RAINEY, ON DENTAL TISSUES. 221 
join to form continuous wavy lines. These lines, after getting 
more defined and sharper, coalesce into the ordinary formsof | 
enamel, in which all appearance of the antecedent stages be- 
comes more or less completely effaced, or, in some cases, totally 
obliterated. The verification of these facts can be easily made 
by a careful examination of the cusps of the foetal calf in the 
earliest stages of calcification ; and for this purpose the portion 
of cusp examined should be split longitudinally into two equal 
pieces, one presenting the enamel and the other the dentine 
surface to the observer, so that they may be seen together 
side by side. Several cusps should be split up for this pur- 
pose, and the examination will be facilitated by the employ- 
ment of glycerine.* It is scarcely necessary to say that this 
examination requires good illumination and great nicety of 
adjustment. At the commencement itwill be made more easily 
by tracing the film of enamel backwards from the point of the 
cusp towards the edge of the matrix. The matrix receiving the 
enamel particles can generally be seen for some distance, but 
it gradually disappears, becoming blended with and concealed 
by the contiguous layers of enamel. The films of newly formed 
enamel soon show a disposition to break up into irregularly 
quadrilateral forms; but in no instance have I met with re- 
gular hexagons, as described by some authors. The laminated 
character of dentine and enamel will, from the explanation 
just given of their mode of formation, admit of being easily 
accounted for; the degree of its distinctness depending upon 
the completeness or incompleteness of the coalescence of the 
dentine and enamel particles, will vary in different teeth. 
Some occasional appearances also, such as very distinct inter- 
globular spaces about the extremities of the laminz ; and the 
lines called contour lines or markings, will be explicable on 
the same principle ; as well as the homogeneous form of 
enamel found in some animals, and the absence of any appre- 
ciable spaces in some parts of all teeth, the dentine being in 
these parts said to have no tubules, as before noticed. 
The next dental tissue is the osteo-dentine or “ crusta 
petrosa.”” The mode of formation of this structure can be 
beautifully seen in the molar teeth of the foetal calf at the 
free margin of the pulp-cavity, where a thin scale of this sub- 
* T have not had an opportunity of judging whether these would preserve 
their natural appearance if kept in glycerine for many months. But I may 
observe that 1 had a piece of oyster-shell which showed beautifully the 
coalescing carbonate of lime by polarized light; of which I put one piece into 
, Canada balsam, and the other into thick glycerine—the former remains now 
as when first. put up, but the latter, after some months, began to lose its 
natural appearance, and now the large globules of carbonate have altogether 
disappeared. 
