134 



A MANUAL OF PHYSIOLOGY 



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 augmentation 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 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, 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, while they are readily 

 stopped by stronger stimulation. In other cases the 

 strongest stimulation may not produce complete standstill. 

 Again, a heated heart may be more readily brought to 

 standstill by stimulation of the vagus than a heart at the 

 ordinary temperature or a cooled heart. 



But there are other points of importance to be noted in 



FIG. 45 (AFTER FOSTER). 

 DIAGRAM OF EXTRINSIC 

 NERVES OF FROG'S HEART. 



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 ganglia. 

 The dark line indicates the course 

 of the sympathetic fibres. The 

 arrows show the direction of the 

 augmentor impulses. 



