CHAP, iv.] 



THE VASCULAR MECHANISM. 



185 



and flaccid. If the duration of the current be short and the 

 strength of the current great, the standstill may continue after the 

 current has been shut off; the beats when they reappear are 

 generally at first feeble and infrequent, but soon reach or even go 

 beyond their previous vigour and frequency. A wholly similar 

 inhibition may be seen in the mammal, and indeed in man: 

 Gzermak, by pressing his vagus against a small osseous tumour 

 in his neck, and thus mechanically stimulating the nerve, was able 

 to stop at will the beating of his own heart; it need hardly be 

 added that such an experiment is a dangerous one. 



The effect is not produced instantaneously; if on the curve the 

 point be exactly marked as at a (Fig. 40), when the current is 



IIWMJU 



FIG. 40. INHIBITION OF FEOG'S HEABT BY STIMULATION OF THE VAGUS. 

 The contractions of the ventricle are registered by means of a simple lever, so 

 that each rise of the lever corresponds to a beat. The interrupted current was 

 thrown in at a, and shut off at 6. It will be seen that one beat occurred after a, and 

 that the pause continued for some time after 6. To be read from right to left. 



thrown in it will frequently be found that one beat at least occurs 

 after the current has passed into the nerve. In other words, the 

 inhibitory action of the vagus has a long latent period; this has 

 been estimated by Bonders to last in the rabbit '16 sec. The in- 

 hibitory effect is at a maximum soon after the moment of applica- 

 tion of the current, and diminishes gradually onward; so much so 

 is this the case, that when the current is applied for more than a 

 very short time the heart recommences beating before the current 

 is removed. 



It is obvious that the normal beat of the heart may be in- 

 terfered with in two distinct ways : on the one hand the systole of 

 the auricles and ventricle (or of either) may be diminished in 

 vigour, on the other hand the diastole or passive interval may be 

 prolonged. The vagus is able to act upon the heart in both these 

 directions; and sometimes the one, sometimes the other effect 

 is most prominent. Thus at times, as in the instance shewn in 

 Fig. 40, the most conspicuous result is the total suppression for 

 some time, of all visible contractions of the ventricle; and the 

 beats, when they appear again, are separated by diastolic intervals 



