124 EXPERIMENTAL PHYSIOLOGY 



again increased — 8 mm. before as compared to 10 during stimulation. 

 There is again an increase in the rate of conduction of the contraction 

 wave from auricle to ventricle. There is also less delay between the 

 commencement of stimulation and the production of its effect than in 

 the preceding case. 



In tracing 4 the same points are again to be observed, but it is 

 noticeable that the slowing becomes less marked as stimulation proceeds. 

 Rate of beating was 36 before and 24 during stimulation ; the height 

 before, 8 mm. ; during stimulation, 9 mm. 



In tracing 5 we see that after two beats the heart comes completely 

 to rest. There is a delay before the effect of the stimulus is apparent. 

 After stimulation ceased a short pause occurred, and then the heart 

 recommenced to beat. The rate gradually increased. Thus the time 

 interval between the commencement of the second beat and corre- 

 sponding point of the first is 3| sees. ; between 3rd and 2nd, 3 sees. ; 

 between 4th and 3rd, 2| sees. It then quickly regained its original 

 rate of 43 per minute. The height of the beat also shows a gradual 

 increase. The measurements are : For the 1st, 4^ mm. ; the 2nd, 

 5| mm. ; the 3rd, 6 mm. ; the 4th, 6^ mm. ; the 5th, 7 mm. ; and then 

 in a few beats it attained a height of 9 mm. as compared to one of 

 8 mm. before stimulation. Thus inhibition has had a beneficial 

 effect upon the ventricle, enabling it to beat a little more forcibly for 

 a time, but this gradually dies away, and in about 20 beats the height 

 is once more 8 mm. The same holds true even to a more marked 

 degree for the auricular contraction. The rhythm between auricle 

 and ventricle beats also shows a very interesting change. In the 

 first beat after the stimulation the ventricular contraction commences 

 shortly after the commencement of the auricular relaxation ; in the 

 second beat at a rather later time ; and as the beats follow one another 

 the ventricular beat gradually falls later in the auricular diastole. 

 Associated with the gradual slowing of the ventricular beat with 

 respect to the auricular it is seen that the auricular contraction begins 

 progressively earlier with respect to the ventricular diastole, and that 

 at last it occurs when about one half of the diastole is completed. As 

 this happens the auricular systole becomes progressively less and less 

 marked, and finally is only represented as a break in the descent repre- 

 senting the ventricular systole. These facts tend to show that con- 

 duction of the contraction wave from auricle to ventricle is at first 

 rapid, but gradually becomes slower than normal as an after-effect of 

 the vagus inhibition, once more running to its normal rate as that 

 after-effect gradually wears off. 



We must remember that in the results we have just been examining 

 we are not dealing with the result of stimulation of pure vagal fibres, 



