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TEETH OF THE VERTEBRATA. 357

the dentinal sheath, which accompanies it throughout all its plexiform
radiations. The structure of these dentinal sheaths is not certainly
known, owing to the impossibility of isolating them without decalcifica-
tion of the dentnie. Some histologists believe that they are calcified,
while others express doubt as to the correctness of this conclusion. One
very marked peculiarity which they possess is their great indestructibil-
ity. Dentine when submitted to the action of strong acid for a suf-
ficient length of time to completely destroy the intervening cartilage,
or when boiled in caustic alkali, will still exhibit these dentinal sheaths,
for it is indeed only in this way that their presence can be demonstrated
satisfactorily. One writer (INIagitot) denies their existence altogether.
Enclosed within each dentinal sheath is a soft fibril, the dentinal
fibrils, which take their origin from the cells of the odontoblastic layer
of the pulp, presently to be noticed, and of which there are sufficient
reasons for believing them to be nothing more than processes or pro-
longations. There is, however, considerable discussion upon the exact
nature and relationship of these fibrils. JNIagitot maintains that they
are continuous with a layer of reticulate cells which lie beneath the
odontoblasts ; these freely communicate with processes of the odonto-
blasts, so that there is a very direct communication between the den-
tinal fibrils and the nerves of the pulp. He would therefore ascribe to
them a sensory function. Klein, on the other hand, holds that the
odontoblasts are concerned only in the formation of the dentine matrix,
and that the dentinal fibrils are long processes of deeper cells extended
between the odontoblasts. Whichever of the various views now held
may ultimately prevail, this much appears to be settled— viz. that the
dentine is extensively invaded, so to speak, by soft plasmic material
derived from the pulp, by which it is not only nourished, but also ren-
dered highly sensitive.
In the outermost layer of the dentine, which underlies the cementum,
numerous globular spaces are found, in which many of the dentinal
tubes end ; these are filled with soft living plasma. These spaces,
if such indeed they may be properly termed, give to this layer a dis-
tinctly granular appearance, whence it w^as called by Tomes the "gran-
ular layer." Other structures, known as the interglobular spaces, pos-
sessing a ragged outline and short pointed processes, may frequently
be seen in dried sections of dentine. They are said by Tomes to be
most abundant at a little distance below the surface, and he believes
them to pertain rather to a pathological than to a normal condition.
The Tooth-pulp.—It appears best to describe in connection Avith the
dentine the pulp or formative organ, in consequence of the intimate
relation which exists between them. As has already been stated, it is
lodged in the pulp-cavity, and is the principal, if not the only, source
of blood- and nerve-supply to the dentine. In the young and growing
tooth, especially about the time calcification begins, it is largest and
assumes its greatest functional activity and importance, from the fact
that it is through its mediation that the dentine is formed ; in fact, in
the early stages of dental development, as we shall hereafter see, it is
coincident with the dentine organ itself, of which in the adult tooth it
is the inconsiderable remnant. As senile changes supervene it gradually