APPLICATION OP MECHANICAL PRINCIPLES. 



425 



Fig. 177. 



nearer to the fulcrum than to the end of the bone on which the weight 

 rests; the arm of the power P F, (supposing, for a moment, that it 

 is acting at this part with every advantage, which we shall see pre- 

 sently it is not,) is, consequently, 

 much shorter than that of the resist- 

 ance W F, which, as in all levers of 

 the third kind, occupies the whole 

 length of the lever. In estimating 

 the effect from this cause alone upon 

 the power to be exerted by the del- 

 toid, we may suppose, that the arm 

 of the power is to that of the resist- 

 ance as 1 to 3 ; the deltoid being 

 inserted into the humerus about one- 

 third down. Now, if we raise a 

 weight of fifty-five pounds in this way, and add five pounds for the 

 weight of the limb (which may be conceived to act entirely at the 

 end of the bone) the power, which the deltoid must exert to produce 

 the effect, is equal not to sixty pounds, but to three times sixty or one 

 hundred and eighty pounds. 



Fig. 178. 



Action of the Deltoid. 



Action of the Deltoid. 

 A. The scapula. B. The os humeri . C. The deltoid. 



Figures 177 and 178 exhibit the disadvantages of the deltoid, so far 

 as regards the place of its insertion into the lever; but many muscles 

 have insertions much less favourable than it. The biceps, D, for exam- 

 ple, in Fig. 179, the muscle which bends the forearm on the arm, is 

 attached to the forearm ten times nearer the elbow-joint or fulcrum 

 than to the extremity of the lever ; and if we apply the argument to 

 it, supposing the weight of the globe, in the palm of the hand, to be 

 fifty-five pounds and the weight of the lirnb five pounds, it would 

 have to act with a force equal to sixty times ten, or six hundred pounds, 

 to raise the weight. 



Muscles, again, are attached to the bones at unfavourable angles. 

 If they were inserted at right angles in the direction P P, Fig. 177, 

 the whole power would be effectually applied in moving the limb. On 

 the other hand, if the muscle were parallel to the bone, the resistance 

 would be infinite, and no effect could result. In the animal it rarely 



