74 PRINCIPLES OP VETERINARY SCIENCE 



influence of wear, until at the age of two and one-half years there 

 remains only a loosened shell. 



Table op Aveeage Periods of Eruption op Teeth in the Horse 



Teeth Eruption 

 A. Deciduous: 



1st incisor (Dil) Birth or first week. 



2d incisor {Di2) 4-6 weeks. 



3d incisor (Di.3) 6-9 months. 



Canine (Dc) 



1st premolar (Dp2) 1 Birth Qr &gt 



2d premolar (DpS) f weeks ' 



3d premolar (Dp4) J 



B. Permanent: 



1st incisor (71) 2\% years. 



2d incisor (12) 3}4 years. 



3d incisor (/3) 4}^ years. 



Canine (C) 4-5 years. 



1st premolar (or wolf-tooth) (PI) 5-6 months. 



2d premolar (P2) 2J^ years. 



3d premolar (P3) 3 years. 



4th premolar (P4) 4 years. 



1st molar (Ml) 10-12 months. 



2d molar (M2) 2-2^ years. 



3d molar (M3) 3^-4 years. 



(The periods given for P3 and 4 refer to the upper teeth; the lower ones 

 may erupt about six months earlier.) 



To determine the age of a horse the time of eruption and wear of 

 the teeth are used as guides. In young horses the permanent 

 incisors have a crown which is broad transversely; later in life 

 the two diameters are about equal; in extreme age the antero- 

 posterior diameter may greatly exceed the transverse. The 

 angle at which the upper and lower incisors meet becomes rapidly 

 less as age advances. The cup of an incisor tooth disappears 

 about three years after the tooth erupts, and the infundibulum 

 from ten to fifteen years after. Both central incisors in the lower 

 jaw lose their cups when the horse is about six years of age, 

 the laterals at seven, and the corners at eight. At nine years the 

 cups in the upper central incisors disappear. At ten years a yel- 

 lowish-brown groove appears at the top of the upper corner 

 incisors, and reaches the wearing surface when the horse is 



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