac pulsations were counted with the aid of a stethoscope, 

 the number being taken previous to and during the stimulation of the nerve. 



VOL. XXVI. PART I. 2 G 



