THE MUSCLES. 217 



ture the muscles of the lower limbs and back are perpetu- 

 ally 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. 



676. Use of the Muscles.-* -Some of the uses of the 

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

 are also the grand organs of motion, by which the system 

 is moved from one place to another, constituting the instru- 

 ments 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 similar motion of 

 the muscles. Even the act of respiration is carried on by 

 these moving powers, and therefore life cannot be sustain- 

 ed, even for a moment, without their action. 



677. Mechanism of the Muscles. In the muscles con- 

 cerned 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. 



678. 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 

 fulcruiL, and the part that is moved constituting the 

 weight. 



679. Lexers are divided into three kinds, according 'to 

 Ih' relative position of their three essential parts, the 

 weight, the power, and the fulcrum. In the first kind the 

 fulcrum 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 ? What are the uses of the muscles ? 

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

 the lever? What part the power? What the fulcrum? What the 

 weight ? How do the three kinds of levers differ from each other? 



19 



