Hairs differ in respect of hardness and softness, length and shortness, straightness and curliness, abundance and scantiness and in addition to these qualities, in their colour, whiteness and blackness and the intermediate shades (grey colour) also according to age, as the animals are young or growing old. Human beings also go grey or change in colour on the head as they grow old but this is not visible in practically any other animal, though it is so in the horse than others. Each part of the body has its own special heat. If the heat is inadequate, the part loses its efficiency, and destruction or disease results. The hair in man has naturally little heat and too much moisture enters it and its own proper heat is unable to concoct the moisture and so it is decayed by the heat in the outside air. All decay is caused by heat — not the innate heat but external heat. And as there is a decay of water, of each, and all such material bodies, so there is also of the earthy vapour, for instance, what is called mould (for mould is a decay of earth vapour). Thus the liquid nutrient in the hair also decays because it is not concocted and what is called greyness results, which can occur visibly in horses more than in other animals. Usually hair change occurs according to the nature of the skin, that is, thickness and due to heat and cold factors of parts of the body. With the advance of years of life, all animals are alike in some points, but are opposed in others as in the case of the colour of the hair. Some do not grow grey visibly in old age, while man is subject to this more than any other animal.
