英文科學(xué)讀本 第五冊(cè)·Lesson 11 Locomotion in Mammals

Lesson 11 Locomotion in Mammals

Man possesses two pairs of limbs, and the vast majority of the vertebrate animals are like him in this respect. All mammals have both pairs, except the whale family, and in these the hinder pair are entirely wanting.

The human arm consists of three parts—the upper arm, the forearm, and the hand. The framework of the upper arm is a single long, hollow bone; in the forearm there are two bones.

The leg, in like manner, consists of three parts—the thigh, the lower leg, and the foot ; the thigh containing one long, hollow bone, the lower leg two, as in the arm.

The first great purpose of limbs is to supply the means of locomotion, or moving about. In man the lower limbs only are adapted for support and locomotion, as may be plainly seen by a comparison of the hand with the foot. The hand, with its opposable thumb, is meant for prehension, or grasping purposes.

Most mammals run and walk on the ground, but there are others that climb trees, others that burrow in the earth, others that fly in the air, and others, again, that swim in the water.

Let us take these one by one, for their limbs must be specially adapted to their special mode of life.

First among the climbing mammals are the four-handed family, all of whom spend their lives in the trees. A glance at the skeleton of one of them will show the great similarity in its structure to that of man. There are the same bones, arranged in the same way, except in the hand-feet. To such creatures a foot like ours would be a hindrance in their tree-climbing. In most of the family the thumbs of all four limbs are opposable like ours, and are thus admirably adapted for grasping, and clinging to the branches of the trees. In some, however, the thumb is placed side by side with the fingers, and not opposite to them. These individuals cannot use their hands for climbing, but make their way by leaping from branch to branch. In all of them there is an unusual length of arm. The arms reach below the knee.

The squirrel lives in the trees, and is a good climber, but he depends upon his long, sharp talons in his climbing, and so also does the cat.

Chief among the burrowing animals is the mole, and we have already noticed the special modification of the neck bones to fit him for his life. Let us look now at his limbs. Those broad, shovel-like paws might seem to a careless observer out of all proportion to the size of the body. It is the paw only that is enlarged; the limbs themselves are extremely short. There are the usual bones, but they are short and thick, and built for strength. The paws themselves are thus close to the body and turn outwards. They are his digging implements.

The next remarkable hands are those of the bats— the flying mammals. Suppose we examine the skeleton of one of these. The upper arm and the fore-arm are seen to consist of the usual bones, except that they are considerably lengthened, as compared with the lower limbs. It is the hand itself, however, that shows the most wonderful development. The bones of the fingers are lengthened out, till they are longer than any other bones in the body—longer than the body itself. These bones form the framework of a sort of wing. Between the fingers themselves, up to the very finger-nails, a thin skin or web is stretched to form the wing. This is the creature's means of locomotion in the element in which it is meant to live and find its prey.