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?
2d. The plica polonica, of which I spoke just now, in which the hairs of the head become, when they are cut or even when they are not, the seat of a bloody exhalation, and in which they have a remarkable excess of life, evidently resides in the internal substance; the epidermoid covering has no connexion with it. Some authors even say that this internal substance acquires sometimes a fleshy nature; then their covering is raised up in scales.
3d. We know the danger of cutting the hair after many acute diseases. I have already seen a melancholy instance of it. Many physicians, Lanoix in particular, have related others. Now, to what are these accidents owing? It is certainly not to the contact of the air, from which the hair defends the head; for these accidents take place, though the head may be covered. It can only be owing to this, that the growth of the hairs that are cut, calls to these organs a vital activity which the internal viscera soon sympathetically feel; hence the pains of the head, the affections of the eyes, &c. observed in these cases. It is a species of active sympathy exerted by the hair upon the viscera; now, every organ which sympathizes has a real vitality, and enjoys very distinct vital properties. The epidermis never takes part in sympathies, because it is almost completely inert, is hardly organized, is not at the level of the other organs, and cannot consequently correspond with them. The danger of cutting the hair after severe sickness, gives me opportunity to observe that it is often as dangerous to remove suddenly the vermin from the heads of children during these diseases. I have seen three or four instances of accidents from this cause.
4th. The hairs not only influence other systems, but are also influenced by them. This is what we often see after acute diseases, in which the roots sympathetically affected, repel the fluids that come to nourish them, die, and the hairs fall out. Observe that this falling out of the hair very rarely takes place at the same time with the desquamation of the epidermis; which proves, that the generally admitted opinion of the origin of the external covering of the hairs is entirely false, and that, though very analogous to the epidermis, this covering does not arise from it, as I have said.
5th. Many animals lose at one season of the year their hairy covering, which is afterwards reproduced; now the period of its regeneration is often that of many diseases, and almost always that of a greater weakness than at other times. We might say that the nutritive work which then calls to the exterior much vital force, diminishes this force in the other regions. Man is not subject to these annual renewals of the external productions which cover his body, like birds, many quadrupeds, reptiles, &c. It is a cause of less diseases. In fact, a thousand different causes would no doubt have frequently deranged these renewals in society, as a thousand causes disturb the menstrual evacuation, &c.; hence the various diseases we escape by the want of this renewal. Man is in general subjected to fewer causes of natural revolutions, than most animals.
6th. Heat and cold have also oftentimes an influence upon the internal substance of the hairs. We know that in some animals, as rabbits, hares, &c. they become white in the winter and resume their original colour in the summer.
7th. A short time after painting the hairs of the head black, a fashion now more common in France than at the period in which they powdered them, there is often experienced pains in the head and a swelling of the hairy scalp, though the skin has been in no way concerned, has not been pulled, and the hair only has been affected.
It follows from all we have just said, that the hairs analogous, by their external covering, to the epidermis; foreign by means of it, if we may so say, to life, belong to it much more particularly by their internal substance, a substance whose nature is yet but little known, as I have already said. What moreover evidently proves this assertion, is that the phenomena of which I have just spoken, and to which I could add many others, cease to be evident in persons, in whom the hairs having become white, have no longer any thing but the epidermoid covering, the internal substance having in part disappeared; particular observation proves this. It may be however that in this case that portion alone of this internal substance, corresponding to the colour, is destroyed, whilst that which is the seat of the exhalations continues to live as usual; and, in this respect, white hairs may experience vital phenomena, of which, I believe, there are a few examples. But all this is subordinate to the future experiments, which will elucidate the pilous structure more than it now is.