On this circumstance depends the easy motions of the fingers in performing music, and of the feet and arms in dancing and fencing, and of the hands in the use of tools in mechanic arts, as well as all the vital motions which animate and nourish organic bodies. On the contrary, many animal motions by perpetual repetition are performed with less energy; as those who live near a waterfall, or a smith's forge, after a time, cease to hear them. And in those infectious diseases which are attended with fever, as the small-pox and measles, violent motions of the system are excited, which at length cease, and cannot again be produced by application of the same stimulating material; as when those are inoculated for the small-pox, who have before undergone that malady. Hence the repetition, which occasions animal actions for a time to be performed with greater energy, occasions them at length to become feeble, or to cease entirely. To explain this difficult problem we must more minutely consider the catenations of animal motions, as described in Zoonomia, Vol. I. Sect. XVII. The vital motions, as suppose of the heart and arterial system, commence from the irritation occasioned by the stimulus of the blood, and then have this irritation assisted by the power of association; at the same time an agreeable sensation is produced by the due actions of the fibres, as in the secretions of the glands, which constitutes the pleasure of existence; this agreeable sensation is intermixed between every link of this diurnal chain of actions, and contributes to produce it by what is termed animal causation. But there is also a degree of the power of volition excited in consequence of this vital pleasure, which is also intermixed between the links of the chain of fibrous actions; and thus also contributes to its uniform easy and perpetual production. The effects of surprise and novelty must now be considered by the patient reader, as they affect the catenations of action; and, I hope, the curiosity of the subject will excuse the prolixity of this account of it. When any violent stimulus breaks the passing current or catenation of our ideas, surprise is produced, which is accompanied with pain or pleasure, and consequent volition to examine the object of it, as explained in Zoonomia, Vol. I. Sect. XVIII. 17, and which never affects us in sleep. In our waking hours whenever an idea of imagination occurs, which is incongruous to our former experience, we feel another kind of surprise, and instantly dissever the train of imagination by the power of volition, and compare the incongruous idea with our previous knowledge of nature, and reject it by an act of reasoning, of which we are unconscious, termed in Zoonomia, "Intuitive Analogy," Vol. I Sect. XVII. 7.