Each hair is composed ()Imo distinct parts, an exter nal or peripheral canal, and an internal or central medul lary part. The former of these resembles the cuticle in its nature and chemical properties, and like it, is of a white colour, whatever may be the colour of the hair itself. The central or medullary portion of the hair is that which gives to this substance its particular colour.* Its nature is unknown, but it is conjectured by Bichat to be composed of extremely delicate vessels, containing a peculiar fluid that stagnates within their cavities. This portion is essentially distinguished from the peripheral tube, by its possessing vital properties.
We know that the passions of the mind have a re markable influence on the hair. In a very short time, from grief or terror, it has become white, an effect that can be attributed only to a change in the distribution of that fluid with which the central portion of the hair is filled. Sometimes the quantity of this fluid is greatly increased, and its quality so much changed, as to have the appearance of blood. In some cases it is found even to assume a fleshy appearance. These phenomena con stitute the characters of that formidable disease called Pica polonica, in which the hair bleeds on being cut, and then becomes matted together. The hair is susceptible of renovation when lost, as appears from a growth of new hair taking place after a recovery from fevers, du ring which the patient had become bald ; and from the phenomena of moulting that yearly takes place in most quadrupeds, as well as birds. These and some other arguments are adduced by Bichat to prove the vitality of the hair. The growth of the hair appears to take place from its roots.
The hair is possessed of very little extensibility, and contractibility, though, considering the smallness of its diameter, it is very strong, and is not easily broken. When drawn between the fingers from root to point, its peripheral surface appears quite smooth, though when rubbed in the contrary direction, it gives a sensation of roughness and resistance. This is found to be owing to small scales or imbricated cones, of which the peripheral tube is composed, and which lie over each other in such a manner that their attachments are towards the root of the hair. By chemical analysis, the hair is found to contain a peculiar oil, which is supposed by Vauquelin to be the colouring matter of the hair, as he observed it to be of a blackish green in black hair, red in red hair, and of a whitish colour in white hair. Besides this oil, hair is said to contain chiefly inspissated mucous, though probably its peripheral membrane also contains albumen. Vauquelin also detected in black hair, (which was the sub ject of his analysis) iron in an unknown state, oxide of man ganese, phosphate of lime, a small proportion of carbonate of lime, and a sensible quantity of silica, and of sulphur.
The above account of hair applies equally to dial of every part of the body ; and indeed the varieties that are found in the hair of the same individual, are not con siderable, consisting chiefly of coarseness or fineness of texture, and darker or lighter colour. In individuals of the different races of mankind, natives of different countries belonging to the same race, and even among those of the same country, the varieties in the hair form a striking mark of distinction. Europeans have in ge neral long flowing hair, of a pretty firm texture, though seldom harsh or wiry. The hair of the Negro is short, woolly, and of a black colour ; that of the native Ameri cans, and most of the Asiatics, is thick, straight, black, and shining. Among the Europeans, the Dimes, and other Scandinavian nations, are remarkable for the pre valence of red hair.
It seems now fully ascertained that the males of all nations have beards, though this appendage has by some writers been denied to the native Americans, because these people take care to eradicate the hairs from their chins as soon as they appear. It is, however, not strictly true, that women have no beards. Most of them have a perceptible clown on the chin and upper lip, and in many women, especially when advanced in years, the beard is very remarkable. It is, indeed, always short, soft, and thin. Eunuchs, it is well known, have either no beards, or such as are very short and scanty.
The state of the hair at different ages, though fami liar, is too curious to be omitted. For some time before birth the head of the f‘rtus is covered with a soft white down, concealed below a fatty matter, which at that pe riod besmears the whole surface of the body. At birth, the hair is often very perceptible, though of a very pale colour. In many children, however, the appearance of hair is very late, a circumstance which indicates a weak and delicate habit of body. In most young people the hair of the head grows very fast, but nothing more than a soft down appears on any other part of the body before the age of puberty. At that period, the beard of the male begins to sprout, and hair skews itself in the arm pits, the limbs, &c. During middle age, this system undergoes little change ; but as the powers of life de cay, the vessels that supply the cavity of the hairs be come obliterated, the colouring matter is no longer se creted, and (to use the language of Bichat) the internal substance dies. Hence the hairs become white, and those of the head generally fall off.