306 



THE ANIMALS AND MAN 



c~ 



PC_Ji 



hard as bone, are not bone but a hardened epithelium tissue. 

 The part above the gum is the crown and is but a part of 

 the whole tooth. Under the gum are the neck and roots. 

 The roots are inserted into sockets in the jaws. The in- 

 cisors and canines have but 

 one root, the premolars have 

 usually two roots and the mo- 

 lars have from two to five. 



If a tooth is cut lengthwise 

 as shown in the figure, there 

 is exposed a central chamber, 

 or pulp chamber filled with 

 blood-vessels and nerves which 

 have entered the cavity 

 through the roots. The pulp 

 chamber is surrounded by a 

 layer of dentine (D), an ivory- 

 like substance which makes up 

 the bulk of the tooth. The 

 dentine is protected on the 

 crown by enamel (E), the 

 hardest substance in the body, 

 and on the roots by cement 

 (C). The enamel protects 

 the teeth from mechanical FIG. 158. Vertical section of a 

 injury and from the effect of tooth in Jaw. E, enamel; D, 

 chemicals and bacteria. 



Care of the teeth. As the 

 teeth masticate or grind the 

 food and prepare it for the (After Stirling.) 



action of the digestive juices, their condition has much to 

 do with the health. 



Decay of teeth is caused by injury to the enamel. This 

 is due mainly (i) to the action of bacteria or particles of 

 food that lodge between the teeth; (2) to tartar, a deposit 



N 



dentine; P. M., peridontal mem- 

 brane; P. C., pulp cavity; C, 

 cement; B, bone of lower jaw; 

 V, vein; A, artery; N. nerve. 



