i8 95 . THE TEETH OF THE HORSE. 253 



from the gums in consequence of continual wear, the length is 

 increased, not by development of dentine and enamel, but by the 

 addition to the root of large quantities of cement ; the root of a very 

 old tooth, in fact, is composed almost entirely of cement. As age 

 advances the teeth project more and more forwards, so as even to 

 project between the lips, and the upper tend to bite in front of the 

 lower ; the teeth also tend to become more horizontal in position, 

 more especially in the lower jaw, and these facts are not lost sight of 

 when judging the age of an old horse. 



In a young horse the roots of the incisors are very close together 

 and the alveoli are continuous ; but as the teeth get shorter with 

 increasing age, the roots separate while the crowns remain in contact ; 

 and this divergence of roots may usually be detected in a living horse 

 of considerable age by the presence of interdental spaces at the edge 

 of the gum. 



A shallow groove running longitudinally along the outer surface 

 of the upper corner incisor has been employed with satisfactory 

 results by Galvayne for determining the age of the horse after the 

 obliteration of the marks in the lower incisors. This groove is of 

 limited length, and, in a middle-aged horse, does not extend to either 

 extremity of the tooth. It is developed on the embedded portion of 

 the crown, and, as the tooth very gradually emerges from its socket, 

 the lower end of the groove is the first to become exposed. This occurs 

 at about ten years of age. At sixteen years the groove extends halfway 

 down the exposed face of the tooth, and reaches the lower edge at 

 twenty-one years : that is to say, that part of the tooth which is 

 visible at ten years has been completely worn away in eleven years. 

 The upper extremity of the groove now issues from the gum, and the 

 groove gets shorter and shorter, until at thirty years it simply forms a 

 notch on the outer border of the table, and the exposed portion of the 

 external face of the tooth is once more nearly smooth. 



The start which the first permanent incisor has over the 

 second, and the second over the third, is maintained throughout life, 

 and since the teeth wear out in the order in which they were developed, 

 the first incisor in an old skull is shorter than the second, and the 

 second than the third, although in youth the order of length is the 

 reverse of this. 



The deciduous incisors (Fig. K.) are much smaller than the per- 

 manent, and exhibit a marked constriction or neck between the crown 

 and root. The crown is of a milk-white colour and is quite smooth on 

 its anterior surface, whereas that of the permanent tooth is less white 

 in colour and is marked on its front surface by a shallow longitudinal 

 gutter. The infundibulum is shallow and the root is gradually ab- 

 sorbed as the permanent tooth forms behind it. The first and second 

 deciduous incisors begin to calcify 1 at five or six months before birth, 



1 The origin of a tooth is here considered to date from the commencement of 

 calcification, and not from the formation of the dental capsule and other soft parts. 



