as ■indicative of the Age of the Animal. 295 
mersion in different fluids, which can be seen passing along the 
tubes displacing any air they contain, and removing their opacity. 
The dentinal tubes, at their openings from the pulp cavity, are 
larger than elsewhere ; and according to Retzius, their average 
diameter near to the cavity is tt'od of an inch. The distance 
between each of them, at tlie same place, is about twice that of 
their diameter. Their direction varies much in different parts 
of the tooth. Those which come immediately from the superior 
part of the cavity take a vertical course ; those which arise a 
little removed from the apex pass obliquely upwards, terminating 
on the borders of the structure with a gentle curve outwards and 
downwards ; those springing from the sides and lower parts have 
a direction almost horizontal ; while in some teeth tlie tubes pro- 
ceeding from the most inferior part of the pulp cavity are seen 
to pass immediately downwards. 
Towards the terminal portion of the fang the dentinal tubes, in 
some animals, of which the sheep is an example, are often collected 
into little bundles, liaving between them a comparatively large 
space, which is occupied with the intertubular structure. The 
tubes thus compacted together run towards the periphery of the 
dentine, before reaching which, however, they suddenly disperse, 
pursue their course at regular distances from each other, and end 
in the ordinary manner. As this is only seen in some teeth, it may 
depend on an interruption to the regular formation of the dentine 
from eitlier local or constitutional causes. 
In their course the dentinal tubes always describe two or three 
bends or curves ; besides which each tube is more or less wavy of 
itself. The former of these bendings have been called the primary, 
and the latter the secondary curvatures, by Professor Owen and 
other writers on the structure of the teeth. Besides this, it has to 
be remarked, that each tube splits into several branches. Usually, 
its first division is into two. These by further separation become 
exceedingly numerous, their size diminishing with each division, 
till at length many of them terminate in immeasurably fine and 
inosculating branches. In some animals, however, as seen in the 
annexed figure of a section of dentine and enamel from the 
sheep, the tubes for tlie most part end in minute cavities {lacunas). 
From these cavities tidmli, as previously stated, pass into the 
enamel or the crusta, whichever of these structures may happen 
to be the covering to that particular part of the dentine. The 
whole of this arrangement is beautifully adapted to carry nutrition 
to every part of the tooth. In the illustration (fig. 8) the den- 
tinal tubes are marked d ; the lacunae, d * ; and the enamel, e. 
The intertubular substance is tolerably transparent, and accord- 
ing to Mr. Tomes is finely granular. Both it and the walls of 
the tubes contain the earthy matter of the dentine. 
