214 



ACTION OF THE MUSCLES. 



cle are not attached to the same bone, but to different 

 bones. Of course, the approximation must be effected 

 by the intervention of a joint, the bones turning upon 

 each other as upon a hinge. 



Emily. Then the bones may represent so many 

 levers ; the parts with which they are connected, the 

 weight to be raised ; the muscle, the power which acts 

 on the lever ; and the joint will represent the fulcrum. 

 Why, I had no idea before, that the movements of the 

 body are produced by forces acting so strictly on me- 

 chanical principles. But levers, you know, are of sev- 

 eral kinds ; on which of these do the muscles act? 



Dr. B.A\\ the three 

 different kinds of levers are 

 used in the body, but the 

 most common, is that ol the 

 third kind. 



Emily. This is where 

 the power is between the 

 weight and the fulcrum. 

 Why, this is the most disad- 

 vantageous of them all. 



Dr. B. -That indeed ; 

 but though it requires a grea- 

 ter quantity of motion, yet 

 this disadvantage is amply 

 compensated for, by greater 

 convenience, in the con- 

 struction of the body, every 

 consideration has been sac- 

 rificed to convenience. As 

 an instance of the manner 

 in which motion is produc- 

 ed by muscular action, here 

 is a figure of the bones of 

 the arm with one of the 

 muscles. Now you see 

 how the muscle shortening 



