There is no doubt as to the relations of the nervous with the other systems, of those which it has with the muscles and with the brain. In fact, these relations are natural; for the one cannot be affected without the other's feeling it. These three organs can in this respect be considered under one point of view. Thus too, the pulsation of the arteries is always connected with the action of the heart, &c. Every idea of sympathy excludes that of a natural connexion of functions. Barthez is mistaken upon this point. I speak only of the unnatural relations of the phenomena that take place between an organ and a portion of the nervous system which is not connected with it by the natural order of life; now, thus considered, the nervous sympathies are very numerous. 1st. Two nerves of the same pair often sympathize with each other. We know in medicine the relation there is between the two optic nerves; the one being disordered in its functions, the other frequently becomes so. This happens more rarely with regard to the ears, the nostrils, &c. though it sometimes takes place in them. Often in neuralgia (a word which I adopt very willingly and which is wanted in the science to express a class of diseases every genus of which has a distinct name) often I say in neuralgia a nerve being painful, the corresponding one becomes so sympathetically. I have an example of this at present; it is a woman, who for two months has been afflicted with sciatica of the left limb. In changes of weather, a pain precisely similar spreads itself along the course of the nerve of the opposite side. I applied two blisters upon the thigh first affected; the pain disappeared in both sides at the same time at the end of twelve hours. Thus, in order to cure pains in both eyes, it is oftentimes sufficient to act only upon one, &c. 2d. Sometimes two nerves of the same side sympathize without belonging to the same trunk. Thus an injury of the frontal nerve has been many times followed by a sudden blindness in consequence of the affection of the optic nerve, &c. 3d. In other cases it is the branches of a common trunk which influence each other reciprocally, as when a branch of the superficial temporal is wounded in the operation of arteriotomy, the whole face, which also receives its nerves from the fifth pair, becomes painful, &c. 4th. At other times, it is not among themselves that the nerves sympathize, but with other organs; and then sometimes they influence, at others they are influenced.