HYGIENE OF THE TEETH. 137 



tissue in the body,* and on the fang the cement, which is a 

 thin layer of bone. 



The pulp cavity opens below by a narrow aperture at the 

 tip of the fang, or at the tip of each fang if the tooth has 

 more than one. Through these openings its blood-vessels 

 and nerves enter. 



Hygiene of the Teeth. The teeth should be thoroughly 

 cleansed night and morning, by means of a tooth-brush 

 dipped in tepid water ; once a day soap should be used, or 

 a little very finely powdered chalk sprinkled on the brush. 

 The weak alkali of the soap or chalk is useful. A large 

 proportion of a tooth consists of carbonate of calcium, which 

 readily dissolves in weak acids ; and decomposing food 

 particles lodged between the teeth develop acids, which 

 eat away the tooth slowly but surely. Hence all food 

 particles should be carefully removed from between the 

 teeth ; as this cannot always be effected completely it is 

 important to brush the teeth with alkaline substances 

 which will neutralize and render harmless any acid.f 

 Good manners forbid the public use of a tooth-pick, but 

 on the earliest privacy after a meal a wooden or quill 

 tooth-pick should be employed systematically and carefully 

 to dislodge all food remnants which may have remained 

 wedged between the teeth. 



Where is the pulp cavity open? What things pass through the 

 opening? 



When and how should the teeth be cleansed? What substance 

 forms a large part of the teeth? In what is this substance soluble? 

 Why should food particles be carefully removed from between the 

 teeth? Why are weak alkaline substances useful in cleaning the 

 teeth? 



* Enamel will strike fire with flint. 



t Acid medicines should always be sucked up through a glass tube and 

 swallowed with as little contact as possible with the teeth. After each dose 

 the mouth should be thoroughly rinsed with water. 



