11G T. CHARTERS WHITE ON THE 



the hexagonal character of the prisms may readily be seen, but, as 

 yet, I have not been able to demonstrate more than a polygonal 

 arrangement — a sort of irregular reticulation ; the structure in a 

 thin section is so friable that it generally breaks up before ground 

 sufficiently thin for microscopical examination. But in some sections 

 figured by Mr. Tomes, which have been cut and then decalcified, 

 the dilute acid has acted first on the centres of the prisms, those 

 being the last portions calcified in the developing tooth, and tbe 

 peripberal portions remaining untouched present a fenestrated ap- 

 pearance, which more approximate to squares than hexagons. 



This, then, may be considered a fair description of what may be 

 seen in a specimen of normal properly-developed enamel ; but 

 appearances may present themselves which will need a word of 

 explanation. In examining a section cut longitudinally, the enamel 

 will present to your view arched lines of a dark colour, and granular 

 in structure ; in transverse sections these appear as rings — these are 

 the contour-lines of Owen, and are the expressions of the stratified 

 depositions of the lime in the development of the enamel. Again, 

 the enamel fibres may be imperfectly joined, leaving interspaces 

 and breaks in their continuities, this appearance is most fre- 

 quently met with in the teeth of unhealthy subjects. 



Another appearance may also be noted, which, while normal in 

 some animals, especially in the Marsupialia, is not so in Man. I 

 mean the extension into the enamel of the dentinal tubuli, which, 

 crossing their proper termination, form dilated extremities for a 

 considerable distance in the substance of the enamel, and generally 

 accompany a want of proper development in the dentine. These 

 I believe to be the principal points which will strike you in your 

 examination of ordinary sections of enamel. We will now proceed 

 to the consideration of the dentine, and its normal and abnormal 

 characters. The dentine makes up the principal portion of the 

 tooth, and is capped by the enamel above, and clothed by the ce- 

 mentum beneath the gum. Upon examining this structure in a 

 fracture, it presents a silky, fibrous appearance, readily distinguish- 

 able from the fracture of the enamel, which appears crystalline. 

 This tissue is best examined in longitudinal and transverse sec- 

 tions; and I will now proceed to describe in more detail the appear- 

 ances presented to us in a longitudinal section of a perfectly healthy 

 and well-formed tooth. Such a section should show us not only a 

 sound cap of enamel free from all irregularities of formation, but a 



