All anatomists have heretofore considered the nervous system in an uniform manner; but if we reflect a little upon the forms, the distribution, the texture, the properties and the uses of the different branches that compose it, it is easy to see that they should be referred to two general systems, essentially distinct from each other, the one having the brain and its dependancies for its principal centre, and the other, having the ganglions. The first belongs especially to animal life; it is on the one hand the agent, that transmits to the brain the external impressions that are to produce sensations; and on the other it serves as a conductor of the volitions of this organ, which are executed by the voluntary muscles to which it goes. The second, almost every where distributed to the organs of digestion, of circulation, of respiration, of the secretions, belongs more particularly to organic life, in which it performs a part much more obscure than that of the preceding one. Neither is strictly confined to the organs of either life. Thus the cerebral nerves send some branches to the glands, to the involuntary muscles, &c.; and the nervous system of the ganglions have ramifications in the voluntary muscles. It is from the general arrangement, without regard to particular exceptions, that the division of the two nervous systems is founded, between which I shall not draw a parallel here to show their difference, because the description of each will be sufficient to do this. The nervous system of animal life is exactly symmetrical, like all the organs of that life. The brain and spinal marrow, which are the double origin of this system, have this character in a remarkable degree. Nerves precisely similar go from them; hence the name of pair, by which is designated the double, corresponding trunk, a name, that we should not be able to employ commonly in the system of ganglions. There are then really two nervous systems of animal life, the one right, the other left; the median line separates them. Their distinction is apparent not only from dissection, but from their diseases. At one time exactly one half of the body is deprived of motion, and the whole nervous system of that side remains passive, the other retaining its ordinary activity; at another, one side only has an unnatural energy and becomes the seat of convulsions, while the other remains calm. In both cases, sometimes the phenomenon is general; often it is limited to a greater or less number of lateral organs; but always there is an evident separation between the two nervous systems, the right and left. The kind of partial paralysis, of which I just spoke, and the principal character of which arises from the symmetry of the nervous system of animal life, is wholly different, as it regards this character, from that in which the lower parts of the body are deprived of motion in consequence of a fall upon the sacrum, or any other analogous cause.