60 HUMAN PHYSIOLOGY 



III. Mode of Action of Muscles. 



The skeletal muscles act to produce movements of th 

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

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

 work to cause definite movements of the various joints. 



In relationship to each joint the muscles are arrangec 

 in opposing sets one causing movement in one direction 

 another in the opposite direction and named according t< 

 their mode of action, flexors, extensors, adductors, abductors, 

 &c. But in the production of any particular movement 

 say flexion of the forearm at the elbow not only are the 

 muscles manifestly causing the movement in contraction, 

 but the opposing group, the extensors, are also in action to 

 guide and direct the force and extent of the movement. 

 This Co-operative Antagonism of groups of muscles is of 

 very great importance, since it explains many of the results 

 observed in paralysis. Thus, if the extensors of the hand 

 be paralysed, as in lead palsy, it is found impossible to 

 clench the hand, although the flexors are intact. Again, 

 if 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 extensors brings about 

 an extension of the hand. 



The muscles round the various joints act on the bones, 

 arranged as a series of levers, of the three classes (Fig. 27). 



FlQ. 27. The three types of lever illustrated by the movements 

 at the ankle joint. 



1st Class. Fulcrum between power and weight. In the 

 ankle this is seen when, by a contraction of the gastro- 

 cnemius, we push upon some object with the toes. 



2nd Class. Weight between fulcrum and power. In 

 rising on the toes the base of the metatarsals is the fulcrum, 



