A CTION OF THE INHIBITO R Y NE RVES. 211 



fitted with a piston and membrane as in Roy's tonometer. The chamber a is 

 hermetically sealed with a vulcanite stopper, which holds a cannula e, 

 which can be fitted into the vena cava inferior, a small glass cannula h, 

 which is fixed into one of the aortic trunks, a cork slab /', 011 which the 

 heart is fixed, and a glass tube <j, containing the platinum electrodes, which 

 is closed, except for two small holes which are blown exactly opposite each 

 other in the middle of this tube. Through these holes the vagus nerve can 

 be drawn by a thread attached to its extremity, and by pinning the thread 

 on to the cork slab /, the nerve will always remain in contact with the 

 electrodes. The fluid within the chamber a cannot penetrate within this 

 glass tube, and therefore the nerve lies on the electrodes surrounded with 

 moist air, although the chamber a is tilled with salt solution or other fluid. 

 The lower extremity of the reservoir /; is on the same level as the top of the 

 chamber a, so that b can be filled with oil without any fear of contamination 

 from the salt solution or other fluid in a. A vulcanite plug is provided in 

 one of the holes in the vulcanite stopper, for the purpose of removing all air 

 from the chamber a. The tap and indiarubber tube d is for the purpose of 

 allowing oil to pass out of b, and so, by means of the negative pressure thus 

 caused, of raising the piston until the lever is in the horizontal position. In 

 tying the superior vena cava care must be taken not to injure the vagus nerve. 

 By means of this apparatus the heart can be kept beating for days, if 

 necessary, and the vagus stimulated. The fluid which flows through the 

 heart may be varied at will, and the effect of the vagus tested with different 

 fluids. The heart may be placed in a fluid of a composition different to that 

 which is flowing through its cavities. The whole heart may be used, or only 

 the ventricle or apex. 



The curves so obtained are difficult to interpret, owing to the fact 

 that the volume measured at any moment is the volume, not of the 

 ventricle or auricle alone, but of the whole heart ; so that if, as seems 

 the case in the normal beat of the frog's heart, there is no moment of 

 time when both auricles and ventricle are simultaneously fully relaxed, 

 it necessarily follows that an action like that of the vagus, which causes 

 a simultaneous cessation of their contractions, must cause a marked 

 increase in the volume registered by the apparatus. As mentioned in 

 my paper, I have endeavoured to eliminate the action of the auricles by 

 raising the pressure within them to a height sufficient to prevent their 

 contraction, and therefore to estimate the change of volume caused by 

 the stimulation of the vagus nerve in a heart consisting of quiescent 

 auricles and a contracting ventricle. Also, owing to the presence of the 

 accelerator nerves in the frog's vagus, I have been able to obtain 

 curves in which an increase of volume has been distinctly shown 

 without any slowing of rate of beat, or any indication of auricular 

 contractions. From these curves I conclude that the vagus is able to 

 diminish the tonicity of the frog's ventricle and a fortiori of the frog's 

 auricle. 



So far I have described the primary effects of the vagus nerve upon 

 the beating heart, and shown that such effects are produced by the 

 direct action of the nerve upon the active muscular tissue. It is there- 

 fore of great interest to show the nature of this action upon the cardiac 

 muscle when that muscle is at rest. Such an experiment is possible in 

 the case of the tortoise, owing to the presence of the coronary nerve, for 

 by means of that nerve the vagus in the neck still remains in com- 

 munication with the auricle and ventricle when the sinus is cut away 

 without injuring the coronary nerve. In such a preparation — equivalent 



