220 RAINEY, ON DENTAL TISSUES. 
supposed physiological importance of the soft material which 
is left after the action of acids on hard structures; these 
residua having been regarded as the formative organs and 
receptacles of the removed earthy matter. Hence has origi- 
nated the idea of different kinds of dentine, as “ well- 
formed consistent dentine,” “secondary dentine,” “ tubeless 
and uncalcified dentine,” according to the relative quantities 
of earthy matter thought to exist in combination with the 
soft tissue. 
Now I have no doubt but that the whole of this is erro- 
neous, and that there is only one kind of dentine which, 
even in its molecular state, is as perfect as it is in the so- 
called tubular dentine, the latter being formed by the 
coalescence of the particles of the former, exactly im the 
same way as the larger globules of earthy matter occurring 
in the deep layers of the shells of Crustaceans are formed by 
the coalescence of the smaller ones. 
As for “ uncalcified dentine,’ I know no other part of a 
calcifying tooth which could be taken for such a form of 
dentine but that which I have designated “ matrix’’—that 
‘upon which the primary particles of dentine are precipitated ; 
but it seems to me that this has no more right to be consi- 
dered as dentine than the membranous border of the bones 
of a foetal cranium has a right to be considered as bone. 
Having now described the formation and structure of den- 
tine, I will proceed to the consideration of enamel. The 
membranous matrix (see fig. 8m) was described as at first 
single, but soon dividing into two layers—one the dentine, the 
other the enamel layer. The examination of the mode of 
formation of the enamel must be commenced from the same 
point, and followed in the same direction as that of dentine. 
The enamel is first perceptible as extremely minute bright 
particles, lying so near to the primary particles of dentine, 
and so similar in appearance, as not to be distinguishable 
from them (fig. 7). Soon, however, the particles of these two 
substances assume their characteristic differences ; the den- 
tine particles bemg known by their coalescing into rows of 
globules, or congregating in spherical masses, as has been 
explamed—the enamel particles by their parallel linear ar- 
rangement. Sometimes the matrix is seen to divide suffi- 
ciently near to its lower border to enable the enamel particles 
to be distinguished from those of dentine prior to their as- 
sumption of the linear disposition, as shown in fig. 8. 
The particles of enamel, after becoming disposed in dotted 
lines, lose much of their brightness, having coalesced into oval 
flat portions, which are at first separated, but which afterwards 
