英文科學(xué)讀本 第五冊(cè)·Lesson 31 Coverings of Mammals

Lesson 31 Coverings of Mammals

Man is the only mammal, with the exception of those belonging to the whale family, whose skin is naked—that is, unprovided with a natural covering. This does not show that other animals require a warm covering more than man. It shows man has used superior intelligence. He is able to clothe himself, and to vary his clothing to suit the variations of climate.

In the lower animals Nature does it all. Every creature is provided with a covering of some sort, to suit the conditions under which it is intended to live. Nature, too, provides for her creatures a change of clothing to suit the returning seasons. You must have noticed the difference in the thickness of the cat's coat in summer and winter. In the hot weather much of the coat becomes loose and is shed, leaving the rest thinner in consequence, but before the winter returns there is a new growth from below as thick as ever.

You cannot take the cat or a rabbit in your arms in the summer time without finding your clothes covered with loose hairs. This is not so in the winter.

The whale family have already been alluded to as exceptions to other mammals in this respect. These creatures have a naked skin, but beneath the skin is an undercoat, so to speak, of solid blubber or fat. This fat wraps the animal at all points like a thick blanket, and he does not feel the icy cold of the polar seas in which he lives.

We have only to think of the great bulky body that has to be propelled through the water, and the wonderful adaptation of such a covering will be at once seen. The smooth, slippery skin glides easily along, and, in fact, assists locomotion, where a thick outer woolly coat would be an impediment. While providing in this way, however, for its easy movement in the element in which it lives. Nature has not been unmindful of its needs in other directions. A thick woolly overcoat was out of the question; therefore the under-jacket of fat was provided, so that the creature might not be left unprotected against the cold in those icy seas.

Land mammals have various coverings of hair, fur, or wool, but all these are in reality the same substance, for fur and wool are only hair somewhat modified.

Let us examine the structure of a hair. Each hair consists of a root, a shaft, and a point. The root is bulbous in form, and is embedded in the dermis, or true skin, where it is nourished with blood from numerous tiny blood-vessels. The little pit or hollow which holds the root of the hair is provided with a delicate lining called the root-sheath. This sheath closely envelops the bulb, and when a hair is pulled out by the root it tears away the sheath with it.

The shaft or stem of the hair consists of a long conical fibre, and, if examined by means of the microscope, is seen to be made up of an outer layer of scales, which appear to overlap each other like the scales of a fish or a snake. Deeply embedded in the dermis, and opening by the side of each hair, is a little gland, which has the power of separating from the blood a fatty or oily fluid to serve the purpose of a natural hair-oil, in keeping the hair moist and supple. These are known as the sebaceous or fat glands. If we place the hand on the woolly coat of a sheep, we find that it is greasy or oily to the touch. This is due to the oil sent out over the wool from the sebaceous glands, which are thickly spread in the skin of the animal.

Let us now look at the hairy coverings of animals from an economic point of view—that is, having regard to their usefulness to man.

The hair of animals does not enter largely into the manufacture of clothing fabrics. We import, however, for this purpose, goat's hair from Turkey and South Africa, and the hair-wool from the alpaca and llama of South America.

The hair of all our domestic animals, too, is turned to account in many ways; but perhaps none is so useful as the long hair of the tail and mane of the horse.

The best of this hair is spun into a coarse thread, and woven into a rough kind of cloth much used in the arts and manufactures.

Another kind of hair-cloth, used for seating chairs and sofas, is made of the same material, but for this the hairs are not twisted, and they run in one direction only of the fabric, the cross threads being strong flax or hempen yarn.

Some of the best tail-hairs are used in making violin bows and fishing lines, and also for sieves.

The short hair, which is not available for any of these purposes, is used for stuffing chairs, sofas, and mattresses.

The springy, elastic properties of the hair admirably adapt it for making a comfortable seat. In preparing it for this purpose, the hair is first spun into a thick rope, and these ropes are then plaited and twisted very tightly, one in the other. In that state they are put into a slow oven and gently heated. This treatment has the same effect as the heated curling-tongs of the hairdresser. It enables the hair to keep the curl that the twisting has given it. The curly character of the hair adds to its elasticity, and therefore to its suitability for stuffing purposes.

Even the short hair which is removed from the skin of the horse and ox in the process of leather-making is turned to good account. The builder mixes it with his mortar for ceilings and all sorts of plaster work. The hair holds the mortar together, and helps to bind it to the laths which support it.