CHAP, iv.] THE VASCULAR MECHANISM. 297 



spondingly rises in successive leaps until the normal mean 

 pressure is regained. The size of these returning leaps of the 

 mercury may seem disproportionately large, but it must be 

 remembered that by far the greater part of the force of the first 

 few strokes of the heart is expended in distending the arterial 

 system, a small portion only of the blood which is ejected into the 

 arteries passing on into the veins. As the arterial pressure rises, 

 more and more blood passes at each beat through the capillaries, 

 and the rise of the pressure at each beat becomes less and less, 

 until at last the whole contents of the ventricle pass at each 

 stroke into the veins, and the mean arterial pressure is established. 

 To this it may be added, that, as we have seen, the force of the 

 individual beats may be somewhat greater after than before inhibi- 

 tion. Besides, when the mercury manometer is used, the inertia 

 of the mercury tends to magnify the effects of the initial beats. 



In the mammal inhibition may be brought about by impulses 

 passing along fibres which, starting in the medulla oblongata, run 

 down over the vagus nerve and reach the heart by the cardiac 

 nerves. It would appear however that the inhibitory fibres do 

 not belong to the vagus proper but leave the central nervous 

 system by the spinal accessory nerve. Thus if the roots of the 

 spinal accessory be divided, those of the vagus proper being left 

 intact, the spinal accessory fibres in the vagus trunk degenerate, 

 and when this takes place stimulation of the vagus trunk fails to 

 produce the ordinary inhibitory effects. In the mammal as in 

 the frog inhibition may be brought about not only by artificial 

 stimulation of the vagus trunk, but by stimulation in a reflex 

 manner or otherwise of the cardio-inhibitory centre. Thus the 

 fainting which often follows upon a blow on the stomach is a 

 repetition of the result just mentioned as obtained on the frog by 

 striking the stomach or stimulating the nervi mesenterici. So also 

 the fainting, complete or partial, which accompanies severe pain or 

 mental emotion is an illustration of cardiac inhibition by the 

 vagus. In fact cardiac inhibition so far from being a mere 

 laboratory experiment enters repeatedly into the every day work- 

 ing of our own organism as well as that of other living beings. 



Indeed there is some reason for thinking that the central 

 nervous system by means of the cardiac inhibitory fibres keeps 

 as it were a continual rein on the heart, for, in the dog at least, 

 section of both vagi causes a quickening of the heart's beat. 



In the dog the augmentor fibres (Fig. 57) leave the spinal cord 

 by the anterior roots of the second and third dorsal nerves, possibly 

 also to some extent by the fourth and fifth, pass along the rami 

 communicantes of those nerves to the ganglion stellatum, first 

 thoracic ganglion, or respectively to one or other of the ganglia 

 forming part of the thoracic splanchnic or sympathetic chain 

 immediately below, and thence upwards through the annulus of 

 Vieussens, passing along one or other or both loops, to the inferior 



