From what we have just said upon the internal substance of the hairs, it appears that it has a true analogy with the reticular body of the skin, and that, like it, it arises from two sorts of vessels, one in which the colouring matter stagnates, the other which gives passage, in some cases at least, to fluids, and in which there is consequently a kind of circulation. The colouring substance of the hairs has some analogy with that of the skin. Thus we observe that the first, like the second, is blacker in warm climates and nearer the equator than in colder ones; thus red hair is frequently found with freckles which are more or less abundantly spread upon the skin of some people, and which are evidently seated in the reticular body, as I have ascertained in many patients who had these marks, and in whom the epidermis was raised up either by erysipelas or a blister. The acids however change the colour of the hair more than they do that of the skin of negroes. The muriatic whitens at first the hairs of the head which become yellow in drying; the nitric yellows, and the sulphuric leaves them black. That which especially interests us in the internal substance of the hairs, is the real vitality which it enjoys, and which essentially distinguishes it from the external covering. It is to this character that must be referred the following phenomena. 1st. The different passions of the mind have a remarkable influence upon the internal substance of the hairs. Often, in a very short time, grief has changed the colour of it, and whitened it by occasioning no doubt the reabsorption of the fluid contained in the small capillary vessels. Many authors have related facts of this kind. Some, even Haller, have doubted them. But I know at least five or six instances in which a discoloration has taken place in less than eight days. The hair of a person of my acquaintance became almost entirely white in the course of a night upon the receipt of melancholy intelligence. In these changes, the epidermoid covering remains the same, preserves its texture, its nature and its properties; the internal substance only is altered. It is said that terror can make the hair stand an end; painters express it even by this external attribute; I know not to what extent we should give belief to this phenomenon which I have never seen; but it is an opinion too generally received not to have some real foundation. Now if fear acts so powerfully upon the hair, if it can give it a real motion, is it astonishing that grief and pain should suddenly change the fluids that are found in it, and deprive it even of these fluids?