MECHANICS OF LOCOMOTOE APPAEATUS 105 



These statements refer not merely to the flexor and extensor 

 muscles of the forearm, but have a general value. When the 

 knee is bent, not only does the leg move backward, but the thigh 

 bends simultaneously forward. Generally speaking, it may be 

 stated that a uni-articular muscle produces a movement in the 

 neighbouring articulation in the opposite direction to that which 

 occurs in the articulation lying between its points of insertion. 



The whole of the force on the muscles 

 is not utilised in the movements of the 

 skeleton. This occurs only in the case 

 when the insertion of the muscles is 

 approximately at right angles to the bone, 

 as in the masseters which are able to em- 

 ploy their full strength in bringing the 

 jaws together. But the great majority 

 of the muscles are inserted more or less 

 obliquely, the direction of their fibres 

 forming a more or less acute angle with 

 the principal axis of the bone. In all 

 these cases a great part of the traction 

 force of the muscle is lost in the move- 

 ment. This disadvantage is frequently 

 diminished by the fact that many bones 

 have prominences at the point of attach- 

 ment of the muscles over which the tendons 

 of the muscles pass as over a pulley, and 

 become attached to the bone at a favour- 

 able angle. 



In every case, whatever the form and 

 size of the angle of insertion of a muscle 

 upon the bone, it is possible by resolving 

 the total traction force into its components, 

 according to the law of the parallelogram 

 of forces, to estimate how much is utilised 



) -i i_ i FIG. 66. Diagram of the resolu- 



in displacing the mOVing bone, Supposing tion ofi muscular force into 



the other bone to be rigid. !n S texiT nents ' Explanation 



Let it be supposed that AC and AB in 



Fig. 66 represent the long axes of two bones, which are movable 

 round the axis A perpendicular to the plane of the figure ; that 

 MM' are the points of insertion of a muscle, M being fixed, M' 

 movable ; lastly, that the line M'D represents the total traction 

 force of which the muscle is capable. If we resolve the line M'D 

 into its two components M'E and M'F, which are vertical to each 

 other, then M'E represents the force utilised by the muscle in 

 moving the joint A, called by mechanicians the moment of force, 

 while M'F is the amount of force that is spent in pressing the two 

 articular surfaces at A against one another, so as to render the 



