THE DOUBLE ACTION OF A MUSCLE. 



255 



a frontal and horizontal axis ; and rotation inwardly, as a plus movement in 

 a horizontal plane and about a vertical axis. If we represent the muscle 

 by a line, the length of which represents in force and direction the action of 

 the muscle, we may resolve these into forces acting in three directions by 

 the ordinary parallelepipedon of forces. Thus, in Fig. 143, if a represents 

 the insertion, and b the origin of the muscle, and the line ab represents the 

 action of the muscle in force and direction, we can resolve ab into forces 

 acting along axes intersecting at the point a. The lines ad, ae, and ac will 

 represent the three components in three planes, at right angles to each other. 



In practice we may proceed as follows : — 



The bone from which the muscle arises is fixed in space by a clam}). 

 The limb is then brought into the position at which the action of the muscle 

 is to be studied. A cord is then attached to the bone at the point of insertion 

 of the muscle, and the cord passes through an eye at the point of its 

 origin, and to the end of this is attached a weight to keep it taut. By 

 this means the distance from 

 the point of insertion to the 

 point of origin of the muscle 

 may be measured along the 

 cord, which can be graduated 

 for that purpose. If the 

 muscle is a pure flexor, 

 when the limb is flexed it 

 will be found that the points 

 are at their nearest possible 

 approximation. If it be 

 mainly an adductor, and a 

 flexor to but a slight 

 extent, then movement in 

 the direction x (flexion) 

 will but slightly approximate 

 the points ; while movement 

 through the same angle in 

 the direction y (adduction) 

 will produce a relatively 

 greater approximation. The 

 components are, therefore, 

 found by moving the bone 

 through a small angle, say 

 10°, in each of the co- 

 ordinate planes, and reckon- 

 ing the shortening of the 



Fig. 143. — Tlie resolution of the force of contraction of 

 a muscle, represented by the line ab, into three 

 forces acting in three co-ordinate planes, namely, 

 into ac along zz, ad along xx, and ae along yy. 



cord which occurs in each case. 



The double action of a muscle at both insertion and origin. — 



So far we have, for the sake of simplicity, considered the point of origin 

 of a muscle as being fixed in space and not free to move. The latter is 

 sometimes the case, as when the upper arm is fixed by muscles from 

 the shoulder, and then the forearm is moved upon it. But, in the great 

 majority of cases, both the points of origin and of insertion are more 

 or less free, and move as a result of the muscular pull applied to 

 them. If a muscle passes directly between its point of origin and 

 insertion, and if it contracts with a force F, this acts equally upon the 

 point of origin and of insertion. Even if the muscle by its tendon 

 passes round a corner, the extensors of the thigh over the knee, or the 

 long head of the biceps over the shoulder-joint, the same still holds good, 



