another dog with hostile intentions, namely, with erected ears, eyes intently directed forwards, hair on the neck and back bristling, gait remarkably stiff, with the tail upright and rigid. So familiar is this appearance to us, that an angry man is sometimes said "to have his back up." Of the above points, the stiff gait and upright tail alone require further discussion. Sir C. Bell remarks 1 that, when a tiger or wolf is struck by its keeper and is suddenly roused to ferocity, "every muscle is in tension, and the limbs are in an attitude of strained exertion, prepared to spring." This tension of the muscles and consequent stiff gait may be accounted for on the principle of associated habit, for anger has continually led to fierce struggles, and consequently to all the muscles of the body having been violently exerted. There is also reason to suspect that the muscular system requires some short preparation, or some degree of innervation, before being brought into strong action. My own sensations lead 1 'The Anatomy of Expression,' 1844, p. 190. [page] 123 me to this inference; but I cannot discover that it is a conclusion admitted by physiologists. Sir J. Paget, however, informs me that when muscles are suddenly contracted with the greatest force, without any preparation, they are liable to be ruptured, as when a man slips unexpectedly; but that this rarely occurs when an action, however violent, is deliberately performed. With respect to the upright position of the tail,