250 MOTION AND COORDINATION 



(the non-striated muscle in arteries), against pressure. 



But in most cases, muscles work against muscles. 



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 



FIG. 114. The muscle pair extensors straighten joints (Fig. 



that operates the forearm. For j j^\ 



names of these muscles, see Fig. ., , , , ., , , 



2. Adductors and Abductors. 

 119. 



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 Sfhincter 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 



