It is generally understood by medical men, and taught in the schools, that there are in the animal economy four distinct systems of nerves. 1st system. This consists of the sensitive nerves, which are distributed to all parts of the animal economy endowed with feeling; and all external impulses are reflected to the medulla oblongata, &c. (See _Dadd's work on the Horse_, p. 127.) In short, these nerves are the media through which the animal gets all his knowledge of external relations. 2d system. The motive. These proceed from nearly the same centre of perception, and distribute themselves to all the muscles of voluntary motion. It is evident that the muscle itself cannot perform its office without the aid of the nerves, (electric wires;) for it has been proved by experiment on the living animal, that when the posterior columns of nervous matter, which pass down from the brain towards the tail, are severed, then all voluntary motion ceases. Motion may, however, continue; but it can only be compared to a ship at sea without a rudder, having nothing to direct its course. It follows, then, that if the nerves of motion and sensation are severed, there is no communication between the parts to which they are distributed and the brain. And the part, if its nutritive function be also paralyzed, will finally become as insensible as a stone--wither and die. 3d system. The respiratory. These are under the control of the will only through the superior power, as manifested by the motive nerves. For the animal will breathe whether it wishes to or not, as long as the vital spark burns.