APPARATUS OF MOTION IN GENERAL. 125 



permits the bones to move simply by reason of its elasticity ; 

 in other joints, the bones slide over each other, and are held 

 in their place by ligaments attached to both bones, and per- 

 mitting of motion only in certain directions ; this mode of 

 articulation is called articulation by contiguity, and it 

 always exists where extensive motions take place. To meet 

 the friction caused by this, the extremities of the bones are 

 further provided with a cartilage of incrustation, and a syno- 

 vial membrane representing a serous membrane in minia- 

 ture, and is constantly bedewed with an unctuous fluid, called 

 synovia, serving the purpose of a joint-oil. By means of 

 this membrane the air is effectually excluded from the joints, 

 so that the relative position of the bones to each other is 

 further secured by the pressure of the external air on the ex- 

 terior of the limb. 



265. Action of the Muscles on the Bones. All the 

 muscles destined to perform extensive movements are attached 

 to the bones by their two extremities, so that by contracting 

 they displace the more moveable bone in the direction of that 

 which is less moveable or fixed. Thus, generally speaking, 

 the muscles intended to move the fingers proceed from the 

 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 pro- 

 ducing the same movement they are called congenerous; 

 antagonistic if they produce opposite movements. Finally, 

 they are named, from their uses, flexors, extensors, ab- 

 ductors, &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. 70), 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 sup- 

 port r, it will have to overcome only the weight of the bone, 



