430 



MUSCULAR MOTION. 



Action of Intercostals. 



? 86 - sent a straight mus- 



cle, passing directly 

 from the one rib to 

 the other; and D E 

 an oblique muscle. 

 The levers of D B, 

 according to the 

 mechanical princi- 

 ples laid down, will 

 be AB and CD, per- 

 pendiculars drawn 

 from the centres of 



motion to the line of direction of the power. These levers being pa- 

 rallel are of course equal; but the levers of D E will be C F and A Gr, 

 also perpendiculars drawn from the centres of motion to the line of 



direction of the power. 



Fig- 187. These levers are of dif- 



ferent lengths; and, ac- 

 cordingly, the muscle 

 must act with different 

 degrees of force on the 

 two ribs;, so that it will 

 cause C D, on which it 

 acts with the longest 

 lever, to, approach A B 

 faster than it makes the 

 latter approach the for- 

 mer, in the ratio of C 

 F to A C, or with three 

 times the velocity. 

 Action of Biceps. In all muscular mo- 



tions, the levers of the 



power and resistance are undergoing variations; so that the degree 

 of power, necessary to be developed in one position of the member, 

 may be much less than in another. The case of the biceps already 

 referred to, elucidates this. Let E C, Fig. 187, represent the os 

 humeri; E A the forearm; E the elbow-joint; W, a weight or resist- 

 ance hung at the wrist, and D the biceps muscle, inserted at b, a 

 tenth of the distance down the forearm. It is manifest, that the force, 

 necessary for bending the arm, must be much greater when it is in the 

 position A E than in that of E a. The lever of the resistance, in the 

 former case, is the whole length of the forearm; or, in other words, the 

 perpendicular drawn from the fulcrum to the line of direction of the 

 weight W; but, when the arm is raised to #, the lever of the resistance 

 is no longer E A, but E H: but not only is the lever of the resistance 

 shortened; that of the power is augmented. The lever of the biceps, 

 when the forearm is horizontal, is the dotted perpendicular drawn from 

 the fulcrum at the elbow to the line of direction of the muscle; but 

 when the forearm is bent to the position E $, the disposition of the 

 muscle is also modified. It assumes the position occupied by the dotted 



