The arteries also are under the influence of two antagonistic sets of nerves. Those which increase their tonic contraction are almost universal in their distribution. It may be that those which actively check it are equally widespread, but the evidence is not altogether free from ambiguity. On certain organs—such as the salivary glands, already instanced—which require great variations in the amount of blood supplied to them, the influence of dilator nerves is very marked. The simplest hypothesis as to the mode of action of vaso-constrictor and vaso-dilator nerves leaves the initiative with the muscle-ibres of the vessel-wall. The distending internal pressure of blood is the stimulus which induces the muscle to contract. In some invertebrate animals—the snail, for example—if blood be prevented from entering the heart, so that there is no distending pressure, the heart stops. In higher animals the heart has acquired a habit of contracting, which keeps it going in the absence of its proper stimulus. The two classes of nerves exercise opposing influences on the muscle. Vaso-constrictor nerves increase the excitability of its fibres; vaso-dilator nerves diminish it. Only thus can we explain their action on a common basis. A good deal might be said as to the reasonableness of such an explanation. Our views as to the relation of nerve-influence and muscle-contraction are apt to go astray, owing to the fact that generations of physiologists have observed the phenomenon of a spasm of a muscle following on a sudden stimulus to a nerve. The two events are evidently related. The stimulus appears to set up a new condition in the nerve—to initiate a process which was not occurring before the electric current was passed through it. The muscular spasm equally appears to be an isolated event. As usual, we are misled by the analogy of human inventions. We compare the nerve-impulse to the fall of a hammer, the muscle-spasm to the explosion of gunpowder. We forget that nerve and muscle are in permanent connection; that the impulse is a sudden exaggeration of an influence which the nerve is continuously exerting, the contraction an exaggeration of metabolic changes which are constantly occurring in muscle. (See in this connection the explanation of muscle-tone, p. 273.) In the case of plain muscle, nerve stimuli do not cause contraction; they merely increase the excitability of the muscle. It may be more difficult for us to figure to ourselves the way in which dilator nerves diminish excitability; but the existence of such an anabolic influence is beyond the reach of doubt. Heart and bloodvessels are part of the same system. The heart has its accelerator and inhibitory nerves, the bloodvessels their constrictor and dilator nerves. For both vessel-wall and heart the stimulus to contraction is the distending pressure of blood—although it is not altogether necessary that this stimulus should be acting at the time.