/S THE PRI^X"IPLES OF M V( )DV\ AMICS. 



THE FORE-ARM AND THE El .BOW-JOINT. 



64. The bones of the fore-arm can act as a 

 lever of the first, second, or third order. 



(i.) Elevate the arm and lift a weight with the 

 hand ; the triceps brachii will move the ulna and 

 carry along the radius, making the bones of the 

 fore-arm a lever of the first order. 



(2.) Rest the hand on a resisting surface, as a 

 fulcrum, and the bones of the fore will be a lever 

 of the second order, whose power comes from 

 the triceps brachii. It is evident that the me- 

 chanical principles involved in these two cases 

 are similar, the weio^ht and the fulcrum onlv 

 changing places, while the power continues to 

 act at the same point. 



(3.) Under the action of the biceps brachii 

 and the brachialis anticus, the bones of the fore- 

 arm act as a lever of the third order. 



65. The fore-arm has three motions : 



(i.) The rotation of the fore-arm has already 

 been described. 



(2.) The fore-arm moves forward and back- 



