132 ZOOLOGY. 



fore-arms; those moving the fore-arms are attached to the 

 arms or shoulders ; and those acting on the arms have their 

 fixed points in the trunk. But they may of course move the 

 parts to which they are attached by either extremity, or both. 

 To a certain extent also, the direction of the movement deter- 

 mines the position of the muscles. Thus, the flexors of the 

 fingers are situated on the front of the arm ; the extensors on 

 the back of the limb. When different muscles act in producing 

 the same movement they are called congenerous; antagonistic 

 if they produce opposite movements. Finally, they are named, 

 from their uses, flexors, extensors, abductors, &c. 



266. The strength of a muscle depends no doubt, 

 cceteris paribus, on its size ; but the effect depends also, in a 

 great measure, on its mode of attachment to the bone. 



Thus, all things being equal, the movement of a muscle 

 will be so much the more extensive the less obliquely it is 

 attached to a bone. 



In fact, if the muscle, m (Fig. 83), whose force we shall 

 consider as equal to 10, be fixed perpendicularly to the bone 

 I, whose extremity a is moveable on the point of support r, it 

 will have to overcome only the weight of the bone, and will 

 carry it from the position a b in the direction of the line a c, 

 thus making it traverse, to the point to which it is inserted, 

 a space which we shall represent by 10. But if the muscle 

 acts obliquely on the bone, in the direction of the line n b, it 

 will then have a tendency to carry it in the direction of b n, 

 and consequently to cause it to approach the articular surface 



r, on which it rests as a point 

 of support. But this being 

 an inflexible body, this dis- 

 placement cannot take place ; 

 r the bone can only turn on 

 the point r as on a pivot, and 

 the contraction of the muscle 

 n, without losing any of the 

 energy assigned to it, can 

 d b l only carry the bone in the 



Fig. 83. direction of a d. Three- 



fourths of its strength will 



be lost, and it will only be equal to effect a displacement for 

 which one-fourth of the strength expended would have 

 sufficed had its attachment been perpendicular to the bone as 

 the muscle m. 



