OF THE MUSCLES. 221 



ture, the muscles of the lower limbs and back are per- 

 petually active, in order to keep the upright position ; for 

 the instant they are relaxed, as from faintness, or a fit, 

 we fall to the ground. 



Use of the Muscles. Some of the uses of the mus- 

 cles are obvious, from what has just been said. They 

 are also the grand organs of motion, by which the sys- 

 tem is moved from one place to another, constituting the 

 instruments of locomotion. It is by the muscles, indeed 

 that all the motions of the body, whether general, or 

 local are performed : not a finger moves ever so slightly 

 without the contraction of some fibres ; nor is a word 

 spoken, or any sound of the voice heard without a simi- 

 lar motion of the muscles. Even the act of respiration 

 is carried on by these moving powers, and therefore 

 life cannot be sustained, even for a moment without their 

 action. 



Mechanism of the Muscles. In the muscles concern- 

 ed in locomotion, and in the other voluntary motions of 

 the body, the rise or origin of the muscle is from one 

 bone, and the insertion into another, the thick part being 

 between these two points, and the motion is performed 

 by the intervention of a joint. 



The bones, must therefore, be considered as levers, 

 acted upon by the muscles ; the part where the tendon 

 is inserted representing the power ; the joint itself the 

 fulcrum, and the part that is moved constituting the 

 weight. 



Levers are divided into three kinds according to the 

 relative position of their three essential parts, the weight, 

 the power, and the fulcrum. In the first kind, the ful- 

 crum is between the weight and the power, or moving 

 cause ; in the second, the fulcrum is at the end of the 

 lever, the weight being between it and the power ; in the 

 third, the power is in the centre, between the weight 

 and fulcrum. 



When are all the muscles relaxed 7 What are the uses of the muscles 7 

 Considering the bones and muscles in a mechanical relation, what part 

 is the lever 1 What part the power 1 What the fulcrum! What the 

 weight 1 Ho w do the three kinds of levers differ from each other 1 

 19* 



