MOTION. 23 



Summary. I . Motion is involved in nearly every activity of the body. 



2. The action of muscle is a shortening, accompanied by a thicken- 

 ing and hardening. 



3. Muscle consists of fibers with a connective tissue sheath for 

 each fiber, bundle of fibers, and for the muscle as a whole. 



4. The skeletal muscle fibers are striated. 



5. The muscles make about half the body's weight. 



6. Muscles may shorten one third their length. 



7. They cannot remain shortened long for a time. 



8. The muscles should be developed symmetrically. 



9. In the limbs the muscles are fusiform and have their greatest 

 diameter opposite the central, or narrower, portions of the bones, con- 

 cealing the fact that the bones are largest at the ends, as is so manifest 

 in the skeleton. 



10. The bones serve as levers by which the muscles exert their force. 



11. The bones of the limbs are hollow cylinders combining light- 

 ness and strength. 



12. The joints have a smooth motion due to the cartilage and 

 synovia. 



13. Locomotion is brought about by reaction. 



Questions. i. What effect is produced by carrying a heavy satchel 

 for a long 'distance without resting? 



2. Which is more tiresome, standing still or walking? Why? 



3. When the boy, who thinks he can strike a hard blow, says, 

 " Feel my muscle," does he usually call attention to the muscle used in 

 striking? 



4. Find other examples of levers in the body. 



5. Find examples of the three kinds of levers, not in the body, 

 which we use often. 



6. Why is it easier to sit with one leg crossed over the other? 



7. What is the effect on muscles of light clothing? 



8. How may the arms be used to illustrate the three kinds of 

 levers ? 



9. Analyze and explain jumping, hopping, etc. 



10. What is "curvature of the spine"? How caused and how 

 avoided? 



11. What makes people bow-legged? 



12. Why are the sides of the body often sore after walking on icy 

 pavements? 



