MUSCLE 231 



4. Mode of Action of Muscles. 



The skeletal muscles act to produce movements of the 

 body from place to place, or movements of one part of 

 the body on another. This they do by pulling on the bony 

 framework to cause definite movements of the various 

 joints. 



The muscles are arranged in opposing sets in relation to 

 each joint — one causing movement in one direction, another 

 in the opposite direction — and named according to their 

 mode of action, flexors, extensors, adductors, abductors, etc. 

 In the production of any particular movement at one joint 

 — say flexion of the phalanx of a foot — the opposing 

 muscles, the extensors, have their activity suppressed or 

 inhibited (p. 86). 



Other muscles which give the support needed for the 



Fig. 115. — The three types of lever illustrated by the movements 



at tne ankle-joint. 



movement also come into action. This may be called their 

 Co-operative Antagonism. These muscles, when acting alone, 

 cause a movement in the opposite direction to that being 

 produced, e.g. extension instead of flexion. If the part of 

 the brain which causes flexion of the hand of the monkey 

 be stimulated and the nerve to the flexors divided, the 

 co-operative action of the supporting extensors brings about 

 an extension of the hand. 



It is very probable that the red muscles act specially in 

 this fixation of joints to enable the pale muscles to bring 

 about difterent kinds of movements. Thus, with the ankle 

 fixed, the gastrocnemius may flex at the knee ; with the knee 

 fixed, it may extend at the ankle. 



The muscles act upon the bones, arranged as a series of 

 levers of the three classes at the various joints (fig. 115). 

 These may be illustrated by the foot in man. 



