THE NERVOUS REGULATION OF THE HEART 



157 



X 



force of the beat or in the conductivity. Both the inhibitory and 

 th3 augmentor impulses arise in the medulla oblongata, and perhaps 

 a 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, there- 

 fore, divide the extrinsic or external nervous mechanisir of the 

 heart into a cardio-inhibitory centre with its efferent inhibitory 

 nerve-fibres and a cardio-augmentor centre with its efferent acceler- 

 ator or augmentor fibres. Both of thosg centres, as we shall see, 

 have, in addition, 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 seventy years 

 ago, and even now our knowledge of the 

 cardiac nervous mechanism is more com- 

 plete 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 sympa- 

 thetic ganglion, and thence find their way 

 along the sympathetic cord to its junction 

 with the vagus, in which they run, mingled 

 with the inhibitory fibres, down to the heart. 



When the vago-sympathetic in the 

 frog or toad is cut, and its peripheral 

 end stimulated, the heart in the vast 

 majority of cases is stopped or slowed, 

 or its beat is distinctly weakened without, it may be, any marked 

 slowing. In other words, the rate at which the heart was 

 working before the stimulation is greatly diminished, or reduced 

 to zero. Such an effect, a diminution of the rate of working, 

 we call Inhibition. What precise form the inhibition shall take, 

 whether the stoppage shall be complete or partial, and if partial 

 whether the beats shall be simply weakened without being slowed 

 or both slowed and weakened, appears to depend partly upon the 

 strength of the stimulus used and partly upon the state of the 

 heart itself. Some hearts it may be impossible to stop with weak 

 stimulation, although other signs of inhibition may be distinct; 



Fig. 69. Diagram of Extrinsic 

 Nerves of Frog's Heart (after 

 Foster). Ill, srd spinal 

 nerve; AV, annulus of Vieus- 

 sens; X, roots of vagus; IX, 

 glosso-pharyngeal nerve; VS, 

 combined vagus and sympa- 

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

 2nd, and srd sympathetic 

 ganglia. The dark line indi- 

 cates the course of the sym- 

 pathetic fibres. The arrows 

 show the direction of the aug- 

 mentor impulses. 



