FUNCTION OF THE MUSCLES IN THE BODY. 



585 



point of application of the weight; for example the triceps, the muscles 

 of the calf. In both instances the muscular force necessary to overcome 

 a given resistance is calculated according to the laws of the lever. Equi- 

 librium will be established when the static factors that is the product 

 of the force in its vertical distance from the fulcrum are equal; or 

 when the force and the weight are inversely proportional to their ver- 

 tical distances from the fulcrum. 



In determining the amount of muscular force and the weight , especial 

 attention should be given to the direction in which these act on the arms 

 of the lever. Thus, it often happens that the direction that was perpen- 

 dicular to the arm of the lever in a certain position may act obliquely 

 upon it during movement. The static factor of a force or weight acting 

 obliquely on the arm of the lever is obtained by multiplying the force by 

 the perpendicular dropped from the fulcrum upon the line of direction 

 in which the force is acting. 



I. 



11 



IK. 



FIG. 199. Diagrammatic Representation of the Action of Muscles on the Bones. 



In Fig. 199, I, B x represents the humerus, and x Zthe radius; A y the direc- 

 tion of traction of the biceps. If the biceps acted only in the rectangular position, 

 as in holding horizontally a weight (P) attached to the forearm or the hand, then 

 the force exerted by the biceps (A) could be determined by the formula A . y x = 

 P . x Z; whence A = (P . x Z) : y x. It is evident that in the depressed position 

 of the radius x C, the conditions are different; then the force of the biceps Aj 

 = (P! . v x) : o x. 



In Fig. 199, II, T F represents the tibia, F the ankle-joint, M C the foot in 

 the horizontal position. The force (a) of the calf -muscles necessary to neutral- 

 ize a force p directed from below against the anterior extremity of the foot would 

 be: a = (p . M F) : F C. If the position of the foot is changed to the direction 

 R S, then the force of the calf-muscles a t = (pj . m F) : F c. 



From the foregoing the amount of force with which muscles that, like 

 the coraco-brachialis, are stretched over the angle of a hinge-joint, act 

 on the arms of their levers is also evident. Here also the static factor is 

 equal to the force multiplied by the perpendicular dropped from the 

 fulcrum upon the line of direction of the force. 



In Fig. 199, III, H E represents the humerus, E the elbow-joint, E R the 

 radius, B R the coraco-brachialis muscle. The factor in this position is A . b E. 



