THE PERMANENT TEETH 



397 



In the mare the canines usually are very small and do not erupt, reducing; the num- 

 ber to 36 or 38. ^ 



Incisor Teeth.— These are twelve in number. The six in each jaw are placed 

 close together, so that their labial edges form almost a semicircle'' in the young 

 horse. They have the peculiarity (not found in existing mammals other than the 

 equKke) of presenting, instead of the simple cap of enamel on the crown, a deep 

 mvagmation, the infundibulum, which becomes partly filled up with cement. 

 Hence as the tooth wears, the masticatory surface (or "table") has a central ring 

 of enamel surrounding this cavity in addition to the peripheral enamel. The 

 cavity becomes darkened by deposits from the food, and is commonly termed 

 the "cup" or "mark." Each tooth is curved so that the labial surface is convex 



Fig. 336. — Uppeh Incisor Teeth of House. Fig. 337. — Lowee Incisor Teeth of Horse. 



The labial surfaces of the teeth have been exposed by removal of the bone. Subject was five years old. 



and the embedded parts converge. The average total length of an incisor at five 

 or six years of age is about two and a half to three inches (ca. 7 cm.). They taper 

 regularly from exposed crown to apex, without any constriction, and in such a 

 manner that in young horses the masticatory surface is broad transversely; toward 

 the middle the two diameters of a cross-section are about equal; near the apex 

 the antero-posterior diameter is considerably greater than the transverse. 



This fact is of value in the determination of age by the teeth, since the mas- 

 ticatory surfaces at different ages represent a series of cross-sections. As the ex- 

 posed crown wears down, the embedded part (reserve crown) emerges from the 



' EUenberger found, as the result of extensive observations (8000 subjects), that about 

 2 to 3 per cent, of mares have erupted canines in both jaws; that 6 to 7 per cent, have them in 

 the upper jaw; while 20 to 30 per cent, have them in the lower jaw. The numerical variation in 

 the above formula results from the fact that the first premolar ("wolf-tooth") is often absent and 

 further that it is doubtful to what set it belongs when present. It is commonly not included in 

 the enumeration. 



