177 
medicatrix naturce, and ample provision made when the pro- 
cess is normal. In the latter case no such result is antici- 
pated, and no constant saving provision instituted ; on the 
contrary is infrequent and accidental, happening, probably, 
in certain temperaments only. 
In both cases the result is the same, so far as these pro- 
cesses are viewed under the microscope — that is, the fila- 
mental arrangements of the nerves are in both cases intact, 
and show beautifully the plexuses of nerves occupying the 
interior of the pulp already described. 
The processes may be compared to petrifying processes in 
inorganic chemistry, soluble lime salts filling up the cells and 
around the nerve fibrils, leaving these structures perfect and 
intact. Cell walls also remain, only their contents being re- 
placed by lime, the blood vessels generally being filled with 
lime, or are absorbed and disappear. 
The precise nature of these changes is not well understood, 
though the result may be readily comprehended. Without 
the aid of these secondary formations the true system of 
pulp structures never could be satisfactorily studied. 
Specimens may be sawed with main-spring saws through 
crowns and roots when the pulp is ossified, including the 
pulp, showing the filamental system through the primary and 
secondary dentine in a perfect state of preservation. Section 
after section may be cut through the tooth and pulp in thin 
slices, and dressed down on a fine hone, first sawing through 
the centre ; then saw off thin slabs on each side, dress down 
and saw as before ; dress down again and saw until all the 
pulp is sawn up. These sections may be cemented to slips 
of glass with Canada balsam : pitch the dressed side to the 
glass, then dress down on a hone until light readily passes 
through it, then introduce underthe instrument, and if not thin 
enough to show the nerves distinctly, dress down until they 
can be seen ; if too thick, same as in preparing ordinary 
tooth specimens. This secondary nodes, or nodular dentine, 
is not true secondary dentine, though very similar, and might 
be included, perhaps, under the head of calcification, which 
may take place in any tissue, especially in advanced life, 
depending on similar causes, that is stasis in the part, and 
deposit of lime, and is an accidental circumstance, and mav 
serve as a protection to the pulp against further ravages of 
decay. In both forms of secondary dentine the remaining 
soft portions of the pulp retain their vitality to a certain 
extent, varying in different cases. 
It will be found that in case of wearing of teeth of old 
persons there is evidence of tubular disturbance, especially 
VOL. IX. — NEW SER. M 
