38 TflETH. 



your eyes on the branch of the jaw which you see in this 

 figure below, (seep. 40,) and observe how it terminates in, 

 a convex knob of an oblong shape, called by anatom- 

 ists the coronoid process. These processes are inserted 

 into corresponding cavities in the head, just under the 

 ears. The cavities being a little longer than the process- 

 es, these latter move freely from side to side. In the 

 jaw of a cat however, we find that the cavity is no lon- 

 ger than the process, and of course all lateral motion is 

 prevented. 



Emily. But why is this Dr. B ? Some end, no doubt, 

 is answered by this peculiar conformation of the jaw. 



Dr. B. Man does not possess it exclusively, but has 

 it in common with many of the inferior animals. If you 

 will listen with patience till you have learnt a little more 

 of the other parts of the mouth, I trust you will be able 

 to explain the cause of this difference without my as- 

 sistance. As for their structure the teeth differ from all 

 other bones in superior hardness, and in having the sur- 

 face of the upper portion protected by a thin layer of a 

 substance the hardest of all others in the body. By 

 means of the enamel as this is called, the teeth are effec- 

 tually preserved from the influence of the atmosphere 

 and the chemical action of the various secretions of the 

 mouth. When the latter are heedlessly suffered to ac- 

 cumulate on the teeth, the enamel is sooner or later de- 

 stroyed (for the time depends on the acrimony of the 

 secretions which vary according to the state of the con- 

 stitution,) and as it is never renewed, the parts beneath 

 are invaded and the whole tooth begins to decay. 



Emily. A forcible hint lor us, certainly, to cultivate 

 assiduously the friendship of brushes and dentifrices, if 

 we would avoid the attentions of the dentist. 



Dr. B. The portion ol the tooth below the gums is 

 called the roots or fangs ; that above being styled the 

 crown of the tooth. The enamel covers only the crown, 

 for the roots are sufficiently protected by the surrounding 



