THE VASCULAR MECHANISM. 205 



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 some- 

 what greater after than before inhibition. Besides, when the mercury 



Tracing showing the Influence of Cardiac Inhibition on Blood-pressure. From a Rabbit, x, 

 the marks on the signal line when the current is thrown into, and y, shut off from the vagus. The 

 time-marker below marks seconds, the heart, as is frequently the case in the rabbit, beating very 

 rapidly. 



monometer 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 stimula- 

 tion 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 

 working 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. 76) 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 upward through the 



