SPECIAL PHYSIOLOGY OF THE MUSCLES 



205 



of insertion can, by the contraction of the muscle, go farther 

 apart. This is the case, e.g., in the superior oblique muscle, 

 whose insertion on the eyeball is removed by the contraction of 

 the muscle from its insertion in the optic foramen, because the 

 trochlea serves as a pulley. 



All the force of a contracting muscle is effective for 

 mechanical work only when the insertion points move in the 

 longitudinal direction of the fibres. In all other cases only 

 a part of the muscle force is effective. This part is found 

 by resolving the force into its components according to the 

 law of the parallelogram of forces. 



EXAMPLE. In Fig. 13, let AB and AC be two bones which by 

 the hinge-joint A move around a line passing 

 through A perpendicular to the plane of the 

 paper, yand^ are the insertion points of a 

 muscle fibre m, by the contraction of which 

 the point J^ is moved forward in the direction 

 perpendicular to AC (f is supposed to be 

 immovable). If the force of the muscle is 

 represented by the length of the line J^D, that 

 part of the force which causes J l to move 

 iorward is found by resolving J^D into its 

 components. According to the law of the 

 parallelogram of forces, draw JJE and its per- 

 pendicular J^F. J^E indicates the amount of 

 force which causes / t to move forward. 



If, in a hinge-joint, the direction of pull of 

 a muscle acting upon a given point of the 

 movable bone does not fall in the plane of 

 the circle described by the moving point, the 

 muscle force must be resolved into three com- 

 ponents. One of these, the active component, 

 falls in the direction of the moving point, the other two, inactive, 

 are perpendicular to this and also to each other, and one of these 

 lies in the plane of the circle. 



In a ball-and-socket joint the muscle force is resolved into two 

 components whose directions fall in the plane of traction of the 

 muscles and the centre of the ball. One of these components, 

 the active, lies in the direction in which the supposed movable 

 insertion point moves in this plane; the other is perpendicular to 

 this. 



If two or more muscles work upon a movable bone, we 

 first determine the active component of each force, and of 



FIG. 13. 



