THE INCLINED PLANE. 



19 



neither invariable nor certain ; the insertion of the forces neither well established 

 nor very precise ; their intensity always approximative. Whatever may be the 

 exactness of the idea which may exist in regard to volume, length, direction, the 

 structure of the muscle, its angle of insertion, the arm of the lever which it 

 moves, the amount of contractility — that is to say, the value of the force itself — 

 escapes us. 



" In the organized machine resistance is only a power disguised alone by 

 the name. It is indeed weight at first, but it is also muscular contraction Avhich 

 opposes and resists the action of the lever. It is an unknown quantity to be 

 overcome by another equally unknown." 



Let us add that the forces of extension have their maximum of intensity at 

 the beginning of their action, whilst those of flexion have it at the termination. 

 This is explained by the difference of the result which they should produce. The 

 first resist the weight of the body at the same time that they overcome in loco- 

 motion the inertia of the regions situated below them ; the second only have to 

 raise the member to permit it to advance over the ground, and are nearly dis- 

 charged from the role which the first fulfil during station. The insertion of the 

 extensors is ordinarily more advantageous than that of the flexors, because these 

 latter have but to employ velocity against the others, which should, in addition, 

 develop force. 



CHAPTER IV. 



THE INCLINED PLANE. 



The inclined plane is, with the lever, the other of the two simple machines 

 employed by the organism in the construction of the locomotory apparatus. 



It is known that in the inclined plane (Fig. 10) a solid, 0, incited by the 

 weight F, is drawn the 

 length of this plane by 

 a force,/, which is cal- 

 culated by the follow- 

 ing formula : 

 /= Oi^ sin F = sin a. 



In other words, 

 the force _/' is the jirod- 

 uct of the weight of 

 the body by the sine 

 of the angle of inclina- 

 tion of the plane. 



The larger this 

 angle becomes, the 

 more the component / 



increases at the expense of the other component, Od, destroyed by the resistance 

 of the plane. The articular surfaces represent a multitude of iticl hied planes which 

 decompose the weight of the mass upon the bony levers and the tendinous and 

 ligamentous cords situated in the vicinity of the articulations. 



Fig. 10. 



