We distinguish two things in every nervous filament, 1st. an external membrane in form of a canal, in which is contained the medulla; 2d. the nervous medulla itself; I shall now treat of each separately.
This membrane forms for each nervous filament a true canal which contains in its interior the medulla; as the veins and arteries contain the blood, with this difference, that this medulla is stagnant, while the blood circulates.
The origin of the nervous coat is very evident at the spinal marrow. It is continued with the dense and compact membrane which covers its white substance, and which is called the pia mater, though it does not resemble the membrane of that name which surrounds the cerebral circumvolutions. To see this origin well, this spinal membrane should be cut longitudinally before or behind. The medulla then appears whitish, soft and easily raised up. If it is raised and scraped with a scalpel or any other instrument, the immediate covering of the spinal marrow is thus separated from either side, especially if precaution be taken to wash it. It might be had in the form of a sac, by cutting out a piece of the medulla of a certain extent and then pressing out the medullary substance at the two ends. In this double experiment, the nerves remain attached to the membrane separated from its medullary substance, because their nervous coat is continued with it. It is exactly as if a number of small arterial filaments went from the aorta; the parietes of this artery would be to those of these filaments, what the pia mater of the spinal marrow is to the coat of the nerves which go from it. Only the nerves are white, because their medulla fills them; whereas the canal to which they belong is transparent, because it is deprived of its own medulla. I do not pretend however, that there is a perfect identity between these two membranes, since we do not exactly know the nature of either; I refer only to their anatomical arrangement.
As to the origin of the nerves contained in the cranium, those coming from the tuber-annulare and its dependancies, that is to say, the elongations that it receives from the cerebrum and cerebellum, have an arrangement analogous to that of the nerves of the spine. However, the difference of thickness and density of the pia mater establishes differences. In fact the pia mater which covers these parts is different from that which serves as a canal to the spinal marrow; it is much softer, less adherent, is torn with more ease, and appears to be analogous to that which covers the cortical substance of the brain. The coat of the nerves of the tuber annulare, which is manifestly continued from this portion of the pia mater, exhibits partly this character. At the place of their union, it is more soft than in the canal, hence the extreme facility with which, as I have observed, the origin of these nerves is broken. Moreover, the continuity with the pia mater is proved by the facility of raising the nerves by raising this membrane; almost always both are attached together.