302 DIGESTIVE ORGANS. [SECT. 141. 



dentine, and thus forms the first rudiment of the crown. The 

 scale of dentine extends further over the pulp and becomes thicker, 

 so that it is soon seated like a cap upon the pulp, and lastly, 

 embraces it closely and completely like a capsule, whilst the pulp 

 becomes smaller and smaller the more the ossification advances. 

 At the same time, the deposition of enamel follows, and soon pro- 

 ceeds from the entire surface of the enamel-organ, becoming 

 thicker and thicker. Thus, at length, the whole of the enamel 

 is formed around the dentinal rudiment of the crown, whilst the 

 enamel-organ and the tooth-pulp constantly decrease in volume, 

 until the former is reduced to a thin membrane, and the latter 

 approaches the proportions which it exhibits in the fully developed 

 tooth. At this period, there is still no trace of cement or of the 

 fangs; it is not till the crown is tolerably far advanced in de- 

 velopment and the tooth is ready for eruption, that they make 

 their appearance. The tooth-germ now grows powerfully in the 

 longitudinal direction, and, the enamel-organ becoming atrophied, 

 only dentine is deposited upon the growing part, viz., the root. 

 The tooth thus forced upwards, begins to be pushed against the 

 upper wall of the tooth-sac and the gums which are firmly united 

 with it, gradually breaks through these parts, which also undergo 

 spontaneous absorption, and, at length, makes its appearance 

 externally. Now the gum contracts around it, whilst the re- 

 maining portion of the tooth-sac becomes closely applied to 

 the fangs, and is converted into the periosteum of the alveolus. 

 The milk-tooth is completed, 1, by the growth of the fang in 

 its due length, by which means the crown rises to its normal 

 height ; and, 2, by a deposition from the tooth-sacs, which now 

 coalesces with the periosteum of the alveolus ; this deposition 

 begins even before the eruption of the tooth, and is converted into 

 the cement, whilst at the same time the tooth becomes still more 

 thickened from within, and the germ correspondingly diminished. 

 In teeth with several fangs, the originally simple germ becomes, 

 during its elongation, divided at its base or adherent part, and a 

 fang is then developed around each division. The eruption of the 

 teeth takes place in the following order; central incisors of the 

 lower jaw from the sixth to the eighth month ; central incisors of 

 the upper jaw some weeks later ; lateral incisors from the seventh 

 to the ninth month, those of the lower jaw first; anterior molars 

 from the twelfth to the fourteenth month, those of the lower jaw 

 first ; canine teeth from the sixteenth to the twentieth month ; 

 second molars between the twentieth and the thirtieth month. 



