英文科學(xué)讀本 第四冊·Lesson 10 How the Bones Move

Lesson 10 How the Bones Move

Our lesson on the joints showed us the bones of the body in action—some moving with a simple backward and forward hinge-like movement, some swinging round in a circle by means of a ball and socket, and some, again, rotating on a peg or pivot. But have you asked yourselves why these bones do not slip out of their places with their work? Let me tell you the reason. At each joint there are strong gristly bands, called ligaments, which hold or bind the bones together, while at the same time they do not interfere with their freedom of movement.

We will now turn our attention to the force which puts this machinery in motion. If the white skin all over the body were stripped away the red flesh would be seen beneath. This red flesh is muscle. The muscles are the agents by which all movements of the body are carried out. Wherever the greatest power of movement is required, there we will find the largest and strongest muscles. In some parts of the body, such as in the legs and arms, the muscles are placed in large solid-looking masses around the bone. In other parts we see that the muscles extend between the bones. The skull has a very slight covering of muscle beneath the skin, for the simple reason that the bones of the skull do not move, and huge muscles are not required.

A lean piece of flesh from the body of an animal would be found on examination to consist of separate bundles of flesh, arranged side by side in such a way that each has the power of moving or sliding about independently of the rest. These separate bundles of flesh are the muscles.

A muscle consists of a thick, solid middle part, which is called the belly, and two tapering ends. As a rule the muscle is attached to two bones—one fixed, the other movable. It is not joined directly to the bones. The tapering ends usually terminate in tough, whitish, leather-like cords, which we call tendons. It is the tendon which binds the muscle to the bone.

We have now to inquire how the muscles act. The substance of which they are formed is very elastic, but their elasticity is that of sponge or cork, not that of India-rubber. You remember of course that cork and sponge contract or shrink up with pressure, and expand or spring back when that pressure is removed. India-rubber, on the other hand, expands when pulled, and contracts when let go. The muscles then contract when they are interfered with.

You will understand the action of the muscles if you straighten the right arm, place the left hand on the fleshy part of it above the elbow, and then bend it upwards to the shoulder.

That part which swells up under your hand as the arm bends, is the great biceps muscle. It is connected at one end with the upper arm, and at the other with the forearm. When it contracts it becomes shorter and thicker. This is why you feel it swell up under your hand. Of course, as it becomes shorter, it draws up the forearm. There are more than 500 distinct muscles in the body, and this is their common manner of working.