rill-: noh'SE r.\ />/•;/,' i)()M/-:sr/<\rin\ 



57 



postciioi' (liniiK'lcr of ihc ciowii is also icdiiccd towaid ils liasc. 



l''i«2;iir(' '.y\ sliows an iipixM- ii«ilil «iriii(l(M- (p. in. ' ) of a lioisc icii or 

 eleven y(>ars old. The tootli is cut in fi\-e sections, and icxcais on each 

 cut suifac(\ the pat tein of enamel which will l)e presented on thenalin- 

 ally worn ii'l'indin*;- surface at vai'ious a^(>s. 



Section .1 is a diaiirain of tin* natural <2;iin(lin«i- surface as it apjx'aicd 

 in lif(\ and shows a \-ery c()nij)licat(Ml pattcMii of enamel, and lar<2;e culs-de- 

 sac almost completely filled with cenuMit. 



ScM'tion (' which is cut one and on(>-fouith inches fai ther alon*;- the 

 crown, shows «>;i-eat simplification of enamel pattern, and the culs-de-sac 

 are nuich smaller. The five branches of the i)ulp cavit\' ih) are 0})en, but 

 would have been filled with new dentine before the natui'al^rindinj!; sur- 

 face reached this jioint which takes place wIkmi the horse is ai)])roxi- 

 mately twenty-five years old. 



Section E. The lower border of enamel which marks the ])ase of 

 the crown where this section is cut, is somewhat irregular, so that the 

 enamel line is incomplete. This explains the three spaces which can be 

 seen in the external enamel. The culs-de-sac 

 are entirely eliminated and the enamel edges 

 are reduced to a minimum. The antero- 

 posterior diameter is also greatly reduced. 

 The branches of the pulp cavity are united at 

 this point (h), where the roots l)egin to diverge. 



Fig. 35. Lower pre- 

 molar (p. )n. -j) of horse S 

 or 9 vears old 



This tooth, p. m. ~, being the youngest tooth 

 in the grinding set, would not be worn down to 

 the base of the crown until the horse reached 

 thirty-five or more years, an age very rareh' 

 attained. Sections of grinding teeth on exhibition, particularh^ speci- 

 men Xo. 95, will be of interest as illustrating structure and wear. 



The lower molars in their construction are quite similar to the 

 uppers, one of the most striking differences being the absence of the culs- 

 de-sac. ]\Iuch the same result is acquired however, b}' the very deep 

 folding in of the external enamel, which at various points almost meets 

 toward the center (Fig. 35). The lower molars are also narrower in their 

 transverse measurement . 



There are several points which sometimes lead to a little confusion in 

 observing the grinding surface of the molars. For example, at the bot- 

 tom of the deeper concave surfaces of dentine between the enameUines, 

 there is a dark brow^n mark which is sometimes mistaken for a fourth 

 material, but is merely a coloration of the dentine. 



In the upper molars we generalh' find two small cavities (Fig. 33A 

 and C), one in each cul-de-sac, which sometimes gives rise to question- 



