SEC. 3. ON SOME LOCOMOTOR MECHANISMS. 



920. 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 gener- 

 ally 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 muscles 

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

 have just studied afford a striking illustration of this. The rela- 

 tions 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 muscle, or, and 

 this is perhaps the most common case, may give a special direc- 

 tion 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 opposite directions on parts placed 

 in the middle line ; and the relations of these antagonistic muscles 

 seem to be specially complex. 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 provided by the anatomical disposition of the 

 parts assisted probably, as we saw ( 597), by skeletal tone, but is 

 also further secured by the action of its antagonists, which more- 



