80 



STUDIES IN PHYSIOLOGY 



the tooth is imbedded in a socket in the bone of the jaw. 

 It has no enamel, but instead its outer layer is a modified 

 bone tissue called cement substance. The incisors and canines 

 usually have but a single root ; the bicuspids may have two ; 

 and the molars are often held in the jawbone by three, 

 four, or five fangs. In the region between the crown and 

 the fang is the neck of the tooth, which is surrounded by 

 the gums. 



The internal structure of a tooth is well shown in a verti- 

 cal section. The covering of enamel is thickest over the 

 top of the crown ; it becomes thinner down the exposed 

 sides, and disappears in the neck region. The largest part 

 of the tooth is composed of the bony den'tine, which consists 

 of fine processes extending from the cells in the pulp cavity 



" Anatomy and Histology of the Mouth and Teeth," gives approxi- 

 mately the time when these changes occur. 



