OF THE EXTERIOR MUSCLES. 455 



direction determined by that of the last portion of these mus- 

 cles. The direction of the motion is frequently in a great 

 measure determined by that of the articulations of the bones. 

 Thus the bones articulated by ginglymus and by rotatory ar- 

 ticulation, although most of them have oblique muscles, move 

 in only two opposite directions. Thus, on the other hand, 

 the same muscle, the biceps flexor cubiti, without changing its 

 direction, produces by its contraction the supination and flex- 

 ion of the fore-arm. Thus also, the pyramidales, gemelli, &c., 

 which are rotators of the thigh outwards, when it is extended, 

 become abductors when it is bent. 



732. In many cases the muscular motions are compound; 

 several muscles contracting simultaneously, communicate to a 

 moveable part a motion different from that which results from 

 the contraction of each of them in particular. Thus, if the 

 rectus superior and rectusexternus of the eye contract toge- 

 ther and with equal force, the eye obeying these different 

 forces, the pupil will be directed upwards and outwards. Thus, 

 if the pectoralis major, which carries the arm inwards and 

 forwards, contracts at the same time with the latissimusdorsi, 

 which carries it inwards and backwards, the arm will be car- 

 ried, by a compound motion, directly inwards. The motions 

 of the shoulder are always compound. Many other parts are 

 often so also; and were it not so, the motions, which are so 

 varied, would be extremely limited. 



733. The motions of the voluntary muscles are in fact 

 most commonly combined. In this respect, the muscular ac- 

 tions may be distinguished into isolated motions, resulting 

 from a single muscle in contraction; into associated or com- 

 bined motions, resulting from the action of several associated 

 muscles, whether congenerous or antagonist, to produce deter- 

 minate motions, as those of flexion, extension, &c. ; into co- 

 ordinate motions, as those which by their association produce 

 standing, locomotion, &c.; lastly, actions of the will, or mus- 

 cular actions directed by volition. These variations in mus- 

 cular action depend upon the nervous influence, according as 

 it is voluntary, and according as, without the influence of the 



will, it is determined by irritation of the nervous centre, by 

 5!) 



