322 MOVEMENTS OF THE LIMBS 



armed man may be able to give a quick blow he will be quite 

 unable, unless his brachial muscles are abnormally developed, to 

 give a heavy one. 



This introduces a point to which the author of the " Tarzan ' 

 stories paid little attention. Tarzan was able to hold his own 

 among the tree tops. Now, man has a fore-arm considerably 

 shorter than the upper arm while the anthropoid ape has a fore- 

 arm only a little short of twice as long as its humerus. This gives 

 it a long and quick reach. In swinging and climbing, the upper 

 arm is the lever employed to lift the body, mainly by the con- 

 traction of the anterior brachial muscle, and the insertion of 

 the brachial over half-way up the humerus from the elbow 

 (fulcrum) gives a power-arm with rather more power than speed. 

 That is, a short humerus is a necessity for climbing animals 

 to furnish strength, just as the long forearm is necessary to give 

 agility. To have equal climbing power, man would need to have 

 extraordinarily bulky biceps, etc., and this would not aid him 

 when he desired to swing and seize distant branches surely and 

 rapidly. 



So far, we have dealt with the levers of the body in a general 

 sense, as if they were straight bars. As a matter of fact, none of 

 the bones of the body can be considered as straight levers, and none 

 of the muscles act absolutely at right angles to the length of the 

 bone. The lengths of the effective power and load arms may be 

 obtained by dropping perpendiculars from the fulcrum to the lines 

 of application of power and load. The ratio of these perpen- 

 diculars gives the ratio of the distribution of power and speed by 

 the lever. 



The value of the bone-muscle mechanism depends on the mass 

 of active muscular fibres, their degree of contraction and the angle 

 which they make with the bone to be moved. The very movement 

 of the bone will alter the angle of pull of the muscle. For each 

 of its positions, the lever will have a moment of rotation determined 

 by the size of the angle made by the line of traction (axis) of the 

 muscle and the axis of the bone. By resolving the force of the 

 muscle into two components, one of which acts along the axis 

 of the bone and the other at right angles to it, one can readily 

 perceive that the latter, the effective component, varies in value 

 directly with the sine of the angle of pull. The ineffective or 

 parallel component varies as the cosine of the angle of pull and 

 represents the pressure exerted by the muscle on the fulcrum. 

 As the moment of rotation is equal to the tension developed (F), 



