CHAP, iv.] THE VASCULAR MECHANISM. 253 



not only by artificial stimulation of the vagus, 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 mentioned a little while ago 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. These are familiar examples of 

 more or less complete inhibition ; but simple slowing or weakening 

 of the beat through the inhibitory mechanism is probably an 

 event of much more common occurrence. For instance, a rise of 

 general blood pressure, or, and perhaps more especially, a rise in 

 the blood pressure of the vessels of the brain, sets going inhibitory 

 impulses by which the work of the heart is lessened, and the high 

 blood pressure lowered, the dangers of a too high pressure being 

 thus averted. Again, the inhibition may be brought about in a 

 reflex manner by impulses started in the heart itself and ascending 

 to the central nervous system along afferent fibres which run in 

 the vagus trunk from the heart to the spinal bulb. In this way the 

 heart regulates its own action according to its condition and its 

 needs. 



There is also some reason for thinking that, in some animals 

 at least, 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 for example, section of both vagi causes a quickening 

 of the heart's beat. But we shall have to speak of these matters 

 more than once later on. Meanwhile we may turn to the augmentor 

 fibres. 



So much of our knowledge of the nervous work of the heart and 

 especially of the action of the augmentor fibres has been gained by 

 experiments on dogs that it may be desirable to give a few details con- 

 cerning the nerves of tlie heart in this animal. 



In the dog the vagus soon after it issues from its trunk ganglion 

 (G. Tr. Vg., Fig. 71) is joined by the sympathetic nerve proceeding from 

 the superior cervical ganglion, the two forming the vago-sympathetic 

 trunk. As this trunk enters the thorax, the sympathetic portion bears 

 a ganglion (G.C.) usually called the lower cervical ganglion. To this 

 ganglion there pass from the stellate ganglion (G.St.) of the thoracic 

 sympathetic chain, two nerves, one running ventral to, the other dorsal 

 to the subclavian artery, and thus forming with the two ganglia, the 

 annulus of Vieussens (An. V.). 



A very large number of the cardiac nerves spring from the lower 

 cervical ganglion and from the vagus trunk lying in contact with it, 

 from the vagus trunk below this ganglion, from the annulus of Vieus- 

 sens, chiefly at least from the ventral limb, and sometimes from the 

 stellate ganglion. There are besides cardiac branches passing from 

 the vago-sympathetic trunk between the levels of the superior and 

 of the inferior cervical ganglia, cardiac branches of the recurrent 

 laryngeal, a cardiac branch of the superior laryngeal, and a long 



