Incisors: yrs. mos. yrs. mos. mos. Centrals At birth 2 6 At birth 1 8 At birth, 12 or 3-4 weeks First 4-6 wks. 3 6 At birth 2 9 8-12 wks. 18 laterals Second 5-12 days 3 6 laterals Corners 6-9 mos. 4 6 12-18 days 4 6 At birth 9 Molars: First At birth 2 6 At birth 2 6 7 weeks 5 Second At birth 2 6 At birth 1 6 8-28 days 14 Third At birth 3 6 At birth 3 8-28 days 13 Fourth 10-12 1 6 13 Fifth 2 2 5 Sixth 4-5 2 6 9 Seventh 18 Canines or 4-5 9 tusks IN DETERMINING THE AGE of the different domestic animals by the development and appearance of the teeth, most of the attention is given to the lower incisor teeth. Up to the fifth year, the age of the horse or ox can be easily determined by the eruption and replacement of the incisors. At _one year_ of age the colt has a fully developed set of temporary incisors. The ruminant's incisors at this age all show wear. _The two-year-old colt_ shows a well-worn set of incisor teeth, and the ruminant at this age has replaced the nippers or centrals. _The third_, _fourth_ and _fifth years_ are indicated by the replacement of the temporary nippers, dividers and corners in the horse, and the first and second dividers and corner teeth in ruminants. [Illustration: FIG. 52.--Longitudinal section of incisor tooth: cup; cement; enamel; ivory; and pulp cavity.] In the horse the permanent nippers are full grown and in wear at _three years_ of age; the permanent dividers are full grown and in wear at _four years_ of age; and the permanent corners are full grown and in wear at _five years_ of age. The table surfaces of the incisor teeth of a five-year-old horse show different degrees of wear. At this period in the animal's age, the nippers have been in wear two years, the dividers one year, and the corners are beginning to show wear. In ruminants, all of the chisel-shaped table surfaces of the incisors show considerable wear when the animal is five years old. After the animal has a full set of permanent teeth, we judge the age by the degree of wear or the appearance of the table surfaces of the incisors, their shape, the angle with which they meet and the general appearance of the head. [Illustration: FIG. 53.--Cross-section of head of young horse showing replacement of molar tooth.]