SEC. 3. LOOOMOTOR MECHANISMS. 



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 com- 

 paratively 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 generally 

 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 few movements of 

 the body in which one muscle only is concerned ; in the majority 

 of cases several muscles act together in concert; nearly all our 

 movements are coordinate movements. Where gravity or the 

 elastic reaction of the parts acted on does not afford a sufficient 

 antagonism to the contraction of a muscle or group of muscles, 

 the return to the condition of equilibrium is provided for by the 

 action either elastic or contractile of a set of antagonistic muscles ; 

 this is seen in the case of the face. 



The erect posture, in which the weight of the body is borne by 

 the plantar arches, is the result of a series of contractions of the 

 muscles of the trunk and legs, having for their object the keeping 

 the body in such a position that the line of gravity fails within 

 the area of the feet. That this does require muscular exertion 



