The Vaso Motor Nerves, Or Nerves To The Arterioles, form an important part of the sympathetic system. When in action, they contract the muscular coats of the vessels, and limit the amount of blood to the part supplied by them; and it has been already pointed out (p. 125), that division of the sympathetic in the neck causes paralysis, and consequent distension, of the arteries of the head. But division of the spinal cord in the cervical region produces paralysis of the bloodvessels of the whole body; and other experiments show that the great vaso-motor centre of all the vessels is situated in the neighbourhood of the medulla oblongata; and that while the vaso-motor nerves of the head and neck leave the cord at the base of the neck to reascend in the sympathetic, those to the rest of the body issue by the anterior roots of the spinal nerves.

The heart receives its nervous supply partly from the sympathetic, and partly from the pneumogastric nerve. Irritation of the sympathetic accelerates its action, as also does irritation of the branch of communication between the spinal cord and the inferior cervical ganglion of the sympathetic. Thus the heart may be said to receive its motor supply similarly to the arteries. But what is much more remarkable is, that irritation of the pneumogastric diminishes the frequency of the heart's action, and if carried sufficiently far arrests it. This it does by preventing in some way the normal impulse to contraction; for the arrested heart has its walls relaxed, and contracts on application of direct stimulus, and therefore the inhibitory action, as it is called, cannot be explained either by spasm or exhaustion.

There are other instances of similar inhibitory actions. Thus, if one of the sensory nerves of a rabbit's ear be divided, and its central end stimulated, the vessels of the ear dilate; and in the case of the submaxillary salivary gland of a rabbit, while irritation of the sympathetic contracts the blood-vessels, irritation of another nerve dilates them and increases the secretion. Such phenomena have led to the use of the expression inhibitory nerves; but the use of that phrase must not lead it to be supposed that the inhibitory action is in any case direct. The dilatation of the vessels of the submaxillary gland is explained by the consideration that nerves end in the secreting cells, and that increased action of the cells may well attract more blood; and cases which cannot be explained in a similar fashion may possibly be all of them the result of action on nervous centres, and not on terminal organs. It is not conceivable that nervous impression could have two antagonistic effects on a muscle, according as it came by one nerve fibre or another; but the heart has ganglia within it, and it can be understood that a nervous current from one source might divert another current out of its usual channel, or possibly oppose its passage through a ganglion.