4th. When the cerebral substance is agitated in water, it becomes suspended in it in the form of an emulsion, as has been observed by Fourcroy, then it is precipitated to the bottom of the vessel. A similar emulsion is made by the olfactory nerves, the posterior part of the optics, &c. When the anterior part of these, in which the nervous coat is very evident, has been soaked some time in water, and commonly even without this, a large quantity of whitish substance can be pressed out of them, which is evidently analogous to the medulla of the brain, and which colours the water that receives it. From the other nerves in which the medullary substance is much less abundant, it can often be forced out by pressure, from the cut ends of the filaments, especially if they have been previously macerated in an alkaline solution.
5th. Boiling hardens the brain, and gives it a greyish and dingy appearance, very similar to what is seen in ataxic fevers. The same phenomenon takes place in the soft nerves. In the others, the nervous coat is in too great a proportion to the medullary substance to allow us to see what happens to this last. It is to this property of coagulating by heat, which the brain has, that must be referred the flaky precipitate that is obtained in a heated cerebral emulsion.
6th. All the acids that are much concentrated harden the brain very evidently, the instant it is immersed in them. The sulphuric afterwards softens it, and finally reduces it to pulp, if it is not diluted. The nitric makes it yellow only, in hardening it. The muriatic has the least action upon it. The effect of acids upon the soft nerves is very analogous to this. In those in which the coat is very distinct, the horny hardening of which this coat is the seat, conceals all the sudden phenomena relative to the medullary substance. When the coat is softened and dissolved, this substance has appeared to me to be diminished in consistence and altered by the acids, whereas that of the brain keeps always the same degree of hardness, if the acid be not too much concentrated.
We all know that alkohol hardens the brain. This hardening, the effect of acids, of boiling, and of alkohol, is a phenomenon that the anatomist can avail himself of to give the parts he dissects a firmness, that will enable him to examine them better. It approximates this substance to the albuminous fluids. I say that it approximates it, for there are still great differences between them, of which, I think, we know but little.