AGE BY THE TEETH. 95 



colored bony material. It also forms a protective covering for the 

 tooth, being spread in a thin layer over the surface of the enamel. 

 This cement gives the permanent tooth the color which distinguishes 

 it from the milk tooth. By the time the animal is 7 or 8 years of 

 age, this substance, through constant rubbing by the lips and washing 

 with saliva, has been worn from the enamel, and the teeth conse- 

 quently appear much white!' than they did when the animal was 

 but 5 or 6. 



The grinding surface of the tooth is called the table. In the new 

 unworn tooth this is irregular and is covered with enamel. The irvfun- 

 dibulum is the infolding of the enamel on the table of the tooth. 

 This forms in the incisors a cavity, the bottom of which is lilled with 

 cement to a depth which varies in different animals. The unfilled 

 portion of this cavity forms what is called the cup. The cups are 

 deeper in the upper incisors than they are in the lower ones. They 

 soon become stained by the food juices so that they appear very 

 black in color. Ordinarily after a lower incisor has been in wear 

 for three years its table surface has been w T orn down to the cement 

 filling and the blackened cup cavity has disappeared. It is often 

 difficult for the inexperienced observer to determine when the cup 

 has actually disappeared. He expects to see the table surface per- 

 fectly level and of uniform color, whereas the enamel being so much 

 harder than either the dentine or the cement, stands in relief on the 

 table surface, and envelops a very shallow and sometimes slightly 

 stained depression (of cement) for several years after the black cup 

 cavity is considered to have disappeared. The enamel of the infun- 

 dibulum persists in the lowers usually until the animal is about 15 

 and in the uppers until he is about 18 years of age. Standing in 

 relief on the table surface as it does, this enamel is frequently termed 

 "the enamel island." 



In the center of the tooth, and extending almost its entire length, 

 is the pulp cavity, a channel, which in life is filled with a fleshy tissue 

 or pulp through the medium of which the tooth derives its nourish- 

 ment. As the tooth is worn off with age the outer extremity of the 

 sensitive pulp, which would otherwise become exposed, is changed 

 into a yellowish colored ivory-like substance that completely fills 

 and closes the cavity. Hence, when the tooth has worn down to 

 the pulp cavity, the latter appears on the table surface (just in 

 front of the remains of the cup) as a yellowish colored mark called 

 the dental star. This usually makes its appearance when the animal 

 is 8 years old, although in very hard teeth it is often not apparent 

 until about 11. 



Depending upon the hardness of the dentine and the character of 

 the food, the teeth wear away at the rate of about one-twelfth of an 

 inch per year. As an incisor is not of uniform shape or size from its 

 5417—09 7 



