.50 AMKh'/C.W MCSI'jr.M criDK LEAFLETS 



filled, particulaily toward the holtoni. with a deposit of cenient. It is 

 usually obser\('d 1 hat highly specialized orj!;ans are moie primitive in the 

 yoiin^ individual than in the adult. This is very noticeable in one speci- 

 men ( l*'i^. \M)li), which repi-esents the deciduous or milk incisor of a colt 

 about foui' days old. This toot h has just cut thi()U<ih the ^um, and does 

 not as yet show any weai' fiom use. The enamel cuj) is \-eiy shallow as 

 compared with that of the pernument tooth {(' and D). It also shows a 

 cleaii>' defined line at the base of the crown, which when the tooth is 

 fully ^rown, will coincide with the line of the j^um as in the teeth of the 

 cow and other simi)le teeth. Th(> root of B is not as yet fully ^rown, but 

 presents a lar<i-e pulp cavity which will be reduced to a nai-row nerve 

 canal as the root jirows longer, and tajx'rs to a point, leavinji only a very 

 small foramen for the j^assa^'e of the nerve. Figure 'M)C shows the perma- 

 nent incisor of a colt two yeai's old. which erui)ts and i-eplaces its decidu- 

 ous ])r(Mlecessor at about three and a half years. The cul-de-sac, as men- 

 tioned above, is very deep, running; down almost the full lenj2;th of the 

 crown, the loot not yet having; begun to develoj). There is already con- 

 sid(M-a])le cement in the cul-de-sac, and a very slight deposit beginning on 

 tlie external surface about the upper end of the crown. A series of lower 

 incisor sections and also vertical sections of incisor teeth, exhibited in the 

 wall case, will (^xplain the structure more clearly. 



WEAR AND COMPENSATING MOVEMENT OF THE INCISORS 



WHAT is frequ(Mitly sj^oken of as "growth" in the horse's teeth is 

 not growth, ])ut rather movement. It is true that growth does 

 continue in some few of the teeth until the horse is ten or 

 twelve years old, but only in the completion and closing of the roots 

 (see above), the crowns being all fully developed, including grinders as 

 well as incisors, at about five and one-half years. From the time the 

 tooth ])egins its service it continues to move out of the alveolar cavity, 

 which fills with new ])one, this movement counteracting the rather rapid 

 wear of the cutting or grinding surface, so that in extreme old age some 

 of th(^ teeth may be worn down to the very points of the roots. One of 

 their most remarkable specializations is the extremely long crown which 

 instead of being entirely above the gum line, as with the more simple 

 teeth, human for instance, extends far down into the alveolar cavity, as 

 much as 3''s inches in the large premolars, as a reservoir of grinding 

 material. Figure 31 shows a series of upper incisors to exj^lain the man- 

 iHM- in which 'he tooth is woin as it ])roceeds from the alveolar cavity, 

 the wearing surface altering in contour as it progresses along the crown 

 of the tooth which constantly changes in shape toward the root. 



