THE PHYSIOLOGY OF THE MUSCULAR SYSTEM 



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this would be from the center of articulation between the humerus 

 and the ulna to the center of the 10-kilo ball, which would be, 

 approximately, to the end of the metacarpal bone (36 cm.). 



(2) The short arm of the biceps lever; this would be the distance 

 from the center of the insertion of the biceps to the fulcrum the 

 center of articulation (6 cm.). 



(3) The short arm of the lever for the brachialis anticus. If the 

 brachialis anticus were exactly parallel to the biceps the short arm 

 would be the distance from the insertion to the fulcrum (5 cm.), as 

 in the biceps; but it is not parallel. A line drawn from the fulcrum 

 perpendicular to the axis of the brachialis anticus / a' is shorter 

 than the line / a. The angle between the brachialis anticus and the 

 biceps is approximately 10 degrees; therefore the angle a' f a would 

 be approximately 10 degrees; then of f is the cosine 10 degrees, or 

 98 per cent, of the radius a / (5 cm.), or 4.9 cm. (Fig. 84). 



FIG. 84 



Mechanics of flexion of the forearm. (The upper a is to be understood as a'.) 



(4) The power arm of the supinator longus is the perpendicular 

 distance from the fulcrum to the line of force of the supinator longus, 

 and is represented by the line / s, which is 4.8 cm. Now the carpal 

 and digital flexors which take origin from the humerus act as forearm 

 flexors after having flexed the carpus and digits. In the action under 

 consideration they would not be brought into forcible action as 

 carpal and digital flexors. We may, therefore, ignore them and 

 confine our discussion to the three muscles mentioned above. 



In the action of the biceps the long arm is 36 cm. and the short 

 arm 6 cm.; in the action of the brachialis anticus the long arm is 

 36 cm. and the short arm 4.9 cm.; in the action of the supinator 



