LEVERS 423 



In general, a lever is a rigid bar either straight or eiirved which 

 is capable of a rotatory motion round a fixed point — the i'ulcruni. 

 It is usual to divide levers into three classes depending on the 

 relative positions of power, fulcrum and load. 



Class I. The fulcrum lies between the point of application of the 

 power and that of the load. In this class of lever, if the power arm 

 is equal to the load arm, we have a balance. The application of 

 1 kg. of power will lift 1 kg. of load. If the power arm is lengthened 

 by shifting the fuleriun nearer to the load, thtni power will be 

 increased proportionalltj as speed is decreased. For example, 

 dealing with a straight lever and putting P = point of application 

 of power, F = fulcrum and L = point of application of load, then 

 PF represents the length of the power arm, and LF = length of 

 the load arm of the lever. If PF = 10 times LF, then 1 kg. 

 at P would balance 10 kg. at L, i.e. the load of 10 kg. would be 

 lifted by the exertion of a little over 1 kg. weight. This is the 

 crowbar lever and is very little employed in the body. The 

 most notable example of it is the forwards and downwards move- 

 ment of the head when one is overtaken by unconsciousness, e.g. 

 the nod of sleep. The fulcrum on which the head moves is the 

 atlas, and the weight of the prefulcral part of the head (long 

 power-arm) outbalances the postfulcral portion (short load-arm). 



Generally, speed is the desideratum. The fulcrum is placed 

 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 LFjPF. 

 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 oecipito-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 liy the toes on contraction of the gastrocnemius. 



Class II. The fidcrum 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 



