THE CIRCULATION OF THE BLOOD AND LYMPH 143 



fatigue-products such as carbon dioxide (Lee), so that when the 

 apex is stimulated at regular intervals, each contraction is some- 

 what stronger than the preceding one. To this phenomenon the 

 name of the staircase or ' treppe ' has been given from the 

 appearance of the tracings (p. 648). 



The Extrinsic Nervous Mechanism of the Heart. While, 

 as we have seen, the essential cause of the rrrythmical beat of 

 the heart resides in the tissue of the heart itself, it is constantly 

 affected by impulses that reach it from the central nervous 

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

 two distinct effects : inhibition, 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 ob- 

 longata, and perhaps a narrow zone 

 of the neighbouring portion of the 

 cord ; and they can be artificially ex- 

 cited by stimulation in this region. 

 They pursue their course to the heart 

 by fibres which may in certain animals 

 be mingled together, but are anatomi- 

 cally distinct. We may, therefore, 

 divide the extrinsic or external ner- 

 vous mechanism of the heart into a 

 cardio-inhibitory centre with its effer- 

 ent inhibitory nerve - fibres and a 

 cardio - augmentor centre with its 

 efferent accelerator or augmentor 

 fibres. Both of those centres, as we 

 shall see, have also extensive rela- 

 tions 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 sixty years ago, and 

 even now our knowledge of the cardiac nervous mechanism is 

 more complete in this animal than in any other. We shall, 

 therefore, first describe the phenomena of inhibition and augmen- 

 tation as we see them in the heart of the frog, and then pass on 

 to the mammal. 



In the frog the inhibitory fibres leave the medulla oblongata in 

 the vagus nerve. The augmentor fibres come off from the upper 

 part of the spinal cord by a branch from the third nerve to the 

 third sympathetic ganglion, and thence find their way along the 



FIG. 58. DIAGRAM OF EX- 

 TRINSIC NERVES OF FROG'S 

 HEART (AFTER FOSTER). 



Ill, 3rd spinal nerve ; AV, 

 annulus -of Vieussens ; X, roots 

 of vagus; IX, glosso-pharyngeal 

 nerve ; VS, combined vagus and 

 sympathetic; i, 2, and 3, the ist, 

 2nd, and 3rd sympathetic gang- 

 lia. The dark line indicates the 

 course of the sympathetic fibres. 

 The arrows show the direction 

 of the augmentor impulses. 



