rilF. HORSE rSDF.R domf.stkwtios 



55 



ciiMiiu'l ctliics .-ilmosl as hard as iilass. suppoitcd on otic side hy dciiliiic 

 and on \\\v olhcr l)y (-(Mncnt. 'I'licsc niatci'ials l)('in«>; sonicwiial softer 

 tlian iMianu'l. wear down sliiihtly in adN'ancc. IcaN'in^- llic enamel ed^es 

 siiffi('ieiill>' exposed to «'i\(> a pcMi'ecl self-sliai|)enin«i-, <>iiii(lin<i sm-face, 

 l)ut at the same time not pi'ojiM'tina; so far heyond t he general plane as to 

 he in dantiCM- oi" ehippin*;- off. 



When we compare such a tooth as this with a i)ei'fectly simj)le one, 

 c()mi)osed of d(Mitine and having- a short, 

 either eoiu^-sliaped oi' flattened crown, covered 

 merely with a smooth sIk^U of (Miannd without 

 cement, it is difficult at first sight to see any 

 relation or similarity between them, or to 

 realize that the highly specialized organ is 

 simph' a development of the primitive one. 

 But let us b}' way of illustration imagine for 

 a moment a tooth with a crown in form 

 somewhat like a pillar or post, composed of 

 dentine with two deep depressions in the 

 top, the dentine being completeh' covered 

 with enamel, and a thick deposit of cement on 

 the surface of the enamel. 



When the cement and enamel are worn 

 through on the grinding surface w^e would 

 have the exposed edge of an enamel cylinder 

 filled with dentine, reenforced with cement 

 on the outer surface, and two enamel rings, 

 or rather pockets with the edges exposed, im- 

 bedded in the dentine. These pockets or de- 

 pressions would be lined on their inner sur- 

 faces, or completeh' filled, as the case might 

 be, with a deposit of cement. This w^ould be 

 a construction similar to that we have 

 already seen in the incisor, except that w^e 

 have two instead of the single cul-de-sac. 

 Now let us imagine the enamel walls of these 

 culs-de-sac, and also the enamel CAdinder, 

 deeply crenated or folded and doubled, in 

 some places the latter coming almost in con- 

 tact wdth the former, thus increasing greatly 

 the exposed enamel edges, and we wdll have 

 the design of the grinding surface of an upper 

 deciduous molar after it is somewhat worn 

 (Fig. 33A). And here it must be borne in 



ill cement 



B na.tvt^O'l cO'Vtty 



Fig. 33. Wearing surface 

 of upper grinding teeth. 

 Natural size 



A. Worn surface of deciduous mo- 

 lar {d. m. :*) of colt about 6 months 

 old. e, exposed enamel edges; 

 i, natural cavity in cement 



B. Unworn surface of deciduous 

 molar (d. m. 4) of colt about 3 

 months before birth, showing 

 only enamel, d, cul-de-sac to be 

 filled later with cement; e, ape.x 

 of folded edge of enamel 



C. Premolar (p. m.l) of horse 8 or 

 9 years old 



