TEETH. 



[ 633 ] 



TEETH. 



prisms do not run exactly parallel witb each 

 Fig. 739. 



Enamel-fibres, isolated by the very slight action of 

 muriatic acid ; human. Magnified 350 diameters. 



other, but are arranged in groups or zones, 

 the fibres of which cross each other. The 

 fibres are readily isolated before they have 

 become so developed as to be hard, and 

 when very slightly acted upon by muriatic 



acid. Sometimes the ivory-tubes extend 

 into the enamel. 



Two kinds of dark bands or stripes are 

 seen traversing the enamel (fig. 736). The 

 direction of one of these coincides pretty 

 nearly with that of the fibres, and it arises 

 from the crossing of the zones of fibres, 

 allowing more or less light to pass through, 

 the bands being light and dark. The other 

 set (fig. 736 //) consists of arched, brouTiish 

 stripes, indicating the laminated structure of 

 the enamel. Under the polariscope, a third 

 set becomes visible, arising from the variable 

 inclination of the axes of the fibres to the 

 plane of polarization. 



The enamel is often traversed by cracks, 

 mostly running parallel with the fibres, and 

 containing air in dry teeth. 



Chemically, teeth consist of an organic, 

 cartilaginous basis, agreeing in composition 

 with that of bone, and inorganic matter 

 consisting principally of phosphate of lime 

 with a small quantity of the carbonate. 



Development. — The rudiments or germs 

 of the first (milk) teeth are met with in the 

 sixth week of foetal life, and consist of small 

 papillae, one for each tooth, which become 

 visible in grooves of the mouth, afterwards 

 forming the alveolar processes. Processes 

 from the sides of these dental grooves are 



Fig. 740. 



Fig. 741. 



yfrjr' li 



Fig. 740. Surface of the enamel, with the ends of the enamel-fibres, from the tooth of a calf. Magnified 350 diameters. 



Fig. 741. Diagram showing the development of a milk-tooth, and the corresponding permanent tooth, a, furrow ; 

 h, the same with the papilla ; c, the same closing, with the commencement of the reserve cavity ; d, the same, further 

 closing ; e, follicle completely formed, with the reserv e cavity ; /, the reserve cavity receding ; g, the same, with 

 a tooth-germ J h, the alveoli of both capsules formed, the milk-tooth being through the gum; i, the same, further 

 advanced, the neck of the capsule forming a solid cord. 



