THE HEART 



55 



Centre from without — must pass through the centre from 

 which the inhibitory fibres of the heart arise, and though 



x, Marks on the 

 signal line when th e 

 current is thrown 

 into, and y shut off 

 from the vagus. 

 The time - marker 

 below marks 

 seconds, a corre- 

 sponds in point of 

 time with x ; the 

 heart does not at 

 once cease to beat. 

 The first beat, b, 

 occurs a short time 

 after shutting off 



£~ J ■ the current. The 



notches in the tra- 

 cing are due to the 



~ ' ' ' * ' ' ' ' *" beats of the heart. 



Fig. 24. — Tracing showing Influence of the Arrest o* the Heart on 

 Blood-Pressure, due to Stimulating the Vagus (Foster). 



such a centre has 

 not been defined 

 with exactitude, the 

 existence of a cardio- 

 inhibitory centre in 

 the medulla is un- 

 doubted. From this 

 centre impulses issue 

 which in a normal 

 state throughout the 

 whole life of the 

 animal are passing 

 down the vagus, 

 keeping a constant 

 watch and control 

 over the rate of the 

 heart, and the evi- 

 dence that such a 

 view is not a fanciful 

 one is shown by the 

 result of cutting off 

 these impulses by 

 dividing both vagi, 



when the frequency of the heart-beats is greatly increased. 

 This constant action of a nerve centre is known as tonic activity, 

 and this in all probability, is the outcome of impulses flowing 



Fig. 25. 



-Blood-Pressure Tracings : Rabbit 

 (Stewart). 



Vagus stimulated at 1 ; stimulation stronger in B 

 than in A (Hurthle's sphygmanometer). 



