ACTION OF MUSCLES ON BONES. 459 



fingers are situated on the opposite side. But we often find 

 that the direction of a muscle's action is changed, by the 

 passing of its tendon through a pulley-like groove or loop ; 

 so that it draws the movable bone in a direction different 

 from that of its fixed attachment. This is the case, for ex- 

 ample, with some of the muscles that bend the toes ; these 

 being situated in the calf of the leg, would draw the toes 

 upwards, were it not that their tendons are carried beneath 

 the bones of the heel, working in smooth pulley-like channels 

 hollowed-out in them (fig. 233) ; hence, when the muscle con- 

 tracts, the tendons draw the ends of the toes towards the heel, 

 and thus bend them. 



607. We generally find that even movements of a simple 

 character are performed by the combined action of several 

 muscles ; of which some may be considered as the principal, 

 and others as assistants. Those which are principals in one 

 movement may become assistants in another ; and vice versd. 

 Thus, if we wish to bend the wrist directly downwards upon 

 the fore-arm, we put in action, not only certain muscles whose 

 tendency would be to produce this movement, but others 

 which, acting by themselves, would produce a different motion. 

 One of these would draw the wrist towards the thumb-side 

 of the fore-arm, and the other towards the little-finger-side, 

 and they become the principal muscles in these movements 

 respectively ; but when they act together, their several ten- 

 dencies to draw the wrist to opposite sides counterbalance 

 one another, and they simply assist the principal muscles in 

 bending the wrist downwards upon the fore-arm. 



608. Almost every muscle in the Human body has its 

 antagonist, which performs an action precisely opposite to its 

 own. Thus by one set of muscles, termed flexors, the joints 

 are bent ; by a contrary set, the extensors, they are straightened. 

 One set of muscles draws the arm or leg outwards, or away from 

 the central line of the body; another draws the limbs inwards. 

 One set, again, closes the jaws ; and another opens it. But 

 we find an economy of muscular substance in some of the 

 lower animals, where parts are to be usually kept in a parti- 

 cular position, which has only to be changed occasionally and 

 for a short time; the antagonism being then supplied by 

 yellow elastic tissue ( 29). 



609. We commonly find that, in order to preserve the 



