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 versé. 
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 
_ Tespectively ; 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- 
position, which has only to be changed occasionally and 
a short time; the antagonism being then supplied by 
‘yellow elastic tissue ($ 29). . 
609. We commonly find that, in order to preserve the 
