354 THE HUMAN BODY AND ITS FOOD 



the nose can be closed by the soft palate, which rises 

 up against them. The opening between the pharynx 

 and the mouth can be closed by two upright muscles, 

 which come together like the halves of a sliding door. In 

 passing from the pharynx, food must go over the ' ' wind- 

 pipe/' or trachea. We should soon be strangled if food 

 went into our windpipes; hence the larynx, at the top of 

 the windpipe, is closed by a cover, called the epiglottis. 

 When the food is in the pharynx, and the openings to 

 the nose, mouth, and windpipe are closed, the upper 

 muscles of the pharynx contract above the food, forcing 

 it downward into the esophagus. This is a tube about 9 

 inches long, lined with mucous membrane, and provided 

 with circular and with lengthwise muscles. The esopha- 

 gus muscles contract above the food, and force it down 

 into the stomach. 



362. Exercises. 



1. What advantages has the internal skeleton of vertebrates over 

 the hard external covering of insects and crustaceans? Has it any 

 disadvantages? 



2. What advantages does man gain from his erect position? 



3. Name several levers in the body. 



4. How could you find out how much of the weight of a bone is due 

 to mineral matter? 



5. What devices are there in the skeleton to prevent the jarring of 

 the softer parts of the body? 



6. What kind of a joint do you think there is in your neck that 

 makes it possible for you to shake your head? To nod "yes." 



7. What is the effect of high heels upon the position of the body in 

 walking? 



8. What kind of a joint do the fingers have? What kinds has the 

 thumb? What are the structures in the hand that make it so skillful? 



