RAINEY, ON DENTAL TISSUES. 215 
calcification can be examined with the greatest facility—I 
shall proceed to consider the process of calcification and the 
mode of formation of the hard parts of a cusp; and shall 
first speak of the structure of dentine and its mode of 
formation. 
As the structure of this tissue owes its histological 
characters to the manner in which it is formed, the account 
of the mode of its formation will best precede that of its 
structure ; and, therefore, I shall give first the process by 
which it is formed. The first microscopic indication of the 
presence of dentine is the appearance of very minute, and 
more or less scattered, bright particles on the inferior surface 
of the membranous matrix, a short distance from its lower 
border. They are far too minute to admit of accurate 
measurement, appearing merely like very fine particles of 
dust (fig. 3 6). Examined nearer to the hard part of the 
cusp the dentine-particles are seen to be larger, and of a 
more or less rounded form, and, in the cusp of the feetal calf, 
to be arranged in lines of partially coalesced globules (fig. 
8c), but at the free border of a half-ossified fang of human 
tooth they are collected into globular masses. The com- 
ponent globules of which being, as in the lines, only imper- 
fectly coalesced, spaces, generally considered as tubules, 
are left between them. Afterwards, a still further coa- 
lescence taking place, the lines, which before consisted of 
strings of globules, are now become long fibres, or rods of 
dentine. Now this process, in point of principle, is exactly 
the same as that which takes place in the calcification of the 
claw of the lobster, of which any one can convince himself 
who will take the trouble to examine that part in the proper 
manner. The intervals between the rods, if traced back- 
wards, will be seen to end in, or rather to become continuous 
with, those between the larger rounded portions of dentine, 
and these, with the interstices between the smaller granules ; 
and thus these spaces diminish in size, but increase in number, 
until they are lost among the dusty-looking particles first 
described. These spaces, severally situated either between the 
rods of dentine or between the partially coalesced dentine- 
elobules or granules, are the so-called “dentinal tubules,” 
which, by writers upon the dental tissues, are said to have 
appreciable walls or parietes. As I believe that the intervals 
im question are merely spaces between the uncoalesced rods 
of dentine, exactly like those between the rods of enamel, I 
shall proceed to give my reasons for entertaining this view. 
And this being the negative side of the question, I may 
adduce the following argument. Now, as during the entire 
