218 MENTAL AND PHYSICAL EXERCISE. 



680. So far as mechanical advantage is concerned, the 

 last is by far the less effective, and it is the application of 

 this principle by which the levers are moved by the mus- 

 cles. We shall see, however, that it is not mechanical 

 power alone which is wanted in the construction of the 

 limbs, and that all the circumstances considered, this is the 

 only kind of lever which could be employed consistently 

 with the perfection of our organs of motion 



681. Muscular action of the Jlrm and Hand. The 

 motion of the forearm may be taken as an example of the 

 effect of muscular contraction, and the manner in which it 

 is produced in the animal system. When we raise a 

 weight by bending the elbow joint, this is effected by mus- 

 cles situated below the shoulder, with the tendons inserted 

 into the upper sides of the bones of the forearm just below 

 the joint." 



682. Let a b, Fig. 122, represent the bones of the fore- 

 arm, b d the bone of the arm, d the muscle, e the tendon, 



Fig. 122. 



c the insertion of the tendon into the radius, and b the 

 elbow joint. It is plain that the contraction of the muscle 

 makes c approach towards d, which, as d is a fixed point, 

 is effected by bending the joint 6, raising up the point c, 

 and thus giving great velocity of motion to a, and the 

 weight attached to it. 



683. " The consideration of the manner in which the 

 muscle acts in this case, proves that the mechanism of the 

 animal body is calculated to produce a great loss of abso- 

 lute power. It is an established position in mechan- 

 ics, that in the action of levers, the power is to the 

 weight as the distance between the weight and the ful- 







What and is applicable to the bones ? Explain Fig. 122, and show why 

 much mechanical force is lost in that arrangement. Why is muscular 

 power thus sacrificed? 



