NUTKITION OF ANIMALS 



255 



enamel 



dentine 



-. pulp 



cement 



nerve canal 



the incisors, a name which means " cutters." At the side of 

 the outer pair of incisors is a pair of sharp-pointed teeth, 

 known as the canines, or dog-teeth. Then come two pairs 

 of bicuspids, or teeth each of which has two cusps, or pointed 

 crowns. Back of the bicuspids are the molars, or teeth having 

 four cusps, three pairs of which are present in an adult's 

 mouth. There may also be two additional pairs of molars, 

 known as wisdom teeth, which appear a long time after the 

 other teeth or may not appear at 

 all. Thus a full set of human teeth 

 consists of thirty-two teeth. 



278. Structure of the teeth. The 

 teeth, as seen in. the ordinary sur- 

 face view, appear white and hard. 

 In such a view it is only the hard 

 enamel of the crown that is seen. 

 More thorough examination shows 

 that this crown is joined to one 

 or more heavy roots which extend 

 into the bone of the jaw. This 

 gives the tooth a firm support. 

 When a tooth is cut lengthwise 

 in halves (fig. 123), the following 

 structures are shown : The enamel 



is the extremely hard covering of the crown. Beneath the 

 enamel is the dentine, which composes the larger part of 

 the crown and root. The pulp is inclosed by the dentine. 

 The nerves and blood vessels enter through the tip of the 

 root and extend into the pulp. When the enamel is broken 

 or decayed, it is not rebuilt, and the underlying parts may be 

 destroyed in a short time. 



279. Mastication : the saliva. One who is hungry and comes 

 into the presence of appetizing food soon experiences an in- 

 crease in the liquids within the mouth, or, as we say, his 

 " mouth waters " for the food. This liquid is saliva, which is 



FIG. 123. A vertical section 

 through a molar tooth 



