MOTION AND LOCOMOTION 



105 



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

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

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



cm en CI 



FIG. 53. The skeleton of the foot from the outer side. Ta, surface with which 

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

 of the calf muscle is inserted: M 5, the metatarsal bone of the fifth digit; N, the 

 scaphoid bone; CI, CII, CHI, first, second, and third cuneiform bones; Cb, the 

 cuboid bone. 



that of the whole lower limb acting at its center of gravity, which 

 lies somewhere in the thigh between the hip and knee-joints, that 

 is, between the fulcrum and the point of application of the power. 

 Levers of the Third Order. In these (Fig. 54) 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 move- 

 ment 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. 43) into the radius and of 



Fio. 54. A lever of the third order. F, fulcrum; P, power; W, weight. 



another muscle (not represented in the figure^the brachialis 

 anticus, into the ulna), and the weight is that of the (forearm and 

 hand, with whatever may be contained in the latter, acting at 

 the center of gravity of the whole somewhere on the distal side 

 of the point of application of the power. In the Body the power- 



