MOTION 



47 



Thus, as most striated muscles are affixed to the skeleton in such a way 

 that one of their ends is stationary and the other freely movable, their 

 contraction invariably results in a closer approximation of their points 

 of insertion and attachment. In this way, movements are produced 

 which, if the bones are employed as levers, give rise to locomotion. 



A lever is a rigid bar, one part of which is relatively fixed and the other freely 

 movable. It possesses a point of support, or fulcrum, a point of resistance, or weight, 

 and a point to which the force, or power is applied. In accordance with the relative 

 positions of these points, we recognize three different systems of levers, namely : 



( 1 ) The fulcrum is placed between the power and the weight. When this lever is 

 moved, the weight and the power describe arcs the concavities of which are turned 

 toward one another. 





Fig. 19 a. — Cabdiac 

 Muscle. 



Fig. 19B. — Single Cardiac 

 Cells. Magn. 1000. 



(2) The fulcrum is at one end and the weight between it and the power. The 

 arcs described by the weight and the power are concentric, but the weight moves 

 less. 



(3) The fulcrum is at one end and the power between it and the weight. The 

 arcs are concentric, but the weight moves a greater distance than the power. 



As an example of a lever of the first order might be mentioned the movement of 

 the skull upon the spinal column. The articulation between the axis and occipital 

 bone serves in this case as the fulcrum, the face as the weight and the posterior 

 muscles as the power. As an example of a lever of the second order may serve the 

 foot when employed to raise the body upon the toes. The fulcrum is formed in 

 this case by the toes resting upon the ground, the weight by the body resting upon 

 the ankle-joint and the power by the gastrocnemius and soleus muscles. As an 

 example of a lever of the third order might be mentioned the arm wheji it executes 

 the movement of flexion. In this case, the fulcrum is formed by the elbow-joint, 

 the weight by the hand and the power by the biceps muscle, the tendon of which 

 is inserted in front of the elbow-joint. These three systems may also be illustrated 

 by giving to the foot the three different positions indicated in Fig. 21. 



