as indicative of the Age of the Animal. 
299 
blood-vessels of the socket, and thus it continues, long after 
the obliteration of its pulp cavity, to serve all its purposes as a 
part of the living organism. Fig. 10 shows in transverse section 
the fang of an incisor of an old horse magnified two diameters. 
Its pulp cavity, a, is barely visible, the encroachment inwards of 
the dentine, d, having nearly closed it. On its outer boundary, 
the dentine, which had originally extended to about as far as the 
line marked, b, has become changed to Crusta, c. Sections of 
teeth of this kind, even when viewed without any magnifying 
power, have a peculiar 
white appearance of the 
more recently formed den- 
tine in their centre, which 
exceeds in opacity the 
other parts. From this 
circumstance it might be 
thought that the structure 
here existing was not the 
same as the surrounding 
dentine. The microscope 
however at once dismisses the doubt. When viewed with an 
inch object-glass only, this whiteness is seen to depend on closely 
compacted dentinal tubes, and nowhere can be detected that 
arrangement of the structures which has led to the opinion of 
the canal being filled with osteo-dentine. 
The precise way in which the Dentine at its periphery changes 
into Crusta has yet to be ascertained. It seems that the dentinal la- 
cuna3 undergo dilation and thus become identical with the hollow 
spaces or cells in the Crusta. All the dentinal tubes however do not 
end in lacunae, but many of them, as has been explained, terminate 
in very fine branches, and it is worthy of note that in this Crusta 
bundles of such dentinal tubes are preserved, as if they had 
passed in unchanged. In further confirmation of the opinion that 
the transition is thus effected, in part at least, is the circumstance 
that the cells in the crusta lying near to the border of the dentine 
are circularly arranged row above row. Another feature has 
likewise to be named, which is that a true Haversian system, in- 
dependent of scattered Haversian canals, exists in such Crusta. 
These several things are depicted in the following engraving, 
fig. 11, which gives a magnified view of a small portion of the 
tooth from which fig. 10 was taken. In it, d represents the den- 
tinal tubes, D* the dentinal lacunae, c the Crusta with bundles of 
* Fig. 10. A transverse section of an incisor of an old horse, magnified two 
diameters, showing the conversion of the dentine into crusta. a, pulp cavity; d, 
dentine; c, crusta; b, a line drawn for the purpose of denoting tlie original extent 
of the dentine. 
Fig. 10.* 
