propagation is slow and diffusive; whereas in the nerve it is rapid, and restricted along a definite path. By this rapidity and restriction the force of the impact is increased; and thus a slight stimulus applied to the nerve is capable of disturbing the state of the muscle. 49. Thus while molecular movement is a fundamental condition of Vitality, and is incessant throughout organized substance, the massive movements of the organism, and the movements of particular parts, are the _directed quantities_ of this molecular agitation. They are due to stimulation. We distinguish this from mechanical impulsion. It is a vital process involving molecular change; it is not simply the _communication_ of motion from without, but the _excitation_ of motion within. It is not like the blow which merely displaces an object, but like the blow which disturbs its molecular equilibrium. The effect, therefore, depends on this molecular condition: the blow which scatters a heap of gunpowder will explode a fulminating salt, and this, in exploding, will excite the gunpowder to explode. The stimulus which is too feeble to excite contraction in a muscle will be powerful enough to excite the neurility of a nerve, and _that_ will excite the contractility of the muscle. The nerve-force is simply neural stimulus. It acts upon the other tissues as the nitrogenous salt upon the gunpowder. Although it is now common to speak of nerves as transmitting waves of molecular motion, and to regard nerves as the passive medium for the “transference of force,” whereby the force is thus made an abstract entity, we must always remember that such phrases are metaphors, and that the truer expression will be not “transference of force,” but the “propagation of excitation.” I mean that it is not the _force_ of the impact nor its _energy_ which a nerve transmits, it is the vibratory change produced in the nerve by the impact, which excites another change in the organ to which the nerve goes. We know by accurate measurements that the excitation of a nerve lasts much longer than the _stimulus_, a momentary impact producing an enduring agitation. We know also that the excitation of a centre lasts longer than the muscular contraction it has initiated. We know, moreover, that a nerve may be totally incapable of conducting an external stimulus, yet quite capable of conducting a central stimulus; were it a passive conductor like a wire this would not be so.[99] 50. The nerve is essentially an exciter of change, and thereby a regulator. A muscle in action does not appreciably determine action in any other (except in the comparatively rare cases of anastomosing muscles); a secreting cell does not propagate its excitation to others. The nerve, on the contrary, not only propagates its excitation, and awakens the activity of the muscle or gland with which it is connected, but through the centre affects the whole organism-- “Ein Schlag tausend Verbindungen schlägt.”