Take the bones of the arm; fix the top end of the humerus; tie it to something so that it cannot move. Fasten a piece of string to either the radius or ulna (it doesn’t matter which), rather near the elbow. Bore a hole through the top of the humerus and pass the string through it. Your string must be long enough to let the arm be quite straight without any strain on the string. Now, taking hold of the string where it comes out through the humerus, pull it. The fore-arm will be bent on the arm. Why? =Because you have been working a lever of the third order.= The radius and ulna form the lever; its fulcrum is the end of the humerus in the elbow (Fig. 3, F); the weight to be moved is the weight of the radius and ulna (with that of the bones of the hand if present), and this may be represented by a weight applied at about the middle of the fore-arm; the power is the pull you give the string, and that is brought to bear on your lever at the point where the string is fastened to the radius, _i.e._ nearer the fulcrum than the point where the weight is applied; and you know that when you have the fulcrum at one end and the power between the fulcrum and the weight, you have a lever of the third order. Now, in order to make the thing a little more like what takes place in your own arm, instead of boring a hole through the humerus, let the string glide in a groove which you will see at the top of the humerus, and fasten the end of it to the shoulder-blade or anything you like above the humerus, and let the string be just long enough to let the arm be quite straightened out, but no longer, so that when the arm is straight the string is just about tight, or at least not loose. Now shorten the string by pinching it up into a loop. Whenever you do this you will bend the fore-arm on the arm. Suppose you used a string which you had not to pinch up, but which, when you pleased, you could make to shorten itself. =Every time it shortened itself it would pull the fore-arm up and would bend the arm--and every time it slackened again, the arm would fall back into the straight position.= In your arm there is not a string, but a body, placed very much as our string is placed, and which has the power of shortening itself when required. Every time it shortens itself it bends the arm, and when it has done shortening and lengthens again, the arm falls back into its straight position. This body which thus can shorten and lengthen itself is called a muscle. If you put one hand on the front of your other upper arm, about half-way between your shoulder and elbow, and then bend that arm, you will feel something rising up under your hand. This is the muscle, which bends the arm, shortening, or, as we shall learn to call it, =contracting=.