2d. The cerebral substance and that of the spinal medulla easily become putrid when submitted to the combined action of water and air; they become of a greenish colour and have acid sufficient to redden blue paper. Of all animal substances, I think they exhibit this phenomenon the soonest. The nervous medullary substance, on the contrary, resists putrefaction much longer. The nerves are among the slowest of all the parts of the animal economy to become putrid. During life they are often found untouched in a gangrenous limb, in the middle of an abscess, &c. In a dead body which is putrid, they preserve their whiteness and consistence, while the other parts are black and soft. I have observed that the water in which the nervous system has been macerated has but little odour, but that that in which the brain has been macerated is fœtid. These phenomena clearly would not take place if the medullary substance of the nerve became as easily putrid as that of the brain. It is manifest, however, that it is especially to the nervous coat, that the nerves owe this sort of incorruptibility; for I have observed, that the optic nerve, in which the medullary substance predominates, and the olfactory and auditory, which are abundantly furnished with it, become putrid sooner than the others. I have remarked also uniformly, that whilst the white substance of the spinal marrow becomes putrid, its covering remains untouched. 3d. The medullary substance of the nerves, as well as that of the brain and spinal marrow, does not seem to be susceptible of any kind of horny hardening. This is very evident when we immerse the two last in boiling water, in a concentrated acid, &c. We may be convinced as to the first, by submitting to the same experiment the soft nerves that have their nervous coat pretty distinct. To this also must be referred the following phenomenon; when the anterior part of the optic nerve is put into boiling water, the nervous coat becomes wrinkled, its canals shrink, and the medullary substance not contracting in proportion, is forced towards the extremities, which become consequently enlarged. As this substance is in less proportion in the other nerves, this phenomenon is less apparent in them; it takes place, however, and this explains the small round tubercle that is seen at each end of boiled nervous filaments; it is the medullary substance that produces these enlargements. This phenomenon is very evident in the spinal marrow, which, being immersed in boiling water, suffers the compressed substance to escape, either at the extremities, or at any openings that may be made in its covering. Thus in boiling a head, the dura mater detached from the cranium contracts powerfully in hardening like horn, compresses the cerebral substance which does not contract like it, and sometimes breaks it, so that it escapes into the space that the boiling has produced between the dura mater and the cranium.