THE ORGANS OF THE INNER GERM-LAYER. 



311 



formed by the odontoblasts (0), or dentinal cells ; this cap at the same 

 time acquires a coating of enamel (s) from the enamel-membrane 

 (stn) ; then there are continually deposited on the first layers new 

 ones, until the crown of the tooth is completed. Under pressure of 

 the latter the enamel-pulp (sp) atrophies, and forms only a thin 

 covering to the tooth at birth. The papilla (zp) is converted into a 

 mass of connective tissue containing blood-vessels (g) and nerves, and 

 fills the cavity of 

 the tooth as the so- 

 called pulp. The 

 larger the whole 

 structure becomes, 

 the moi^e it raises 

 up the tissue of 

 the gum, which 

 covers the edge of 

 the jaw, and 

 causes it to be- 

 come gradually 

 thinner. Finally, 

 it breaks through 

 the gum soon after 

 birth, and at the 

 same time casts 

 off from its sur- 

 face the atrophied 

 remnant of the 

 enamel-organ. 



The time has 

 now come in which 

 the third hard sub- 

 stance of the tooth 

 is formed, the cementum that envelops the root. So far as the 

 dentine has received no coating of enamel, the bounding con- 

 nective tissue of the dental sac (zs) begins, after the eruption of the 

 teeth, to ossify and to produce a genuine bone-tissue with numerous 

 SHARPEY'S fibres ; this bony tissue contributes to the firmer union of 

 the root of the tooth with its connective-tissue surroundings. 



The eruption of tJie teeth ordinarily takes place with a certain degree 

 of uniformity in the second half of the first year after birth. First 

 the inner incisors of the lower jaw break through in the sixth to the- 



Fig. 173. Section through the fundament of the tooth of a young 



Dog. 

 k, Bony alveolus of the tooth ; zp, dental papilla ; g, blood-vessel ;. 



o, odontoblast-layer(ruembrana ebons) ; 26, dentine ; s, enamel ; 



sm, enamel-membrane ; zs, dental sac ; sp, enamel-pulp. 



