APPLICATION OF MECHANICAL PRINCIPLES. 



431 



Fig. 188. 



line, which is farther distant from the fulcrum ; and the lever of the 

 power is consequently increased. In this case, then, of the action of the 

 biceps, in proportion as we raise the arm, the mechanical disadvantages 

 become less and less; the lever of the power increasing, whilst that of 

 the resistance diminishes. 



In many of the changes of position of a body, whilst a bone is turn- 

 ing upon its centre of motion, the centre itself is often describing a 

 curve at the same time. In Fig. 188, let A 

 B represent the foot, B C the tibia, C D the 

 thigh-bone, and D E the trunk; and let us 

 suppose it is required to bring the body to 

 the erect position B F ; so that B C shall cor- 

 respond to B G, D to (1 1, and D E to I 

 F. The point C will describe the curve C G; 

 and, whilst it is accomplishing this, the point 

 D is likewise moving ; so that the latter, in- 

 stead of describing the curve D H, which it 

 would do, were the centre of motion C fixed, 

 proceeds along the curve D I: the point E, 

 again, is subjected to the like influence; and 

 instead of describing the curve E K, which it 

 would do if the centre D were fixed,. rises 

 along E F. 



The motions produced by the muscles may 

 be either simple or compound. The simple 

 muscles admit of variety ; some being straight, 

 composed of parallel fasciculi ; others reflected 

 in their course, and others, again, are circu- 

 lar. In the straight muscles, each fibre, by 

 its contraction, draws the tendon in its own 

 direction; and the effect of the whole is to 

 bring it towards the centre of the muscle. In a long muscle, the 

 whole contractile effort is concentrated on the tendon, in consequence 

 of the course of the fibres being parallel to that of the tendon. In 

 most of the broad muscles, on the other hand, as the attachments at 

 both extremities are usually at different points, all the fibres do not 

 concur in one effort. Different sets of fibres may have a very different 

 action from others, and are capable of being thrown separately into con- 

 traction. The ordinary direction in which a muscle acts is from its ten- 

 dinous, back to its aponeurotic, attachment, that is, from the movable 

 to the more fixed part ; and, in a straight muscle, this direction can be 

 accurately appreciated. It must be borne in mind, however, that the 

 muscle can act in an inverse direction also. 



When the whole of the fibres composing a broad muscle are brought 

 to act on the tendon, as in the case of the deltoid, we find, by the com- 

 position of forces, that the middle line of direction must be taken for 

 the purpose of estimating their line of action. A part, however, may 

 act and carry the arm upwards and outwards ; whilst the opposite fibres 

 may move it upwards and inwards. 



Where a muscle is reflected, like the superior oblique of the eye, ai^cl 



Combined Muscular Movements 

 in Rising. 



