796 THE TEETH. 



blance to the crowns of the future teeth, and then the formation of the 

 hard substance commences in them. This process begins very early, and by 

 the end of the fourth month of foetal life thin shells or caps of dentine are 

 found on all the pulps of the milk-teeth, and a little later on that of the 

 first permanent molar, while at the same time the coating of enamel begins 

 to be deposited on each. The cap of dentine increases in extent by a growth 

 around its edges, and in thickness by additions in its interior, at the expense 

 of the substance of the pulp itself, which decreases in proportion. Thia 

 growth of the tooth continues until the crown is completed of its proper 

 width, and then the pulp undergoes a constriction at its base to form the 

 cervix of the tooth, and afterwards elongates and becomes narrower, so as to 

 serve as the basis of the fang. Sooner or later, after the completion of the 

 crown, this part of the tooth appears through the gum, whilst the growth of 

 dentine to complete the fang is continued at the surface of the elongating 

 pulp, which gradually becomes encroached upon by successive formations of 

 hard substance, until only a small cavity is left in the centre of the tooth, 

 containing nothing but the reduced pulp, supplied by slender threads of 

 vessels and nerves, which enter by a small aperture left at the point of the 

 fang after the dentine is completed. In the case of teeth having complex 

 crowns and more than a single fang, the process is somewhat modified. On 

 the surface of the dental pulp of such a tooth, as many separate caps or 

 shells of dental substance are formed as there are eminences or points ; these 

 soon coalesce, and the formation of the tooth proceeds as before as far as the 

 cervix. The pulp then becomes divided into two or more portions, corres- 

 ponding with the future fangs, and the ossification advances in each as it 

 does in a single fang. A horizontal projection or bridge of dentine shoots 

 across the base of the pulp, between the commencing fangs, so that if the 

 tooth be removed at this stage and examined on its under surface, its shell 

 presents as many apertures as there are separate fangs. In alt teeth, the 

 pulp originally adheres by its entire base to the bottom of the sac ; but, when 

 more than one fang is to be developed, the pulp is, as it were, separated 

 from the sac in certain parts, so that it comes to adhere at two or three in- 

 sulated points only, whilst the dentine continues to be formed along the 

 intermediate and surrounding free surface of the pulp. 



Formation of the hard tissues of the teeth. Previously to the commencement of 

 ossification, the primitive pulp is found to consist of microscopic nucleated cells 

 (pulp-granules, Purkinje), more or less rounded in form, and imbedded in a clear 



Fig. 554. DIFFERENT STAGES IN TTTE FORMATION OF A MOLAR TOOTH WITH Two 



FANGS (from Blake). 



1, the distinct caps of dentine for five crowns in the earliest stage of formation ; in 2, 

 and the remaining figures, the crown is downwards ; in 2 and 3, the formation of the 

 crown having proceeded as far as the neck, a bridge of dentine stretches across the base 

 of the tooth-pulp ; and in 4, the division of the fangs is thus completed ; in 5, 6, and 7, 

 the extension takes place in the fangs. 



