THE CIRCULATION OF THE BLOOD AND LYMPH 133 



the staircase or ' treppe ' has been given from the appearance 

 of the tracings (p. 548). 



The Extrinsic Nervous Mechanism of the Heart. While, as 

 we have seen, the essential cause of the rhythmical beat of 

 the heart resides in the tissue of the heart itself, it is con- 

 stantly affected by impulses that reach it from the central 

 nervous system. These impulses are of two kinds, or, rather, 

 produce two distinct effects : inhibition t or diminution in the 

 rate or force of the heart-beat, and augmentation, or increase 

 in the rate or force. Both the inhibitory and the augmentor 

 impulses arise in the medulla oblongata, and perhaps a 



A frog's heart 

 was stimulated at 

 a point correspond- 

 ing to the nick in 

 the horizontal line 

 below each curve. 

 In i and 2 there 

 was no response ; 

 in 3 and 4 there 

 was an extra con- 

 traction, succeeded 

 by a compensatory 

 pause. 



FIG. 44. REFRACTORY PERIOD AND COMPENSATORY PAUSE (MAREY). 



narrow zone of the neighbouring portion of the cord ; and 

 they can be artificially excited by stimulation in this 

 region. They pursue their course to the heart by fibres 

 which may in certain animals be mingled together, but are 

 anatomically distinct. We may, therefore, divide the ex- 

 trinsic or external nervous mechanism of the heart into ? a 

 cardio-inhibitory centre with its efferent inhibitory nerve- 

 fibres, and a cardio-augmentor centre with its efferent 

 augmentor nerve-fibres. Both of those centres, as we shall 

 see, have also extensive relations with afferent nerve-fibres 

 from all parts of the body, including the heart itself. 



It was in the vagus of the frog that inhibitory nerves were 

 first discovered by the brothers Weber more than fifty years 



