ANIMAL MECHANICS. 



25 



If both arms be extended horizontally, with the palm 

 upward, either unloaded or loaded with equal weights in 

 both hands, after a short time, much fatigue is experienced 

 and the experimenter finds himself compelled to terminate 

 the experiment — as the feeling of fatigue passes rapidly into 

 pain, chiefly felt in the supraspinous and central deltoid mus- 

 cles. It is necessary to keep the arms quite horizontal, for 

 if they be held either above or below the horizontal line 

 the distressed muscles are aided by the action of other 

 muscles. 



The total work done in this case may be estimated as 

 follows ; let 



w - weight held in hand ; 

 a - weight of arm ; 



a = length of arm, measured from centre of glenoid ca- 

 vity to centre of weight ; 



t - total time in seconds, until fatigue stops the experi- 

 ment. 



The muscles exert a force capable of sustaining the weight 

 of the loaded arm at its centre of gravity, and if we suppose, 

 this weight to be moved (with an unknown angular velocity) 

 uniformly, the total work done is measured by the weight of 

 the loaded arm multiplied into the arc through which the arm 

 would move in the time during which the experiment lasts. 



Let 



x = distance of the centre of gravity from the centre of 



the glenoid cavity ; 

 0 - the angle through which the arm would move in the 



time t ; 



u) = the unknown angular velocity. 

 The total work done is represented by 

 (w + a) zO = Total work ; 



