The spinal marrow gives origin to thirty or thirty-one pair of nerves, viz. eight cervical, twelve dorsal, five lumbar, five or six sacral, and to a nerve that enters the cranium and goes out of it under the name of spinal. The following are the characters of these nerves at their origin. 1st. They are continuous, like the preceding, with the medullary substance. 2d. They all arise by two cords, one anterior, the other posterior. These cords derive their origin from many filaments, placed above each other, most usually separate and always distinct. 3d. The adhesion is much stronger at the origin of these nerves than at that of the preceding, a circumstance that depends upon a cause that will be hereafter pointed out. 4th. The consistence of the spinal nerves is also very evident in their canals. From what has been said, it is clear, that the nerves do not arise deep in the cerebral substance, at least in an apparent manner, but take their origin from its external surface. Many physiologists however, have admitted an origin more remote than can be proved by examination. They have believed that the nerves of one side arise from the opposite, and that each pair cross each other not only in the brain, but also in the spinal marrow. This opinion is founded upon a singular phenomenon, viz. this, that paralysis almost always takes place on that side which is opposite to the affected side of the brain, a phenomenon that is frequently noticed in diseases and proved also by experiments, as has been shown by Lorry. On the other hand, it is said that convulsions are seated in the side corresponding with the injured side of the brain; but this fact is more uncertain than that of paralysis, which is incontestable. I do not believe that with our present knowledge we can explain this last, and the anatomical opinion pointed out above, is contradicted at the first sight.