PROBLEM 2. The Digestive System Makes Food Usable 



203 



Fig. 207 Longitudinal section of an in- 

 cisor. The different kinds of teeth all 

 have the same three regions. What are 

 they? All are alike in structure. 



Ename 



' Crown 



Neck 



Dentine 



Cement 



Nerve and 

 blood vessels 

 to dental pulp 



Root 



back. Do you note any difference? Now put a grain or two of granulated 

 sugar on your dry tongue. What do you discover.^ Blindfolded, and with 

 nose tightly shut, taste the following substances (someone must put them 

 on your tongue without telling you which is which): salt, lemon juice, 

 vinegar, sugar, something bitter, grains of ground coffee, farina, etc. 

 Rinse your mouth after tasting each substance. Which can you recognize? 

 Repeat, with your nose no longer held shut but still blindfolded. What do 

 you conclude? All of your observations must be carefully recorded. 

 Compare them with your classmates'. Why? Can you now name four 

 different ways in which the tongue functions? 



7. What can you discover about the number and the arrangement of 

 your teeth? It will help you in your study to know that the teeth in the 

 upper and lower jaws are alike. Use a mirror to discover how you bite 

 off a piece of bread. Which teeth do the work? How are they fitted for 

 it? You have four such teeth at the front, in each jaw. They are called 

 incisors. Which group of mammals has the incisors well developed? Right 

 behind the incisors, you have a single tooth on each side which is some- 

 what more pointed. It is a cuTime, the tooth which is so large in cats, dogs, 

 and their relatives. How do the back teeth differ in shape from incisors 

 and canines? How many back teeth are there in each jaw? How do they 

 function? Bite into a piece of hard chewing gum. What impression does 

 each kind of tooth make on the gum? Can you see that the two teeth 

 directly behind the canine differ from those still farther back? How? 

 Those farthest back are the molars. Between the molars and the canine lie 

 the bicuspids. If you have your last molars, called wisdom teeth, and 

 have lost no teeth, you can count 32 teeth in all. How many have you? 

 Write a report of your observations. 



