This property is, of all others, the most strongly marked in the nerves. Exposed and irritated, they cause great pain. By tying, pricking, cauterizing, or exciting in any way a nervous filament, we uniformly obtain that result so well known in practical surgery, and by those who make experiments upon living animals. This property would seem at first to establish a very great difference between the medullary substance of the nerves and that of the brain, especially towards the convexity of this organ; for we can almost with impunity irritate this after having removed the cortical substance. It is only deep in the brain that the animal sensibility becomes strongly marked, and even there it is not so much so as in the nerves. Observe, however, that in the experiments upon the cerebral pulp you destroy the organ itself that perceives, that, without which it could not have animal sensibility, and whose derangement would consequently have an inevitable influence upon this property; whereas the seat of perception being untouched when we irritate a nerve, the pain is more sensibly felt. It is, in fact, principally in the medullary substance of each nervous filament, that animal sensibility resides. The nervous coat possesses a much less degree of it. Hence why simple contact, without pressure, occasions but little pain; hence, also, why a nerve can almost with impunity be immersed in a purulent, ichorous fluid, or even in the sanies of cancer; hence why the contact of the air occasions but little pain when the nerves are merely laid bare, as I have had frequent occasion to see in animals; why, in a variety of cases, different tumours, in whose atmosphere the nerves are situated, have no influence upon them. The membrane of each filament is truly in every case a kind of covering that protects its medullary substance, in which the sensibility particularly resides. As to the cellular texture which enters into the composition of nerves, it has, as elsewhere, no connexion with this property. Hence why we can, as I have often done upon a living animal, separate from each other, with the point of a very delicate scalpel, the different filaments of a nerve of some size, of the sciatic, for example, when they have first been laid bare, without giving the animal much pain. In these experiments, it is easy to be convinced of the kind of insensibility of the covering of each nervous filament. It is necessary to pierce it and arrive at the medullary substance in order to produce pain.