SEC. 3. ON SOME LOCOMOTOR MECHANISMS. 



683. The skeletal muscles are for the most part arranged 

 to act on the bones and cartilages as on levers, examples of the 

 first kind of lever being rare, and those of the third kind, where 

 the power is applied nearer to the fulcrum than is the weight, 

 being more common than the second. This arises from the fact 

 that the movements of the body are chiefly directed to moving 

 comparatively light weights through a great distance, or through 

 a certain distance with great precision, rather than to moving 

 heavy weights through a short distance. The fulcrum is gen- 

 erally supplied by a (perfect or imperfect) joint, and one end 

 of the acting muscle is made fast by being attached either to a 

 fixed point, or to some point rendered fixed for the time being 

 by the contraction of other muscles. 



There are indeed few movements of the body in which one 

 muscle only is concerned ; in the majority of cases several mus- 

 cles act together in concert ; the movements of the larynx which 

 we have just studied afford a striking illustration of this. The 

 relations of the muscles which thus act together are many and 

 varied. When one muscle is contracting, the contractions of 

 another muscle or of other muscles may, as just stated, serve to 

 secure a fixed point, or may enforce the effect of the first mus- 

 cle, or, and this is perhaps the most common case, may give a 

 special direction to the action, the movement effected being 

 the resultant of the forces employed in combination. Many 

 muscles are, either partially or wholly, antagonistic in action to 

 each other, such for instance as the flexors and extensors, and 

 such muscles as those of the face, which act bilaterally in oppo- 

 site directions on parts placed in the middle line ; and the rela- 

 tions of these antagonistic muscles seem to be specially com- 

 plex. When a muscle contracts it is, as we saw in treating of 

 nerve and muscle, of advantage that the muscle should at the 

 moment of contraction be already " on the stretch ; " this is pro- 

 vided by the anatomical disposition of the parts assisted proba- 

 bly, as we saw ( 470), by skeletal tone, but is also further 

 secured by the action of its antagonists, which moreover, after 



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