Physiology and Hygiene for Secondary Schools

by Francis M. Walters

3. Rotators (two kinds).—The rotators are attached about pivot joints and bring about twisting movements.

4. Radiating and Sphincter Muscles. —The radiating muscles open and the sphincter muscles close the natural openings of the body, such as the mouth.

The pupil should locate examples of the different kinds of muscle pairs in his own body.

Exchange of Muscular Force for Motion.—The second difficulty to be overcome in the use of muscular force in the body is due to the fact that the muscles contract through short distances, while it is necessary for most of them to move portions of the body through long distances. It may be easily shown that the longest muscles of the body do not shorten more than three or four inches during[pg 251] contraction. To bring about the required movements of the body, which in some instances amount to four or five feet, requires that a large proportion of the muscular force be exchanged for motion. The machines of the skeleton, while providing for motion in definite directions, also provide the means whereby strong forces, acting through short distances, are made to produce movements of less force, through long distances. The mechanical device employed for this purpose is known as

The Lever.—The lever may be described as a stiff bar which turns about a fixed point of support, called the fulcrum. The force applied to the bar to make it turn is called the power, and that which is lifted or moved is termed the weight. The weight, the power, and the fulcrum may occupy different positions along the bar and this gives rise to the three kinds of levers, known as levers of the first class, the second class, and the third class (Fig. 115). In levers of the first class the fulcrum occupies a position somewhere between the power and the weight. In the second class the weight is between the fulcrum and the power. In the third class the power is between the fulcrum and the weight.

Fig. 115—Classes of levers. I. Two levers of first class showing fulcrums in different positions. II. Lever of second class. III. Lever of third class. F. Fulcrum. P. Power. W. Weight. a. Power-arm. b. Weight-arm.

Application to the Body.—In the body the bones serve as levers; the turning points, or fulcrums, are found at the joints; the muscles supply the power; and parts of the[pg 252] body, or things to be lifted, serve as weights. For these levers to increase the motion of the muscles, it is necessary that the muscles be attached to the bones near the joints, and that the parts to be moved be located at some distance from the joints. In other words the (muscle) power-arm must be shorter than the (body) weight-arm.[86]

Examining Fig. 116, it is seen that the distances moved by the power and weight vary as their respective distances from the fulcrum. That is to say, if the weight is twice as far from the fulcrum as the power, it will move through twice the distance, and if three times as far, through three times the distance. Thus the muscles, by acting through short distances (on the short arms of levers), are able to move portions of the body (located on the long arms) through long distances. Can all three classes of levers be used in this way in the body?