Fig. 113—*Capillaries* of muscles. *Plan of Using Muscular Force.*—Two difficulties have to be overcome in the using of muscular force in the body. The first of these is due to the fact that the muscles exert their force _only when they contract_. They can pull but not push. Hence, in order to bring about the opposing movements(85) of the body, each muscle must work against some force that produces a result directly opposite to that which the muscle produces. Some of the muscles (those of breathing) work against the elasticity of certain parts of the body; others (those that hold the body in an upright position), to some extent against gravity; and others (the non-striated muscle in arteries), against pressure. But in most cases, _muscles work against muscles_. [Fig. 114] Fig. 114—*The muscle pair* that operates the forearm. For names of these muscles, see Fig. 119. The striated, or skeletal, muscles are nearly all arranged after the last-named plan. As a rule a pair of muscles is so placed, with reference to a joint, that one moves the part in one direction, and the other moves it in the opposite direction. From the kinds of motion which the various muscle pairs produce, they are classified as follows: 1. _Flexors and Extensors._—The flexor muscles bend and the extensors straighten joints (Fig. 114). 2. _Adductors and Abductors._—The adductors draw the limbs into positions parallel with the axis of the body and the abductors draw them away. 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 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