

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 the 

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

 pulled up, the effective component will become greater and the 

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

 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 

 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 

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

 foot. 



Opponents. All the muscles attached to levers in the body 

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

 contracts, the opposing group will relax to exactly the same 

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

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

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

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

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

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

 arm may come to any position and remain there without the 

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



Synergists. Movement does not usually take place merely by 

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

 There are numerous other muscles brought into play, synergists 

 whose action, though secondary, helps the primary movements, 

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

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

 an 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 

 by the various muscles brought into action in opening a table 

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

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

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

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



