on the Motion of the Arm. 415 



brought into play, the upper- set, that namely of least velocity, 

 are, it must be admitted, the principal agents, so far as this 

 muscle is concerned : in other words, Nature, according to 

 my explanation, causes a muscle to work at disadvantage, in 

 an action of essential importance. 



This, I think, is only an apparent difficulty, for in this mo- 

 tion as correct a balancing of leverage is displayed, as can 

 anywhere be fonnd throughout the body. 



The fist is thrown forward by a double motion. The 

 humerus, represented by A B, fig. 8, revolves round the 

 point A till it takes the position A C, while the forearm, 

 represented by B D, revolves round the point B till it takes 

 the position B E, so that the resulting position of the whole 

 extremity is A C F. The upper division of the pectoralis 

 major, the anterior fibres of the deltoid, and the coraco-bra- 

 chialis, are the main causes of the first motion ; the triceps, 

 ancona3us, and supinator muscles, of the second. 



The distance which the forearm passes through, repre- 

 sented by the curve 2, exceeds considerably the space tra- 

 versed by the upper arm, represented by the curve 1 ; 

 but as the motion of the forearm round the point B is from 

 above downwards, its extensors have no weight to raise ; on 

 the contrar}', are assisted by gravity. Whereas the humerus, 

 though it moves through a shorter distance, moves upward, 

 and carries with it the forearm, so that its elevators have to 

 raise a considerable weight. In order that these two motions 

 may be completed in the same time, the former requires the 

 greater velocity, the latter the greater force. Accordingly the 

 triceps and its associate extensors, act on the ulna by a lever 

 between one and two inches long, while the three elevators of 

 the humerus act by levers whose respective lengths are about 

 four, five, and six inches. See figs. 8 and 9, in which P 

 represents the tendon of the pectoralis major, D that of the 

 deltoid, C that of the coraco-brachialis, and T that of the 

 triceps. 



I may just add, (for it is interesting to observe the uncon- 

 scious acquaintance which every man has gradually acquired 

 with the precise capabilities and most effective application of 

 every fibre in that complicated machine, his own frame,) that 

 in preparing to strike a blow the elbow never hangs close to 

 the side, as in fig. 11, bui is always thrown out, as in fig. 12; 

 in order that the elevator muscles, all of which draw more 

 or less inward, as well as upward and forward, may act during 

 the strong effort at their full advantage. 



Thus, then, not only is the leverage of the upper and lower 

 portions of the pectoralis major accurately adapted to the ac- 



