It follows from all these considerations and many others for which we are indebted especially to Reil, that the internal arrangement of the nerves varies singularly, that each presents almost a different texture, that under this point of view they do not resemble the arteries and veins, which are every where the same, whatever be their size, their course, &c. These varieties however, do not affect the intimate structure, and our business is to describe this intimate structure even to the last fibres that we can separate. Reil appears to me to have thrown great light upon this subject. I have repeated exactly his experiments; they have given results very analogous to his. Some only have appeared to me so difficult, that I have not even undertaken them. I have added to his researches many new facts as will be easily seen by comparing his work with this article, in which will only be found that which rests on accurate observation; I have omitted all the theoretical ideas that Reil has added to the facts which he offers. 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. _Of the nervous coat and its origin._ 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.