1126 



DIGESTIVE SYSTEM 



The deciduous teeth. — The deciduous [dentes decidui], temporary or milk 

 teeth are twenty in number, corresponding to the following formula: 



di I dc 



1, dm I = 20. 



The deciduous teeth (figs. 882, 883) are much smaller in size than the perma- 

 nent teeth, and their necks are more constricted. The enamel of the crown cap is 

 thicker. In general, their form and structure otherwise is very similar to that 



Fia. 884. — Pulp-cavity of the Upper First Molar, From the Fifth to the Ninth Year 



(Broomell and Fischelis.) 



already described in the case of the permanent incisors and canines. The molars, 

 however, are different. Their cusps on the masticating surface are very sharp and 

 irregular. There are usually three cusps on the first upper molar and four on the 

 second; four cusps on the first lower molar and five on the second. The roots 



Fig. 885. — Showing the Extent of Calcification of Deciduous Teeth, (Peirce.) 



22 months 

 18 months 



12 months 

 6 months 



40week8(newb.)-|'=^i^ 

 30weete(fioefal> 

 18weeks(foeta1> 

 17weeks^et8l> 



correspond to those of the permanent molars (three above and two below), but 

 they are much more divergent, to allow room for the development of the corre- 

 sponding subjacent permanent premolar teeth. The first molar is always con- 

 siderably smaller than the second. 



Fig.' 



-Showing the Extent of Calcification of the Permanent Teeth. (Peirce.) 



Calcification in the dentine and enamel of the teeth does not begin until the anlages of 

 the crowns arc well formed. The process of calcification follows that of the development of 

 the tooth in general, beginning in the superficial portion of the crown and gradually spreading 

 toward the root. Calcification in the d(xi(bi()us teeth begins during the fifth fretal month, and 

 at birth the crowns are nearly completed (fig. 885). Of the pcrrnancnt set of teeth, only the 

 first molar has begun to calcify at birth (fig. S8G). Calcification of the other permanent teeth 

 begins during the second year; excepting the second molar, which begins during the fifth, and 

 the third molar, which begins about the eighth year. There are, however, great variationa 



