296 FORCE OF MUSCULAR CONTRACTION. 



a fixed surface, the bone can only turn upon the point r, as 

 upon a pivot, and the contraction of the muscle 71, having the 

 same force as the muscle m, would only be able to carry the 

 bone in the direction a d, and would require a force equal to 

 40, or four times that of m, to raise it in the direction of the 

 line a c. 



In the animal economy the muscles are inserted most usually, 

 very oblic[uely, and consequently in a manner little favorable 

 to the intensity of the result of their contraction. There is 

 nevertheless a very happy contrivance, which tends to diminish 

 the obliquity of their insertions, without marring the usefulness 

 or symmetry of the limbs. It is the articular swellings at the 

 extremities of the bones, which contribute also to the stability of 

 the joints, the advantage of which is seen in fig. 74. 

 The tendon i of the muscle m, Fig. 74, is inserted as is the case 

 in general immediately below the articula- 

 tion, upon the mobile bone o, in a direc- 

 tion more approaching the perpendicular, 

 thus making the head of the bone a sort ' m """ 

 of pulley over which it acts, by which the 

 effect of the contraction is considerably 

 increased. 



A more striking instance is met with in the deltoid muscle. 

 Baron Haller, has made an interesting calculation of the abso- 

 lute force required to be exerted by the deltoid muscle, in order 

 to raise a weight of 60 pounds at the elbow, reckoning the weight 

 of the arm at five pounds of this. Its insertion is at an angle of 10 

 degrees upon the humerus, and at about one-third of the distance 

 between the shoulder and elbow. The force requisite to raise a 

 weight is exactly in the proportion of the distance, which the 

 weight from the fulcrum bears to that of the power from the 

 fulcrum. Thus, from the disadvantage of insertion, the force 

 requisite to be exercised there is three times as great as it would 

 be if inserted at the elbow ; therefore the actual weight lifted, 

 as far as the muscle is concerned, is equal to 180 pounds. 

 But this is not all. The insertion of the muscle at an angle 

 of 10 instead of 90 degrees, takes off the purchase in the 



