MUSCLES 379 



in midnight hazings, and also develops the traits of bravery, manliness, 

 and self-reliance. There is a special danger of overexertion in com- 

 petitive sports, but with intelligent oversight of the teachers they are a 

 great benefit to all. 



SUMMARY 



1. Cells whose use is to produce motion are called 



muscle cells. 



2. In the arteries and in most of the organs of the chest 



and abdomen are spindle-shaped muscle cells, which 

 are not affected by the will, but are controlled by 

 the sympathetic system. 



3. Muscles covering the bones and moving the body 



under the control of the will form one half of the 

 body. 



4. Voluntary muscles are made of ribbonlike cells which 



are marked crosswise. 



5. Impulses from motor nerves cause a muscle to be- 



.come thicker and shorter, so that it moves any- 

 thing attached to its end. 



6. A muscle ends in a stringlike tendon which crosses a 



joint, and is attached to the lower of the two bones 

 which form the joint. 



7. Muscles are arranged in pairs. Those upon the back 



side of a limb usually straighten the joint, while 

 those upon the front side bend it. 



8. Owing to the manner of their attachment, most muscles 



must put forth far greater force than the weight 

 which they can lift. 



9. A piece of a man's muscle is stronger than any other 



muscle of the same size. 



IO. The power for contraction of a muscle is derived from 

 the heat of oxidation within the body. About one 

 fourth of the heat is thus used. 



