NERVES OF THE HEART. CARDIAC MUSCLE. 267 



The cannula is (Fig. 101) connected by an inelastic tube, c,oi 

 convenient length, also filled with magnesium sulphate, to 

 one end of a U-shaped glass pressure-gauge or manometer, 

 ddgg, containing mercury. On the top of the mercury in the 

 limb gg of the manometer floats a light stem e carrying a pen 

 which writes on a travelling surface. Above the mercury, o, 

 on the side dd, the tube is filled with magnesium sulphate 

 solution. When the pressure on each side of the manometer 

 is alike the mercury stands at the same level in both limbs, 

 but when it is increased on the side dd by taking the clamp 

 off the artery and throwing in the pressure of the blood the 

 mercury in gg rises, carrying the float and pen with it and 

 draws a line such as that at yz, Fig. 102, on the travelling 



g 



FIG. 102. Tracing: of arterial pressure during vagus inhibition of the heart. To 

 be read from right to left: yzpq, blood pressure-line traced by the manometer pen; 

 o indicates on the tracing the instant at which the nerve was stimulated; p, the 

 instant at which the stimulation ended; ae, line traced by a pen marking half 

 seconds; xg, line of no pressure, that is, level at which the pen would write were 

 there no arterial pressure; the distance between it and the part of the manometer 

 line directly above it multiplied by two gives the actual pressure in mercury in the 

 artery at that moment. The small variations of pressure seen on the curve are 

 due to beats of the heart; they are absent during the inhibition and slow for a 

 short time after it. 



surface, the small curves (pulse-waves) on which correspond to 

 the slight increases of arterial pressure following each contrac- 

 tion of the left ventricle. The number of these small curves in 

 a given time gives us therefore the pulse-rate. The pneu mo- 

 gastric is meanwhile exposed in the neck and cut across: the 

 object of dividing it is to prevent stimuli travelling to the 

 brain by the afferent fibres in it, as they would act on the nerve- 

 centres and lead to complicated results. The peripheral end 

 of the cut nerve is then stimulated, the excitation commencing 

 at, say, the instant corresponding to the point o on the tracing. 



