DIGESTIVE SYSTEM. 147 



and in each jaw are divided into, incisors 4, cus- 

 pids 2, bicuspids 4, molars, 6, the dental formula for 

 one side being, I.f C.I B.f M.f = 16. 



Structure of Teeth. The parts of a tooth are 

 crown, neck, roots and pulp. Its calcareous wall 

 consists of enamel, dentin, and cementum. 



Enamel. The enamel is the hardest tissue in the 

 body and covers the exposed portion, or crown, of 

 the teeth. Its function is to mechanically protect 

 the tooth. This enamel is derived from the ecto- 

 derm, while all bone tissues are products of the 

 mesoderm. Bone, if injured, may regenerate and 

 repair the defect. Enamel does not regenerate if 

 injured, the defect being permanent, as the formative 

 enamel tissue disappears before the eruption of the 

 tooth. Chemically, it is composed of phosphates 

 and carbonates of calcium and magnesium, a small 

 amount of fluorides, water, and perhaps a very small 

 amount of organic material. In consequence of the 

 latter, enamel, unlike bone, is soluble in acids, 

 leaving scarcely any residue. 



Enamel is composed of two structural elements, 

 (a) enamel rods or prisms (also called fibers) , and (6) 

 inter prismatic or cement substance^ both of which are 

 calcified. These elements have different properties, 

 both chemical and physical. The interprismatic 

 substance is more readily acted upon by acids, and 

 it is therefore possible to etch enamel sections and 

 produce a disassociation of the enamel rods. The 

 interprismatic substance is not so strong as the rods, 

 and in splitting or breaking the enamel the tissue 

 usually separates along the cement lines; that is, 



