PULLEYS 323 



and the perpendicular distance (d) of the axis of the muscle from 

 the fulcrum, one may write M=Fd. Then the effective com- 

 ponent is equal to F sin a where a is the angle of pull, and tin- 

 parallel component to F cos a. Hence, as the bony lever gets 

 I pulled up, the effective component will become greater and the 

 parallel component will become less. In other words, the more 

 i parallel the axis of a muscle is to the axis of the bone which it is 

 to move, the weaker will be its action the maximum value is 

 obtained when the line of action is at right angles to the bone. 



Pulleys. By means of a single fixed pulley the direction of a 



force is altered, but not its magnitude. In the body, instead of 



reducing friction by means of a rotating pulley the tendon operates 



I in a synovial sheath (q.v.). Good examples of the pulley may be 



found in the cartilaginous loop (trochlea) for the superior oblique 



on its way to the eyeball : and the peroneus longus looping 



I round the outer malleolus on its passage to the inner side of the 



I foot. 



Opponents. All the muscles attached to levers in the body 

 I are set in opposing pairs or antagonistic groups. As one group 

 I contracts, the opposing group will relax to exactly the same 

 degree. The ulna, for instance, is pulled up towards the humerus 

 I by the action of the anterior brachial and it is pulled downwards 

 I by gravity and the action of the posterior brachial muscle. Both 

 I sets of muscles act together and harmoniously, so that in any 

 I position of the ulna relative to the humerus, the opposing (muscular 

 land gravity) forces exactly balance one another. That is, the 

 I arm may come to any position and remain there without the 

 I expenditure of any extra energy (not taking into account gravity). 

 Synergists. Movement does not usually take place merely by 

 Ithe contraction of a muscle and the relaxation of its opponent. 

 iThere are numerous other muscles brought into play, synergists 

 Iwhose action, though secondary, helps the primary movements, 

 (generally by altering the pose of the body as a whole, but some- 

 Itimes by immobilising the bone to which the muscle is fixed. As 

 Ian example of the former, may be cited the action of the trunk 

 [muscles holding the body erect while a weight is being held above 

 |the head. The latter synergetical complex may be illustrated 

 Iby the various muscles brought into action in opening a table 

 Idrawer. " One hooks his fingers into the handle of the drawer 

 Band if it opens easily enough, the contraction of the flexors of the 

 [fingers is sufficient. If it works a little harder the flexors at 

 Ithe elbow contract to hold the bones of the forearm up so that the 



