MOTION AND LOCOMOTION. 147 



other. Here the fulcrum is at the hip-joint, the power is ap- 

 plied at the knee-cap by a great muscle (rectus femoris) which 



Spl 



FIG. 65. The skeleton of the foot from the outer side. Ta, surface with which 

 the leg-bones articulate ; Ca, the calcaueum into which the tendon (tendo Achillis) 

 of the calf muscle is inserted ; M5. the metatarsal bone of the fifth digit ; N, the 

 scaphoid boue-, C/, CII, GUI, first, second, and third cuneiform bones ; Cb, the 

 cuboid bone.\ 



is inserted there and arises from the pelvis; and the weight 

 is that of the whole lower limb acting at its centre of gravity, 

 which lies somewhere in the thigh between the hip and 

 knee-joints, that is between the fulcrum and the point of ap- 

 plication of the power. 



Levers of the Third Order. In these (Fig. 66) the power- 

 is between the fulcrum and the weight. In such levers the 

 weight-arm is always longer than the power-arm, so the power 

 works at a mechanical disadvantage, but swiftness and range 

 of movement are gained. It is the lever most commonly used 

 in the Human Body. For example, when the forearm is 

 bent up towards the arm, the fulcrum is the elbow-joint, the 

 power is applied at the insertion of the biceps muscle (Fig. 

 52) into the radius and of another muscle (not represented 

 in the figure, the brachialis anticus, into the ulna), and the 



W F 



FIG. 66. -A lever of the third order. F, fulcrum ; P, power ; W, weight. 



weight is that of the forearm and hand, with whatever may 

 be contained in the latter, acting at the centre of gravity of 

 the whole somewhere on the distal side of the point of appli. 



