英文科學(xué)讀本 第五冊·Lesson 16 Limbs and Locomotion

Lesson 16 Limbs and Locomotion

Our last investigation in this subject dealt only with the climbing, burrowing, and flying mammals. We will now proceed to examine the rest of the great family as to their special modes of locomotion, commencing with the swimmers.

Among the swimming mammals some, such as the seal and walrus, are fitted to live out of the water as well as in it. We have already spoken of them as fin-walkers. Their four short limbs are called fins, and must be regarded as swimming aids, rather than as a means of locomotion on land. Slow, awkward, and ungainly on land, these creatures have marvellous power as swimmers. The water is their natural home, and in the water they find their prey.

An examination of the skeleton will, in this case again, show that all the usual bones are present in the limbs. Those of the upper and forearm are short and thick, but the bones of the fingers are long. They form the framework of a large hand. The hand itself is webbed, and is the only part of the limb that passes out of the body.

The hind limbs are usually turned backwards in a line with the body. They, too, end in broad webbed flippers, which serve the purposes of a tail.

The whale family live entirely in the water; they are not adapted to live out of the water, for they have no means of locomotion on land. Their front limbs resemble in every respect the fins of the other swimming mammals, but they have no hind limbs. The body ends in a large tail-fin, which is the actual propeller; the fin-limbs merely act the part of balancers to steady the huge carcase in the water.

The running and walking mammals present remarkable divergences according to their several habits and modes of life. Most of the carnivorous mammals are toe-walkers; the bear and badger families alone plant the foot flat on the ground.

A careful examination of the skeleton of the cat will show that what appears at first sight to be the animal's foot is merely the extremity of it, formed by the toes. Behind the toes we may trace the limb to the next joint, but this joint is not a backward-turned knee, as it seems to be. It is the heel, the other end of the foot; and above it, in each case, is the lower leg, with its two bones, just as in other animals. These animals walk with a silent, springy tread on the tips of their toes only, the heels raised well above the ground. No other mode of locomotion could be so suitable to animals of their predatory nature.

In the great majority of the rodents, or gnawing mammals, we find the hind limbs much longer and stronger than the front. In most of them the run becomes a series of leaps. One of the rodents, the beaver, is specially fitted, by means of his webbed feet, for water locomotion. He always lives in and near the water.

The whole of the hoofed animals, including not only the horse, donkey, ox, sheep, and pig, but also the rhinoceros, hippopotamus, zebra, camel, giraffe, and the entire deer and antelope family, are toe-walkers. But the toes are encased in a hard horny hoof.

Taking the ruminants, or cud-chewers, first, we find the number of toes is reduced to four, and not only so, the hinder pair of toes never touch the ground in walking. The animals walk on two toes only, and each toe is encased in its own hoof, which is nothing more than a large nail, the whole giving the appearance of a cloven hoof.

In the horse family we find the phalanges still further consolidated into a single toe, which is enclosed in its own hoof. That is to say, the horse's so-called foot is nothing but the extremity of its single toe.

This, however, is far from being the only modification in the limbs of these hoofed animals. The part immediately above the hoof is not the lower leg. It answers to the back of the hand and the instep of the foot. Strange to say, however, instead of five bones, such as we and most other mammals have, we find here only a single bone, and that is changed beyond all recognition. It is commonly called the canon bone. In the foreleg the bones of the so-called knee are really the wrist bones, and in the hind leg those of the backward-turned knee are the ankle bones.

In the horse there is a small rudimentary bone behind the canon bone. This is commonly known as the splint bone.

The bones of the upper and lower arms and legs are very similar to those of other mammals.

Thus the whole of this arrangement in the limbs of the hoofed animals, while depriving them of all grasping power, clearly increases the firmness, strength, and speed of the limbs.