174 SPECIAL MUSCULAR MECHANISMS 



moves about its longitudinal axis. In the fourth, one bone 

 slides over the other. 



Many of the bones may be looked upon as levers, with the 

 muscles attached as sources of power. Levers are divided 

 into three classes, according to the relative position of the 

 power, the weight to be moved, and the axis of motion or 

 fulcrum. Different movements of the foot offer an illustra- 

 tion of the three kinds of levers. The first kind (Fig. 9), where 

 the fulcrum (F) is between the source of power (P) and the 

 weight or resistance (W), is shown when the foot is raised and 

 the toe tapped upon the ground, the ankle-joint being the 

 fulcrum. The second kind of lever, where W is between F 

 and P, is illustrated when the body is raised upon the toes, 

 which, resting upon the ground, are the fulcrum. The third 

 kind of lever, where P is between F and W, is illustrated when 

 a weight is held up by the toes, the ankle being the fulcrum 

 and the anterior group of muscles of the leg the source of power. 



FIG. 9 



in 



Illustration of levers of all three orders (Huxley): W, weight of resist- 

 ance; F, fulcrum; P, power. 



Standing is a complex coordinated action in which the 

 muscles are continually in play to keep the centre of gravity 

 of the body over the base of support. In walking the centre 

 of gravity is continually being put forward, but the body is 

 kept from falling by the legs, which alternately move forward 

 to sustain it. One foot or the other is continually on the ground. 

 Running differs in that the body is more forcibly moved ahead 

 by vigorous pushes. The body is more inclined, and both 

 feet leave the ground at times. 



Voice. The larynx is a closed cavity, except in its communi- 

 cations with the trachea below and the pharynx above. The 



