VETERINARY PHYSIOLOGY 



III. Mode of Action of Muscles 



The skeletal muscles act to produce movements of the body 

 from place Lo 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 relationship 

 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. 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 



FIG. 28. The three types of lever illustrated by the movements 

 at the ankle joint. 



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. In many movements some of 

 the antagonistic muscles are relaxed under the action of their 

 nerves (see p. 41). 



The muscles round the various joints act on the bones, 

 arranged as a series of levers, of the three classes (fig. 28). 



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

 ankle this is seen when, by a contraction of the gastrocnemius, 

 we push upon some object with the toes. 



2nd Class. Weight between fulcrum and power. In rising. 



