Arthur S. (Arthur Swayne) Underwood.

Aids to dental anatomy and physiology online

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shades off gradually into the less calcified substance of
the sheath, there is no gradual shading off towards the
fibril, but a sudden and strongly marked line of demarca-
tion, and very often a space or interval between the two —
in fact, whatever the mutual relationship of these struc-
tures may be, we may be certain of one thing, that the
relationship between the fibril and the sheath is neither
in degree nor kind the same as that between the sheath
and the matrix.

Thus, then, we have fibrils with attendant nerves, tubes
or holes in which the fibrils lie, sheaths or layers of semi-
calcified matrix through which the tubes go, and, lastly,
calcified matrix.

Fibrils.— The true nature of the dentinal fibril (and
consequently the true nature of dentine) was discovered
by John Tomes— namely, that it consists of soft proto-
plasm, the axis being probably softer than the periphery
(this assumption rests on the statement that when
stretched they break and form a bead at the broken
ends). Up till Tomes' time they had been described as
fluid by some authors, and solid and hard by others.

Towards the outer portion of the dentine there exists
an area in which the soft protoplasmic elements are
disposed in a somewhat special manner, and this tract,
01 area, is called the granular layer. It is not by any


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means always present, and when it is so, it is more
common in the dentine which underlies cementum than
in that which underlies enamel. It consists of a row
of small interglobular spaces ; they are not, and have
nothing in common with, lacunae, but under a low power
they look rather like them. They are excessively common
in certain cetaceans, where there exist sometimes not one
but several of such rows. It is important not to confuse
these spaces with lacunas, and it will be well for every
student to examine them carefully under a high power,
when their real nature will become apparent. The space
occupied by a cell must, in a fresh specimen, contain the
cell, and from this cell, possessing an obvious and stainable
nucleus, branches proceed in many directions, connecting
the cell with its neighbours. The interglobular space
contains no cell and no nucleus ; it is simply a spot where
a failure on the part of the calcospherites to entirely
coalesce has resulted in a hiatus with rounded outlines.
If a million round lead bullets in an iron box were heated
up to melting-point, the resulting mass would be solid
lead. If the melting process stopped just short of com-
plete fusion, there would be interglobular spaces left, of
varying size, but always with rounded outlines. The
bullets must be of all sizes to carry out the simile. This
is the best I can do to explain an interglobular space.
This portion of the dentine is often extremely sensitive to
touch, so that it cannot be removed with an excavator
without causing considerable pain ; but as soon as the
superficial layer has been removed, the tissue underlying
it, though nearer to the pulp, will frequently be found to
exhibit a much lower degree of sensibility.

Another function has been conjectured to appertain to
the dentinal fibrils, and that is a trophic one — that is to
say, that they minister in some manner to the nourishment
of the tissue which they permeate. There is no doubt
that the division of certain nerve trunks is followed by a
degeneration, apparently due to impaired nutrition, in
the peripheral portions of the tract which they supply.
Such changes have been demonstrated in the consequences
of operations on spinal nerves, the Gasserian ganglion,
etc.; moreover, destruction, whether pathological — *'.*.,
the result of disease — or accidental, as in the case of

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injury from a blow, etc., or following on the deliberate
annihilation of the pulp in the course of treatment of the
central nervous supply of a tooth, is followed by changes
in the nutrition of the organ. This undoubted clinical
fact is further emphasized by the anatomical conditions
observable in vaso-dentine. Among fishes we find vaso-
dentine, hard or tubed dentine, and an intermediate
condition. In the hake the dentine is ail vascular with
no fibrils, in the flounder tubed dentine becomes vascular
near the pulp chamber, and varieties are fairly common
among fishes. But we never find both trophic apparatus
in the same tooth in full development. As one fades the
other grows. May we not therefore assume that we are
considering two schemes of nourishment, and that Nature,
always bent on economizing energy, refuses to employ
two forces to do the same thing? It is the old story of
the horns and the canine teeth over again. It would be
a waste of energy to encourage two concurrent systems
of irrigation, so Nature employs the handiest. Thus* in
the tooth of the flounder, the tip consists of ordinary
hard, thickly-tubed dentine ; a little lower down the tubes
become fewer and wider apart, while here and there
a vessel appears. Still nearer to the base of the tooth the
fibrils disappear altogether, while the vascular network
of capillaries becomes dense and thick. In the hake, the
tissue is freely vascular throughout, but destitute of fibrils.
As a general statement, it may be said with truth that the
frequency and regularity of fibrils in dentine is in inverse
ratio to the frequency and regularity of capillaries, and it
seems a justifiable deduction from these observations that
both are serving to some extent the same ends in the
physiology of the part, and that the presence of both in a
high state of efficiency in the same tract of tissue would
be a redundancy and a solecism, not in accordance with
the laws of economy that seem almost always to result
from the workings of evolution.

The fibrils taper towards the outside of the dentine,
and give off throughout their course fine inosculating
, branches. These branches are more numerous as the
fibril becomes more distant from the pulp, and in the
peripheral portions of the tissue the network is exceed-
ingly fine, and no doubt better instruments will soon

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demonstrate a still more delicate system of ramification.
It was supposed by the older writers that the fibrillar
elements did not penetrate to the actual outer margin of
the dentine, but modern anatomists are agreed that they do.

Dentinal Tubes. — These are simply the holes which
contain the fibrils, and in life are quite filled by them.
After death, especially if astringent reagents have been
employed, the soft tissues shrink somewhat, and some-
times become coagulated into a series of more or less
elongated dots. Under these circumstances an interval
may be perceived between the contained soft tissue and
the tube which it once filled.

These post-mortem changes and the changes due to
reagents employed are responsible for much erroneous
and misleading histology. Perhaps the safest fixative
available is formalin. Decalcifying agents, alcohol, heat,
and the effect of the razor, are all possible sources of
error. Mr. Wellings writing on this subject says :

'Decalcifying Agents. — These are always likely to
produce swelling of the tissues, and to bring about
chemical changes in the tissues that must seriously affect
the reaction of staining reagents. Very early calf embryos
and rats and mice at birth can be cut without decalcifica-
tion. Tooth germs with a quite considerable amount of
formed dentine and enamel can be cut at once. When it
is necessary to employ acids, we have found that, in our
hands at least, the most satisfactory solution is 5 per cent,
hydrochloric acid in 1 per cent, formalin. A large amount
of the fluid was used, and changed as frequently as was
found necessary. In this way quite satisfactory de-
calcification was attained without any interference with
the original fixation and with the minimum of swelling.

' Alcohol frequently causes shrinkage and distortion
through too rapid diffusion. To control this effect we
have employed methods of impregnation that do not
require the use of alcohol. Gum mucilage, dextrine mass,
and gelatin were each tried and found serviceable in
conjunction with the Aschoff C0 2 freezing microtome.

' The temperature of the paraffin bath is also the
source of much distortion from shrinkage. In addition
to this, the effect of a temperature above 40 C. on adult
decalcified bone and teeth is to harden them so that

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cutting is impossible. Where it has been necessary to
examine such tissues (as, for instance, in the vital staining
experiments), they have been cut without embedding or
after impregnation with gum mucilage, dextrine, or gelatin ;
or (in the case of sections through the entire skull) they
have been embedded in celloidin.

'The distortion due to the razor it is impossible to
avoid. Every precaution in the matter of quality and
sharpness of instruments, etc., has been taken, but there
is always a liability to the production of artifacts where
the knife has to pass through tissues of different densities
in cutting the section.' *

The course of the tubes is, naturally, the course of the
fibrils, and varies from being perfectly straight to curving
like a bent corkscrew. Two sorts of curves have been
described, the primary curves^ which consist in one sweep
somewhat like the italic letter /, stretching the whole
length of the tubes — these are more easily observed in
the crown of the tooth ; and the secondary curves, which
are smaller undulations of a more or less corkscrew
character, and are better demonstrated in the root tissue.
When the primary curves perform a sudden wriggle in
their course, and these wriggles of neighbouring tubes
coincide — i.e., occur in the same plane — the result is an
appearance of a line running at right angles to the
general course of the tubes. It is as if someone had been
trying to draw a series of parallel lines, and had had his
elbow jogged midway in each line, and the coincidence
of the deviations resulted in a line more or less at right
angles to the general course of the fibrils. These lines
are called the contour lines of Schreger^ and are defined
as due to the coincidence of the primary curves.

The lines of Owen include both these and the lines
formed by rows of interglobular spaces common in certain
cetaceans. The opening of each tube towards the pulp
is called the lumen.

The sheaths of Neumann^ the existence of which is
denied by Magitot and Kolliker, consist, according to
Tomes and others, of the semi-calcified layer of matrix
which immediately surrounds the tubes. Being of the
nature of calcoglobulin, they exhibit all the indestructi-

* Transactions, International Medical Congress, 1913.

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bility of that substance. Although they can best be
demonstrated by decalcification of the tissue, they may
be observed in transverse sections of incipient caries,
when they remain of a faint yellow colour, all the rest
of the tissue being stained with an aniline dye. They
have been described as possessing a double contour —
that is, an observable limit outside and inside. But this
appearance may be due to the thickness of the section
showing the inside margin twice — /.*?., on the upper and
lower surfaces of the slice — the one appearing to be
outside and distinct from the other. Mr. Howard
Mummery has demonstrated recently that these sheaths
do unquestionably exisi-»e-separate structures.

' Vascular patches 1 in human dentine are by no means
o rare as is sometimes stated. I have already collected
w eat many specimens, and have observed some
peculiarities in the arrangement of the dentine in their
immediate neighbourhood, which have not, as far as I
know, been described by other authors. The general
appearance under a low power of a vascular patch in
human dentine strongly resembles that of a knot in a
piece of wood. The tubes, as they approach the vascular
canal, deflect in all directions, crowding together as if to
give the object a wide berth, and after having passed it
they resume their previous relative positions. At first
sight the appearance of the section suggests that the
small area round the vessel is destitute of tubes, but
minuter investigation reveals a very fine system of tubes,
radiating from the vascular centre to the periphery of the
* patch/ thus affording an additional reason for supposing
that the fibrillar system has some trophic business to
perform. These vascular patches are normal in the
walrus, the inner third of whose dentine is entirely
composed of them.

Touching the termination of the fibrils, Mummery holds
that the dentine is everywhere permeated by very fine
branches. Hopewell Smith and Tomes both describe these
branchings as more frequent in the root than in the crown
of the tooth. Mummery says they are more readily
stained in the root, but not more numerous. He says :
'At the margin of the cementum the fibrils terminate in
loops and very delicate, scarcely visible branches, many

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of which enter the granular layer and are finely beaded
to their extremities. Under the enamel many of the
tubules are much coarser, and some appear truncated . . .
as if there had been an alternation of absorption and
deposition during the first stages of development of the
tissues. Many tubes cross the border line and terminate
in various ways in the enamel, some by small round, knob-
like bodies, some by tiny flattened expansions, while others
pass between the prisms in fine whip-like processes, but
I have not been able thus far to trace the fine beaded
fibres into the enamel.'

There are three recognized varieties of dentine, besides
hard or unvascular dentine — namely, plicidentine, vaso*
dentine, and osteodentine.

Plicidentine differs from hard dentine in that, the pulp
from which it is formed being of irregular form, the
dentine has a doubled or folded appearance in order to
fit into the convolutions of the pulp. It is practically the
difference between the skin of a filbert and the skin of a
walnut This complication, in the shape, of the pulp,
varies very much in degree. Thus, in certain lizards
(varanus, tor example) the amount of folding is slight,
rather like the folding of the silk between the ribs of a
half-open umbrella, while in labyrinthodon, an extinct
amphibian, it looks like a wild confusion of small pulp-
chambers and systems of tubes surrounding them.

Sometimes the pulp recalls the shape of the tree
commonly called a ' monkey puzzle,' as in the rays mylio-
bates, aetobates, and zygobates, and in the rostral teeth of
pristis, the saw-fish.

Vasodentine resembles hard dentine and plicidentine,
in so far as that it is, like them, developed from a row
of odontoblast cells. It differs from plicidentine in not
being folded, and from both the above-named varieties in
being traversed with more or less regularity by vascular
canals. Each canal contains one capillary, and nothing
else ; and each capillary fits and wholly fills its canal.
Vasodentine varies much in vascularity. Sometimes, as
in the hake, the vascular supply is very rich, and then
there are seldom any fibrils distinguishable. The two
elements, fibrils and capillaries, being apparently designed
to serve one and the same functional end — namely,

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nutrition — are never present, to any great extent, in the
same tract of tissue. If the vessels are numerous the
fibrils are absent, and vice versd. In the flounder the tip
is pure, hard dentine, finely tubed ; the harder varieties
are always met with in exposed situations, such as the
tip and outside of the tooth. A little lower the tubes
become scanty, and a few vessels appear. At the base
the tubes are absent and the capillary system abundant.

The two forms of dentine present in this creature, as
also the intermediate condition, are obviously formed
by similar odontoblast cells. The manatee, a Sirenian
animal, whose straight enamel prisms have already been
noticed, is possessed of vasodentine, as are the tapir and
the extinct megatherium (a gigantic edentate). The
Indian tapir has vascular canals in the crowns of his
teeth, and the American variety in the roots.

In teeth of perpetual growth, such as rodent incisors,
the dentine that occupies the situation of the obliterated
pulp is generally vascular. It must be remembered that
when vaso and hard dentine are found in the same tooth
the hard is always in the exposed situations — the tip and
outside — the softer vascular tissue forming the central
core and base.

It will have been noticed that in human — /.<?., hard —
unvascular dentine, when vessels are present as an
abnormal phenomenon, their presence profoundly modifies
the course of the tubes in their neighbourhood. In the
dentine of the walrus, where the presence of these canals
is not an abnormality but a constant phenomenon, the
same disturbance of the fibrillar system takes place,
whereas in the case of the fishes where both fibrils and
vessels are present the fibrils pursue their course unin-
fluenced, apparently, by the presence of the capillaries.

The rule appears to be that when no nerves accompany
the vessels the course of the neighbouring tubes is not
modified, but when nerves go with the vessels (and the
tissue is strictly osteo- and not vaso-dentine) the dentinal
system is arranged and focussed with direct reference to
the canals.

Osteodentine differs in a broad and essential way from
the forms of dentine already mentioned. It is not, like
hard dentine, plicidentine and vasodentine, the product

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of the activity of a surface row or rows of odontoblast
cells gradually laid on from within. It is formed very
much like intramembranous bone, by a simultaneous
activity of the cell elements throughout the tissue ;
spiculae of calcification traverse the whole area, encom-
passing and catching, as it were, in the meshes of a net
various soft-tissue elements which exist in the pulp. It is
no longer the outermost cells of the pulp forming dentine
and advancing towards the centre, leaving formed dentine
in their track, but an entirely different process, the cell
activity pervading the whole pulp. The spaces which
remain unobliterated have no special reference to the
vessels, but contain all the elements of pulp tissue,
including nerves, connective tissue, fat, etc.

Mr. Chas. Tomes has formulated a table in which the
main points of the subject of these varieties of dentine
are simply indicated as follows :

(a) Dentine developed from odontoblasts —

1. Hard dentine, tubes, no vessels, simple pulp


2. Plicidentine, the same folded (myliobates,

labyrinthodon, etc).

3. Vasodentine, tubes containing vessels and

nothing else (hake).

(b) Dentine developed from osteoblasts —

Osteodentine, channels containing vessels and
all other pulp tissues (pike).

It must be remembered that the same pulp may make
all these varieties of dentine, and also that the different
results are always due to a special disposition of the soft
tissues around and among which the deposit of lime-salts
takes place. It should also be borne in mind that these
different tissues, between which the sharp lines of
classification are drawn for teaching purposes, shade
off into one another by scarcely perceptible gradations.
Writers of text-books are obliged to present subjects in a
cut-and-dried form to their readers; but Nature does not
respect definitions, and the student who explores for
himself finds out very soon that the best books are little
more than skeletons, and that without personal work and

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observation, though he may pass examinations, he can
never possess anything worth calling knowledge.


The Pulp. — We have already discussed the functions
of the pulp as a formative organ. It remains to consider
its anatomy now that development is finished and the
remains of the dentine germ persist as the nutrient and
sentient centre of the tooth.

The pulp, which is sometimes loosely and erroneously
spoken of as the nerve, is a mass of vascular and nervous
connective tissue, situated in the centre, or core, as it
were, of the tooth, and roughly resembling it in shape,
only on a reduced scale. It is fed by two or three
arteries, and one large and a few smaller nerves. The
latter break up near the surface into an exceedingly fine
plexus called the plexus of Raschkow. Up to quite recent
times various conjectures were advanced to account for
the final distribution of the nerve filaments. That den-
tine was sensitive we all knew. An exposed surface of
dentine at some distance from the pulp cavity could be
shown to be quite capable, at times, of appreciating not
only heat and cold and the touch of instruments, but the
presence of acids. This conveyance of sensation could
only be due either to the presence of nerve filaments or
the presence of nerve endings. I think most observers
inclined with Hopewell Smith to the conclusion that the
fibrils were in themselves nerve endings. I certainly
thought so myself. Boll, more than forty years ago,
thought he saw fine filaments passing between the odonto-
blasts as far as theidentine; and when the fibrils were
pulled out, the nerve filaments seemed to accompany
them, from which fact he concluded that they penetrated
the dentine in company with the fibrils, but he did not
see them in the hard substance. Klein (1883) guessed that
this might be the case. The fact that all nerve cells and
fibres are originally derived from the epiblast (Schafer)
seems a conclusive objection to the odontoblasts
(mesoblastic) being nerve endings. Weil, Magitot, and
others, thought the objection not insuperable ; Tomes
thought it was almost so.

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To Mr. Mummery belongs the glory of demonstrating
these fine non-medullated filaments accompanying the
fibrils throughout the dentine. As Tomes pointed out,
the easier a tissue is to examine, the finer the nerve
distribution observable. The difficulties of observation
in the case of a calcified tissue like dentine no doubt
account lor the delay of this discovery.

The basal layer of Weil is a clear space in the pulp
immediately beneath the odontoblasts. Many blood-
vessels are found in this layer. Mummery stained it
with gold chloride, and found the space to be filled with
fine nerve fibrils and connective-tissue fibres, and it is
not caused by distortion in section-making, nor is it
really a basal layer at all.

Lymphatics have not yet been demonstrated in the
pulp, but it is difficult to believe that they are absent.
Modern methods will doubtless throw light upon this
point. The pulp consists largely of cells, which are for
the most part simple round or oval nucleated bodies.
Those ' very near the surface are, however, somewhat
specialized in form, tending rather to the columnar shape ;
the cells actually on the surface and in contact with the
calcified dentine are very elongated, and because when
the pulp is torn out of the tooth these cells adhere to the
dentine, they were mistaken by the earlier writers for a
membrane and called the * membrana eboris.' '1 hey are
usually called the odontoblast cells, and are described
as possessing processes towards the dentine (fibrils),
towards the cells underlying them, and towards each
other. I have not been successful in demonstrating all
these processes, but there seems to be no reason why
they should not exist. Possibly all the cells of the pulp
inosculate by branches with their neighbours.

I have said that the shape of the pulp is a rough
miniature of the shape of the tooth, and this is generally
the case. But it sometimes happens that the pulp gives
off offshoots, more or less fine diverticula, with no refer-
ence to the contour of the tooth. I have met with several
instances in which fine filaments of pulp tissue have shot
out from the confines of the pulp as far as the under
surface of the enamel, carrying with them acute sensi-
bility. This is a most puzzling abnormality, and results

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in all the symptoms of an exposure of the pulp, when the
pulp proper has obviously not been exposed.

The pulp undergoes considerable changes as age
advances. If decay threatens it from any quarter, the
odontoblast cells frequently resume their formative
functions, and make some rough-and-ready dentine to

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Online LibraryArthur S. (Arthur Swayne) UnderwoodAids to dental anatomy and physiology → online text (page 7 of 13)