CHAP, iv.] THE VASCULAR MECHANISM. 249 



fibres is not to exhaust the heart, but rather to strengthen it ; and 

 by repeated inhibitions carefully administered, a feebly beating 

 heart may be nursed into vigorous activity. 



The other set, those joining the vagus from the sympathetic, 

 are 'augmentor' or 'accelerating' fibres; the latter name is the 

 more common but the former is more accurate, since the effect of 

 stimulating these fibres is to increase not only the rapidity but 

 the force of the beat ; not only is the diastole shortened but the 

 systole is strengthened, sometimes the one result and sometimes 

 the other being the more prominent. These augmentor fibres 

 need a somewhat strong stimulation to -produce an effect, the time 

 required for the maximum effect to be produced is long, and the 

 effect, when produced, may last for some time. A slowly or 

 weakly beating heart is more easily augmented than is a strong 

 one. Further, the augmentation is followed by a period of reac- 

 tion in which the beats are feebler, by a stage of exhaustion ; 

 and indeed by repeated stimulation of these sympathetic fibres a 

 fairly vigorous heart, especially a bloodless one, may be reduced 

 to a very feeble condition. 



By watching the effects of stimulating the sympathetic nerve 

 at various points of its course we may trace these augmentor 

 fibres from their junction with the vagus down the short sympa- 

 thetic of the neck through the sympathetic ganglion connected 

 with the first spinal nerve, G 1 , Fig. 69, through one or both the 

 loops of the annulus of Vieussens, An. V, through the second 

 ganglion, connected with the second spinal nerve, G n y to the third 

 ganglion connected with the third spinal nerve, G 111 , and thence 

 through the ramus communicans or visceral branch of that 

 ganglion, r.c., to the third spinal nerve, ///, by the anterior root 

 of which they reach the spinal cord. 



137. Both sets of fibres, then, may be traced to the central 

 nervous system ; and we find accordingly that the heart may be 

 inhibited or augmented by nervous impulses which are started in 

 the nervous system either by afferent impulses as part of a reflex 

 act or otherwise, and which pass to the heart by the inhibitory or 

 by the augmenting tract. 



Thus if the spinal bulb or a particular part of the spinal bulb 

 which is specially connected with the vagus nerve be stimulated, 

 the heart is inhibited ; if, for instance, a needle be thrust into 

 this part the heart stands still. This nervous area may be 

 stirred to action, in a 'reflex' manner, by afferent impulses 

 reaching it from various parts of the body. Thus if the abdomen 

 of a frog be laid bare, and the intestine be struck sharply with the 

 handle of a scalpel, the heart will stand still in diastole with all 

 the phenomena of vagus inhibition. If the nervi mesenterici or 

 the connections of these nerves with the spinal cord be stimulated 

 with the interrupted current, cardiac inhibition is similarly pro- 

 duced. If in these two experiments both vagi are divided, or the 



