Part 32

The external covering of the hair appears to be of the nature of the epidermis. It has in fact almost all the attributes of it. 1st. The hairs of the head burn exactly like this membrane, give out when burning an analogous odour, and leave after combustion a similar kind of coal; now it is principally to the external portion that these phenomena are owing. 2d. Water penetrates the hairs with great ease; hence very useful hygrometers can be constructed with them; now the same is true of the epidermis; and moistened hairs in foggy weather present in this respect a phenomenon analogous to that of the epidermis softened, wrinkled and whitened by the contact of a cataplasm. 3d. It is by means of the epidermoid covering that the hairs are foreign to life, that they are insensible and never become the seat of any acute or chronic affection. 4th. This covering is white, whatever may be the colour of the hairs. The cause of the colour resides in the internal medulla; thus the epidermis of negroes and that of white people differ but very little. Hence why when the internal substance of the hair has disappeared, the canal remaining alone exhibits a more or less evident whiteness. 5th. In this state, though the interior of the hair may be dead, the epidermoid exterior, which is independent of it, preserves most commonly the faculty of growing when it is cut; thus the cutaneous epidermis is truly foreign to all the subjacent diseases of the skin. 6th. I presume that it is this covering which gives to the hairs of the head the property of remaining so long uninjured. When removed far from the access of the air, they remain unaltered for ages; they have not in them the principle of decomposition of the other animal substances. They never become putrid either in air or water. Thus we have seen that the cutaneous epidermis never undergoes putrefaction, which seizes upon the subjacent parts.

It appears however that the hairs are more unalterable than the epidermis, and that there is even a difference of nature between them. In fact, 1st, maceration and ebullition, which make the epidermis very easy to be broken, though they soften it but little, leave the hairs with their usual resistance, unless carried to degrees that I have not tried. By boiling and macerating them comparatively with the epidermis, we easily make this observation. 2d. The acids act less efficaciously upon the hairs than upon this membrane; but the alkalies dissolve them with as much and even more ease. 3d. A thread of epidermis of equal thickness would be incomparably less resisting than a hair. 4th. The hairs can, like the epidermis, be painted of different colours; but they do not retain them so long, and on this account the colour must be renewed oftener.

Some modern authors have said that there is detached from the external covering of the hairs a kind of scales which form as it were little branches to them. We do not see these elongations. However the experiment mentioned by Fourcroy, and which consists in this, that by rubbing a hair between the fingers, it is raised like the heads of some species of grain in the direction from its base to its point, this experiment, I say, appears to prove the existence of these insensible elongations, which perform also an essential part in the adhesion of the hairs of the head to each other, an adhesion that is such that when they have remained a long time without being separated, as in long diseases, it is only done with the greatest difficulty.

Sometimes the hairs are bifurcated in a very evident manner at their extremity.

It is the greater or less thickness of the epidermoid covering of the hairs, which constitutes the different nature of them. Thick and compact on the genital parts, the chin, &c. it is less easily penetrated with water, and renders the hairs more elastic there and more capable of curling. Loose and thin in the hairs of the head, it makes them more smooth, and gives them more sensibly the property of the hygrometer. It is the peculiar nature of this external covering, which gives to the hairs of the head and the hair of negroes the character which distinguishes them.

From what we have just said it is evident that the external covering of the hairs of the head is the part of them which is essentially inert and foreign to life. It is not the same with their internal substance.

III. _Internal Substance of the Hairs._

This substance is the most important; it is this which essentially characterizes the hairs, which I should have ranked in the epidermoid system, if they had nothing but their external covering, as is the case when they become white.

We are entirely ignorant of the nature of this internal substance. It can only be presumed that there are extremely delicate vessels inclosed in the common epidermoid covering containing a colouring substance, which stagnates in these vessels, or at least is subjected in them to a very slow nutritive motion. Among these vessels, do any of them as on the skin, open outwards to throw off fluids? Many physiologists have thought so, and on this account they have considered the hairs as real emunctories. I do not believe that we have any anatomical data upon this point; but the plica polonica, a singular disease in which the hair when cut pours out blood, evidently proves that they have exhalants in a natural state, which then becoming enlarged and dilated, pour out a fluid that they before refused to admit. Besides, there is no doubt that the pilous exhalants, infinitely less active than the cutaneous, are a much less copious emunctory. As to the absorptions which some have pretended are made by the vessels of the hairs, I think that nothing can prove them.

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?

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.

ARTICLE THIRD.

PROPERTIES OF THE PILOUS SYSTEM.

The hairs experience but a slight degree of the horny hardening when exposed to the action of caloric. They then turn in various directions, curl and twist; but this arises from a cause entirely different from that of the horny hardening of the other organs. The caloric then removes the moisture with which the hairs are constantly penetrated, and thus approximates their particles. Thus when the hair is moistened by fog, a bath, &c. the curls disappear. The oily substances that are used at the toilet, give a coat that is insoluble in water, and preserve the curling, by preventing it from penetrating the hairs. Some time after the head has been washed, they curl more, as we have had occasion to observe since the Grecian head dresses have been in fashion among us. This at first appears to be contradictory, but it is not so. In fact by then rubbing the hairs much, the unctuous substance is removed, which always surrounds them, or this substance combines with the soap, if the water contains it, as is often the case; by this means it easily penetrates the hairs, the pores of which remain open, and by afterwards evaporating with the fluids that were already there, and which the unctuous substance retained, it leaves these organs more dry than they were, and consequently more disposed to curl.

A proof, that it is the epidermoid covering which thus imbibes the moisture that it afterwards loses in the state which succeeds the curling, is, that the detached epidermis can be curled with a hot iron, and afterwards rendered supple by soaking it in water.

The contractility and extensibility of texture are very indistinct in the hairs; it is their resistance which prevents their rupture; they can hardly be stretched at all.

They have no animal sensibility when pulled; the pain that arises from it has its seat especially in the skin through which they pass. Thus when drawn opposite to their direction, we suffer much more than by stretching them in the direction of their pores. I do not deny however that these elongations, which fix their origin to the neighbouring parts, may be also the seat of pain when the hairs are pulled. These organs have no animal contractility.

The organic properties certainly exist in their internal substance. The changes which this substance undergoes can only depend on the different alterations which affect these properties. The organic sensibility and the insensible contractility especially are raised in it in a remarkable degree in the plica polonica; now in order to have the degree of energy which they then do, they must have existed there in a natural state. It is these two properties, that, the sympathies of which we have spoken, put into action. The organic contractility is nothing in the hairs.

Yet we cannot deny that in the natural state, these organs are, next to the epidermis and the nails, those in which life is the least active, those which have the least numerous relations with the other organs. Whilst every thing is destroyed in most of the other systems by diseases, this is most often unaffected by them; it grows as usual, and appears to be in no wise disturbed; it has then a manner of being, of existing, wholly different from the others.

In general, the external productions of animals, as the feathers, the hair, the scales, &c. seem to form a separate class of organs, foreign to the life of the internal organs; it is almost like the different species of mosses that grow upon trees, without making essentially a part of them.

ARTICLE FOURTH.

DEVELOPMENT OF THE PILOUS SYSTEM.

I. _State of this System in the First Age._

In the first months of the fœtus there are no hairs on the skin which is then gelatinous. It is when the fibres of the dermoid texture are formed, that there begins to appear on the head a light down, an indication of the hairs which are afterwards to arise. This down is whitish and concealed by that fatty and unctuous substance, which we have said is deposited on the external surface of the skin at this age. Soon this down, which appears to be but the external covering of the hairs, which is then of extreme tenuity, begins to be coloured black or flaxen, according to the tint that is afterwards to predominate; it is the internal substance that forms it. The colour remains faint until after birth. At this period the hairs are often more than half an inch long. Upon all the rest of the body there is only the down, the precursor of the hairs; the face especially has much of it. The hairs of the head are then in advance at one period of the other hairs, in their growth.

After birth the hairs grow much more rapidly than before. It is precisely the reverse of most of the other parts, whose growth is more rapid in the womb of the mother. During the whole of youth this system has a tint less deep, than it is afterwards to have. The flaxen becomes nearer the chesnut, and this nearer black, and the first tints of the bright red grow many degrees darker towards the period from the twenty-sixth to the thirtieth year. The light tints are to the pilous system in youth, what the imperfectly developed forms are to the muscular, cellular, &c. Oftentimes that which is to be afterwards flaxen, approaches a whitish tint, which is owing only to the nature of the internal substance, and not to its absence in old age. Thus the white of the Albinos depends also upon the peculiar species of this internal substance. Many hairs are wanting upon the body of the young man.

II. _State of the Pilous System in the following Ages._

At puberty there is a remarkable revolution in this system which becomes almost double. The hairs of the genital parts are formed; the beard which is, as I have said, the characteristic attribute of the male in the human species, is also then developed. We might say that there was the same relation between the hairs of the neighbourhood of the testicles and those of the beard, as between the testicles themselves and the organs of the voice, between the womb and the mammæ. The beard is, in this respect, the external sign of virility. Some time before it comes out, we see under the skin the sac which contains the origin of the hairs; it is already very evidently formed, and permits the principle of the organ to be seen which it is to contain, as I have oftentimes ascertained; thus the sac of the tooth exists a long time before the tooth is cut.

At the same time the hairs of the axilla grow also; those of the trunk and extremities, which were then almost in a state of down, become larger, assume a determinate colour, and increase even much in number.

Why does puberty occasion this general growth in the pilous system? This is asking the reason of all the other phenomena which appear at this period. I would only observe that the hairs of the head, the eyebrows, the eyelashes and the hairs at the openings of the body, are those which are the least affected by this revolution. Besides, this growth is gradual; it requires at least two or three years for the beard to become what it is always to be.

In the following ages the hairs undergo but few changes; they grow in proportion as they are cut in different parts, and are the seat of a constant external work; now, observe that this work is more prompt, and the growth of the hairs consequently more rapid, in summer in which the cutaneous organ is especially in action, than in winter in which it is contracted; an additional proof of the real vitality of the organic forces of the internal substance of the hairs.

III. _State of the Pilous System in Old Age._

Towards the end of life, the pilous system is affected by the general obliteration which takes place in almost all the external vessels; it ceases at first to receive the colouring substance. The internal substance dies, the epidermoid covering remains alone; the hairs become white. The hairs of the head appear the first, and are the first to die. The beard, the hairs of the genital parts and then those of all the parts of the body afterwards die. Besides, there is a great variety among men as it respects the period in which the hairs whiten; in some, this phenomenon begins about the thirtieth year, and even sooner, in others it is towards the fortieth, fiftieth or sixtieth. A thousand causes arising from the passions of the mind, from diseases, aliments, &c. can have an influence in society upon this premature death, so common in many men, but which does not take place in animals, who are not exposed from their kind of life, to the same revolutions, until the last years.

The hairs, after remaining white for a longer or shorter time, finally fall out; then the sac which covered the origin of them flattens down and entirely disappears. I have examined many bald heads; the skin of the cranium was perfectly smooth on its internal surface, though it had been separated from the cellular texture. No trace is discoverable there of the innumerable appendices which the canals form, after the hairs they contain have been drawn inwards. I have also dissected a man who after a putrid fever had become almost entirely bald. There were all these little canals entire, and in the bottom of them could already be seen the rudiments of new hairs. There is then this difference between the falling out of the hairs of old people, and that which is the consequence of diseases, that every thing dies in the first, because the vessels which go to the root cease to transmit fluids to it; whereas in the second case the hair alone falls out, and its sac remains.

It is a pretty generally received opinion that the hair, the nails and the epidermis continue to grow after death. We have, I think, but few data respecting this singular phenomenon. I am however certain that I observed a real elongation of the hairs of a chin of a head that had been carefully shaved, and which I macerated eight days in a cellar. An attendant of the dissecting room, who prepares many heads for the bones, informed me that he had often made the same remark, when putrefaction is prevented for some time. What is certain also is, that the growth of the beard is not in the direct ratio of the vital forces; in the diseases which affect these forces with a general prostration, it grows as much as in those in which there is a general exaltation of these forces. We remark this in hospitals where at the side of an inflammatory fever, there is often found a putrid or slow nervous one. Besides, why should there not be sufficient tonic forces left in the hairs to grow some time after general death, as there is in the lymphatics to absorb, &c.?