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The Muscles.The voluntary muscles are in most cases attached to two or more different bones; when they contract or shorten, these bones are brought closer together. Thus the act of bending the forearm is accomplished chiefly by the contraction or shortening of a muscle which is attached above to the shoulder, and below to the radius. Muscles of the Head.-The forehead is wrinkled and the eyebrows drawn upward by a thin muscular sheet which is attached to the top of the head. The eyes are closed by a muscular ring which passes around the opening of the eye and is attached at its inner angle. The eyeballs are moved by six small muscles, which are attached at the bottom of the cavity in which the eye rests, and are inserted into the outer coat of the eyeball at different points around its circumference. The lower jaw is pulled upward by four pairs of muscles, two of which can be seen and felt on the outside of the face when the teeth are firmly pressed together. The jaw is pulled downward by muscles which are attached to the bone of the tongue in the neck. Muscles of the Trunk.-Some of the largest and most powerful muscles of the body are attached along the backbone and ribs. These extend in different directions, some onto the limbs, some upward to the back of the skull. The cavity of the abdomen is enclosed in front and at the sides by broad muscles which pass around from the side of the spinal column and ribs. The muscles of the breast are few but powerful ; two of them extend from the side of the chest to the arm, and by their contraction draw the arm forward over the chest. Muscles inside of the body. - One of the most important of these is the diaphragm. This is a broad sheet which separates the chest from the abdomen. It is attached to the lower end of the breastbone, to the ribs, and to the spinal column. During the act of expiration, the center of the diaphragm is raised above the edges where it is attached to the ribs, so that the muscle has the shape of an inverted wash-basin. When breath is taken into the lungs, the muscle contracts, pulling its central part downward, so that the cavity of the chest is enlarged. The diaphragm therefore is an important agent in breathing; in fact breathing can be carried on by the action of this muscle alone. Movements in walking and running. - " The move ments of walking, running, leaping, etc., are performed as follows : When the body stands upright, the feet are planted flat upon the ground, bearing at once upon the heels behind and the ball of the toes in front, the weight of the body resting between the two upon the arch of the foot. The body is maintained in this position, as we have seen, by the various muscles which act in such a way as to keep its different parts carefully balanced, and to retain the weight of the whole suspended exactly over the ankle-joint. " Now, in walking, when a movement is to be executed in advance, the body is first made to lean a little forward, so that its weight no longer remains above the ankle but is thrown forward so as to rest entirely upon the toes ; the heel is then lifted from the ground by the action of the very strong muscles situated on the back part of the leg ; these muscles, which come down from above, form the fleshy mass which is known as the " calf of the leg. " They terminate in the strong, cord-like tendon, called the " tendon of Achilles," which is easily felt at the back part of the ankle-joint and which is attached to the projecting bone of the heel. When these muscles contract, they draw the heel upward by means of the tendon inserted into it, and lift in this way the ankle-joint and the whole body, carrying it upward and forward, its principal weight resting, as already mentioned, over the ball of the toes. " The action of the leg and foot in this movement is the same as that by which we might lift a weight from the ground with the aid of a lever. Suppose one end of a strong stick to rest upon the ground, and that this stick bears upon its middle a heavy weight. Then by taking in the hand the other end of the stick, we may lift the weight exactly as the body is lifted in walking by the muscles of the leg and the ankle-bones. " At the moment that the body is raised and tilted forward in this way, the other foot is lifted entirely from the ground and swung forward so as to take a step in advance. As soon as the body has been carried far enough in an onward direction, the second foot is also raised in the same manner as before, while the first is swung forward in its turn to take another step. In this way the two legs act alternately, the weight of the body being carried forward first by one and then by the other ; all the muscles, however, upon the two sides combining harmoniously in their action, so as to produce an easy, graceful and continuous movement. 0 In the act of walking as above described, one foot is always upon the ground, and the weight of the body is mainly supported in this way by bearing upon the toes; it is only lifted forward alternately on the two sides by the leverage of the bones of the foot. Consequently no violent muscular exertion is required, and the movement can be kept up for a long time without fatigue. " The act of running, however, instead of being a series of steps, is performed by a succession of leaps or springs, in each of which the whole body is thrown clear of the ground, and carried forward by the impetus which it has received. In order to accomplish this, at the moment the heel is about to be raised by the action of the muscles above described, the knee and hip-joints are first bent, and then instantly straightened by the sudden contraction of their extensor muscles. The whole limb thus acts like a powerful spring, which by a sudden extension throws the entire body off the ground and carries it through the air in an onward direction. The opposite limb is at the same time thrown forward to receive the weight of the body, and to perform in its turn, and with similar rapidity, the same movements. The speed of the runner depends on the vigor of the muscular contractions, and the swiftness with which the successive motions are performed. The act of jumping is accomplished in a similar way with that of running, except the same motions are executed by both limbs together, so that each leap is performed by itself, and is not combined with the others in a continuous movement."-Dalton. But first, if you want to come back to this web site again, just add it to your bookmarks or favorites now! Then you'll find it easy! Also, please consider sharing our helpful website with your online friends.
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