LEVERS 321 



near the power. The power-arm PF is short and the load-arm 

 LF is long. The relative speeds of the points will be as LF/PF. 

 The catapults employed by the ancients to cast stones are examples 

 of this kind of lever. The arm is used as a lever of the first class 

 with a short power member when a cricket ball is thrown. 



Normally, the head is a lever of this order, the power being 

 applied very close to the fulcrum. The quick nod of assent is 

 caused by the contraction of the anterior straight muscles which 

 are yoked close to the fulcrum, while the slower backward move- 

 ment is due to the placing of the effective muscles (splenii and 

 complexi) somewhat further away from the occipito-atlantal 

 joint. The feature of this arrangement is stability. Another 

 good example is when the foot is lifted off the ground and the 

 ground pressed on by the toes on contraction of the gastrocnemius. 



Class II. The fulcrum is at one end of the lever and the load 

 lies between it and the power. That is, the power-arm is always 

 of the same length while the load-arm may vary in length with the 

 position of the load, e.g. nut crackers. The outstanding example 

 of this lever in the body is the foot. On rising on the toes, the 

 base of the metatarsals is the fulcrum, the body-weight, borne by 

 the tibia to the ankle, is the load, while the power is applied to 

 the os calcis by the gastrocnemius. A foot with a long load-arm, 

 i.e. with the load near the power, is designed for speed not power, 

 well adapted for running. On the other hand, the further the 

 load is from the point of application of power, in this case, the 

 longer the heel, the smaller will be the force necessary to lift 

 the body. That this is so, may be inferred from a study of the 

 development of the gastrocnemius muscle compared with the 

 length of the heel bone. Europeans have short heel-bones and 

 well-developed, bulky calves, while Africans have long heels 

 and ill-developed calf muscles. 



Class III. The point of application of the power is between 

 fulcrum and load. This power must always be greater than the 

 load. It is the commonest class of lever in the body, and this is 

 to be expected as its use results in the most rapid action possible. 

 Speed is obtained, as before, by shortening the power-arm. In 

 the arm, the brachial muscle is inserted about one centimetre 

 beyond the fulcrum (elbow) while the total length of the load-arm 

 (forearm) is about 30 cms. The result of this arrangement is 

 that the load (hand) moves with about 30 times the speed of the 

 bone at the point of the application of the power. " Speed is 

 gained at the expense of power." It follows that while a long- 



B.B. 21 



