536 PHYSIOLOGICAL DEMONSTRATION OF THE TEETH. 
In this, and especially near the layer closest to the enamel, 
dentinal cells are sometimes seen, which may probably be 
analogous to the lacunae of bone. If you examine the den- 
tinal curvatures, you will see that they are of two kinds ; one 
set, in bold and evident curves ; another set, not so evident, 
but which, with a little patience and high magnifying power, 
you may see, curves upon the curves already demonstrated. 
The former are called the e primary,’ the latter the secondary 
6 curves’ of the dentinal tubuli (like a biserrated leaf in bota- 
nical description). From the tubuli minute branchlets are 
given off on the sides ; and tow ards the end the tubes termi- 
nate, either in cells, or by anastomosis, or by looping back 
upon themselves. 
“ The Cementum, or c crusta petrosa,’ at first envelopes the 
whole tooth, but soon gets w orn off the crown, as far dow n 
as the neck. It is, compared with the other two structures, 
very soft, and, examined w ith the microscope, more closely 
resembles bone than any of them ; in fact, it is continuous 
w ith the bone of the jaw 7 in some animals, thus proving its 
identity. It contains lacunae and can aliculi, which are easily 
demonstrable ; and, when there is a large mass of it, some- 
thing like Haversian canals. 
a There is, then, a great analogy betw een tooth and bone. 
In the crusta petrosa absolute likeness, and in the dentine 
(the constant tissue of the teeth), — analogies too striking to 
be overlooked, — viz., the tubuli analogous to the canaliculi • 
intertubular cells, analogous to the lacunae ; and intertubular 
substance, analogous to the laminae of bone. In the enamel 
the greatest departure is observable, but not wider than its 
peculiar function suggests ; and it must be remembered, 
first, that it is the least constant tissue of tooth ; and, se- 
condly, that its chemical composition is very much the same 
as that of other parts of tooth, all of which clearly resemble 
bone composition. 
“ Lastly, the analogy is completed in a review of the mode 
of tooth development. Thus, upon a mucous papilla, a large 
quantity of gelatinous matter is observable, in which certain 
cells appear; the gelatinous matter exactly resembles the 
incipient cartilage in w hich ossification commences. This 
papilla is supplied with an artery, w 7 hich nourishes its cells, 
and these gradually so develope, that the older ones are 
pushed outwards, and form the dentine.” — Medical Circular , 
29 th June , 1853. 
