THE ORGANS OF THE INNER GERM-LAYER. 



311 



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

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

 (sm) ; 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 more it raises 

 up the tissue of 

 the gum, which 

 covers the edge of 

 the jaw, and 

 causes it to be- 

 come gradually 



thinner. Finally, 2** 



{< * 



it breaks through j"r> 



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 the 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, odontobla3t-layer(meinbrana eboris) ; zb, dentine ; s, enamel ; 

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



