CHAP. HI. ACTION OF THE NERVOUS SYSTEM. 67 tance must be discussed at some little length; and it is always advisable to perceive clearly our ignorance. The most striking case, though a rare and abnormal one which can be adduced of the direct influence of the nervous system, when strongly affected, on the body, is the loss of colour in the hair, which has occasionally been observed after extreme terror or grief. One au- thentic instance has been recorded, in the case of a man brought out for execution in India, in which the change of colour was so rapid that it was perceptible to the eye.1 Another good case is that of the trembling of the muscles, which is common to man and to many, or most, of the lower animals. Trembling is of no service, often of much disservice, and cannot have been at first acquired through the will, and then rendered habitual in association with any emotion. I arn assured by an eminent authority that young children do not tremble, but go into convulsions under the circumstances which would induce excessive trembling in adults. Trembling is excited in different individuals in very different de- grees, and by the most diversified causes,—by cold to the surface, before fever-fits, although the temperature of the body is then above the normal standard; in blood-poisoning, delimim tremens, and other diseases; by general failure of power in old age; by exhaustion after excessive fatigue; locally from severe injuries, such as burns; and, in an especial manner, by the passage of a catheter. Of all emotions, fear notoriously is the most apt to induce trembling; but so do occasionally great an- ger and joy. I remember once seeing a boy who had just shot his first snipe on the wing, and his hands 1 See the interesting- cases collected by M. G. Pouchet in the * Revue des Deux Mondes,' January 1, 1872, p. 79. An instance was also brought some years ago before the British Association at Belfast.