Arising in the manner we have pointed out, the coat of the nerves passes with them through the cavity of the cranium and that of the spine. It is very distinct in these cavities, because it is not surrounded there with cellular texture, but only with the arachnoides, which may be raised with great ease; instead of using the different preparations that Reil mentions for the purpose of separating the coat of the nerves from the cellular texture, it is infinitely more convenient to examine this membrane upon the last nerves of the spine, which are, as we have seen, remarkably long. _Action of certain substances upon the nervous coat; its resistance, &c._ Without the osseous cavities, the nervous coat embedded in the cellular substance, adheres to it strongly, but appears evidently to be of the same nature as in the interior. We are ignorant what its nature is, whether it is the same as that of the pia mater, of the medulla, of the tuber annulare and its dependancies. It appears to have great affinity with the cellular texture. It is transparent and consequently free from the colour of the nerves; hence why, when they have been deprived, by alkalies, of their pulp, they lose a great part of their whiteness. The coat of the nerves is one of the parts of the animal economy which are hardened with the greatest ease, especially at the instant the nerves are immersed in an acid slightly concentrated, particularly the nitric and sulphuric. I have not observed in any other texture this phenomenon in so remarkable a manner; the nerve is suddenly diminished in size and twisted in different directions; now we shall see that the medullary substance is in no way concerned in this phenomenon. The action of boiling water produces an analogous effect; by it the nerve is wrinkled, contracted and hardened; then, after the ebullition has continued for a certain time, it gradually becomes soft, and its whitish colour is changed to a sort of yellowish tint, very different from that of boiled tendon or aponeurosis. The action of the acids continued for some time, produces an effect analogous to that of ebullition. To the sudden hardening like horn which the nerve undergoes, soon succeeds a softness so great that at the end of a short time it is easily moved under the finger, and afterwards becomes partly dissolved. The alkalies do not produce the horny hardening in the nervous coat any more than in any other texture of the living economy; neither do they dissolve it. Hence, undoubtedly, why Reil, having macerated for some time a portion of nerve in soap-boilers lie, was able to separate accurately the nervous coat from its medullary substance.