ANIMAL SPIRITS are supposed to consist of a fine and subtle fluid, secreted in the cortical substance of the brain, and spinal marrow, which passes through the medullary part, and is conveyed through the cavities of the nerves to every point of the body. In support of this ingenious and probable conjecture, many arguments have been adduced: Dr. Mead, in his Essay on Poisons, contends that the existence of animal spirits is proved by the almost instantaneous diffusion of poisonous venom through the whole body; for the bite of a rattle-snake killed a dog in less than a quarter of a minute, in which time, according to Dr. Keil's computation of the velocity of the blood, the brain or heart could not have been injured by its circulation: the cause of its death must, therefore, be ascribed to a quicker medium, which can be no other than that of the animal spirits.