THE VASCULAR MECHANISM. 203 



is at its maximum soon after the beginning of stimulation, a very prolonged 

 inhibition may be produced by prolonged stimulation ; indeed by rhythmi- 

 cal stimulation of the vagus the heart may be kept perfectly quiescent for a 

 very long time and yet beat vigorously upon the cessation of the stimulus. 

 In other words, the mechanism of inhibition that is, the fibres of the vagus 

 and the part or substance of the heart upon which these act to produce in- 

 hibition, whatever that part or substance may be is not readily exhausted. 

 Further, the inhibition when it ceases is, frequently at all events, followed 

 by a period of reaction, during which the heart for a while beats more vig- 

 orously and rapidly than before. Indeed, the total effect of stimulating the 

 vagus 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 "aug- 

 mentor " 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. In contrast with the 

 case of the vagus fibres, a somewhat strong stimulation is required to produce 

 an effect ; the time required for the maximum effect to be produced is also 

 remarkably long. Moreover, at all events, in the case of a heart in which 

 the circulation is not maintained, and which is therefore cut off from its 

 normal nutritive supply, the augmentor fibres are far less easily exhausted 

 than are the inhibitory fibres. Hence, when in such a heart both sets of 

 fibres are stimulated together, as when the vagus trunk in the neck is stimu- 

 lated, the first effects produced are those of inhibition ; but these on con- 

 tinued stimulation may become mixed with those of augmentation, and 

 finally the latter alone remain. Lastly, the contrast is completed by the 

 fact that the augmentation resulting from the stimulation of the sympathetic 

 is followed by a period of reaction in which the beats are feebler ; in other 

 words, augmentation is followed by exhaustion ; and, indeed, by repeated 

 stimulation of these sympathetic fibres a fairly vigorous bloodless heart 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 sympathetic of the neck through the first 

 splanchnic or sympathetic ganglion connected with the first spinal nerve, 6r l 

 (Fig. 74), through one or both the loops of the annulus of Vieussens, An. V, 

 through the second ganglion connected with the second spinal nerve, 6r n , 

 to the third ganglion connected with the third spinal nerve, 6r ni , 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. 



145. Both sets of fibres may then be traced to the central nervous 

 system ; and we find accordingly that the heart may be inhibited or aug- 

 mented 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 medulla oblongata, or a particular part of the medulla oblon- 

 gata 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 region in question may be stirred into 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 intestines be struck sharply 



