STUDY OF A LONG BONE. 



23 



Ball 



Articular Extremity 



of the body on tiptoes, or as the foot is used in taking each 

 step, the foot is used as a lever of the second class. When 

 one lifts a weight with the toes, the foot is used as a lever 

 of the third class. These three 

 classes of levers are illustrated 

 in the accompanying figures. 



In the earthworm we have an 

 example of what sort of motions 

 we could execute without bones. 

 And even the earthworm has 

 spines that help it in crawling. 

 But we see how slow and rela- 

 tively feeble are its movements. 

 Compare these with the rapidity, 

 force, and precision of our mus- 

 cular actions. 



In the experiment with the 

 frog's muscle we saw what we 

 have mentioned as the action 

 of a muscle by itself. We need 

 levers through which the muscles 

 can act. 



Study of One of the Long 

 Bones. For this take, prefer- 

 ably, a femur or a humerus. Let 

 us suppose we have a femur. 



1. Observe its shape, cy- 

 lindrical, somewhat curved, en- 

 larged at the ends. 



2. The ends have smooth places, where they fitted other 

 bones. 



3. Along the sides, especially near the ends, are ridges and 

 projections, where the muscles were attached. 



Medullary Cavity 



Shaft 



Hard Bone 



Spongy Bone 

 Articular Extremity 



Fig. 7. Longitudinal Section of 

 Femur. 



