consideration. If the table of an incisor tooth be observed, a cavity
will be seen in the centre of it: this cavity, called the infundibulum,
is of variable depth. In the temporary or milk teeth, it extends only
half way down the crown; in the permanent or horse incisors, it is from
three quarters of an inch to an inch and a quarter deep, being of
greater depth in the teeth of the upper than in those of the lower jaw,
and not of the same magnitude in all horses. The infundibula are
indentations in the body of the ivory, and they possess two coverings,
an external one of crusta petrosa, and internal investment of enamel:
the enamel does not materially vary in thickness upon this part, but the
crusta petrosa of the infundibula is, in different subjects, of very
unequal extent. In some animals it becomes very thick, and in such, the
infundibula seem soon to be obliterated, owing to the crusta petrosa
filling up the cavity. A good specimen of the early obliteration of the
cavity, by the thickness of its investing membrane, is shown by the
teeth preserved with the jaw of the horse Leander, which is now in the
possession of Mr. W. Field. That animal, though but four years old, had
lost the "marks" in the central incisors. A white line, however, shows
that the enamel dips deep into the substance of the ivory, and indicates
what would have been the form of the infundibula, had not the crusta
petrosa filled up the cavities. The infundibula have received special
attention from horsemen, and have been thought to indicate correctly the
age of the horse; their lining membrane speedily becomes blackened by
the chemical action of the food, &c., constituting what are called the
"marks" of the mouth; and as the teeth undergo wear, these marks are
ultimately lost. The period at which they may disappear, and the
appearances which they may put on, are well shown in a plate which Mr.
Fores has published, entitled "_The Age of the Horse_," and to that
plate I therefore refer the reader; but, while doing so, I must caution
him not to depend too much upon a sign, which is only true as a general
rule; for as the infundibula in all animals are not of the same depth,
or supposing them to be of one depth, the crusta petrosa is seldom of
the like thickness in any two horses, and the teeth do not wear
uniformly in different subjects, of course the indications they exhibit
cannot be absolutely relied upon. Those indications, however, ought to
be known, and should never be disregarded. Some animals of seventeen
years of age, and even in rare instances of twenty-eight years old, will
retain the marks in two or more of the incisors; nevertheless, as a
general rule, these signs are worthy of attention, and, taken in
conjunction with the illustrations presented in this work, afford all
that can be obtained to confirm the judgment.
17. _There are infundibula also in the molar teeth_, but not in all of
them. Those of the lower jaw have no infundibula, but only grooves or
fissures, into which the crusta petrosa dips, and which it fills up. The
manner of the arrangement will be seen in the table of one of these
teeth represented in figure 2, page 26. In the same place, the wood-cut,
figure 1, depicts the grinding surface of one of the superior molars.
Every tooth of the last description, possesses two infundibula, which
extend almost to the root, and are possessed of the same coverings as
the like cavities in the incisors. The crusta petrosa, however, which
lines the infundibula of the superior molars, is much thicker than the
similar membrane inverting the like parts in the incisors; consequently,
absolute cavities, answering to the marks of the nippers, are of
comparatively slight extent, soon obliterated, and not generally found
in any of these teeth after the horse is six years old; indeed all the
marks are often obliterated before the fifth year.
18. _To distinguish an upper from a lower molar tooth_ is not difficult.
The presence of the infundibula in the first would alone enable it to be
pointed out; but there are other indications that are worthy of a
passing notice. The table of the upper molars is much broader, being
compared to those of the lower jaw as nine is to five. Then the form of
the table is different in each. The table of the lower tooth is indented
on either side, the indentation extending downwards along the fang. The
table of the upper molars present two narrow prominences, and two broad
grooves on the outer side; and also exhibits one broad prominence upon
the centre of the inner side--all of these developments being likewise
contained along the fang. The slant of the tables also differs. In the
upper jaw, the outer edge is the lowest. In the lower jaw, the tables
incline in the opposite direction.
19. _The teeth of the horse are very firmly implanted in the jaws._ The
fangs are of a length which they who have only seen the teeth extracted
from the mouth of a human being would hardly imagine. The length of fang
of course gives the teeth some security; but this is not the only
provision made for that end. The teeth radiate towards a common centre,
being widely separated at their roots, but approximating at the crowns.
They also incline laterally from the perpendicular, so that the force
applied to them never acts in a direct line, or is concentrated upon one
particular point.
20. _The molars of the lower jaw are the active agents of mastication_,
for motion, during this process, only takes place in that part of the
head in which those teeth are placed. The inferior molars are the
instruments that grind--the upper molars are simply the surfaces upon
which the food is ground. The whole of the molars, however, are never
simultaneously employed. The horse can chew but on one side at a time,
for as the inferior maxillary bone is considerably narrower than the
superior, if the teeth on one side are brought into apposition, those
of the opposite side are necessarily separated. Any one who has
observed a horse feeding, will have remarked that the animal, during the
act, continually abducts and adducts, the lower jaw not causing it to
describe a circle, as does the cow, but urging it first to one side and
then to the other. It will be seen, therefore, that the whole force of
those strong muscles, which close the mouth of the horse, is employed on
a portion of the jaw at the same moment, and the power then exerted must
obviously be very great. Recognizing this fact, the reason for all that
has been pointed out--the shelving tables--the grooved or indented
sides--the double inclination, and the great length of the fangs is at
once perceived, for the action is a wrenching one; and the greater
extent of, and more uneven, surface presented by the molars of the upper
jaw, facilitates the comminution, without imposing weight upon the
muscular activity. Had the larger teeth been implanted in the inferior
maxillary bone, the labour imposed would have been much augmented, and
the end attained have been in no way expedited.
21. _Provision has been made by Nature to meet the wear to which the
horse's teeth are subjected._ Such provision, however, I am of opinion,
has been somewhat mis-stated by the majority of writers who have treated
of this matter. Many of these authors dwell greatly upon the growth of
the teeth of the horse, as though this function were in constant
activity during the life of the animal. The arguments by which their
views are supported, are primarily drawn from analogy rather than
deduced from observation. Because the teeth of certain animals,
especially those classed by naturalists as rodentia, and of which the
rat and rabbit are familiar types--because the teeth of these creatures
are known to grow, and those of the elephant are also proved to
increase--the same is assumed of the horse. The inference is certainly
of strength, but nevertheless it is far short of proof; a similarity
between the animals is not demonstrated, and a sameness cannot be shown.
Moreover, in another direction, an evidence is attempted to be made of
the consequence which ensues when one of the molar teeth loses its
vitality; the tooth that so suffers is rapidly worn down, and the
opposing tooth becomes of an unnatural length, projecting far beyond its
fellows. The increase of length in such a case has been supposed to go
on in the sound organ at the natural rate, and the growth of the
diseased tooth only has been thought to be stopped; hence the amount of
wear and of growth is attempted to be measured; but on consideration it
will be seen, that something more than the mere cessation of increase
has to be calculated. The carious tooth had ceased to be a part of the
living body, and with its vitality the organ lost its capability of
self-preservation. The condition of the part had changed, and of the
strength which vitality bestows, all persons must be conscious. To argue
from the results produced upon dead matter, and then apply the
deductions to living organs, is a course which no physiologist will
sanction. The tooth that had ceased to live, might be quickly worn down,
for other reasons than that it had ceased to grow. This is so clear, and
the inference to which allusion was formerly made so obviously
untenable, that I shall not longer detain the reader by dwelling upon
either, but proceed to state my own views of the subject. I admit the
teeth of the horse do grow, and only doubt if the process has been
properly described. When the crown of the tooth first appears in the
mouth, the fang is not completed, and the root has not even been
developed. The horse is seven years old before all the roots are
perfected, and when these are completed, I doubt if the after growth is
material. If the jaw of an old horse be examined, the alveolar cavities
will be found to be shallow, showing that the loss, consequent upon the
wear, was compensated by the tooth being projected into the mouth, and
not by any increase of substance. Again, those animals, the incisors of
which retain the marks in extreme old age, may show long teeth, but not
of that excessive length which growth would suppose; only such as the
want of wear would occasion, supposing the increase to cease when the
root was perfected. The fang, in fact, is so much tooth in reserve, and
as such answers the purpose for which growth was supposed to be
necessitated. I have by me, specimens of old teeth, but the measurement
of none of them contradicts the opinion I have advanced, there being but
a material increase of the crusta petrosa, which at the root blends with
the ivory, and cannot be clearly separated from it. The thickening of
the crusta petrosa around the root and neck, probably never ceases
during the health of the animal; but to this substance alone is
confined the imaginary growth of the horse's teeth. When the roots are
perfected, the length of the tooth is completed, and the only after
process consists in a gradual deposition of earthy matter, within the
body of the member, the bulk of which is defined. In old teeth, the
ivory becomes very dense; and he who attempts to cut through an old and
young tooth, will be made aware of a contrast. The pulp in the teeth,
after the formation of the root, gradually diminishes and ultimately
disappears, in consequence of the ossific deposition. In fact, when
growth ceases, consolidation has taken place, and rendered its
continuance no longer necessary.
22. _The cavity of the pulp_ has been always stated to exist in the
tooth of the horse. In the human tooth it occupies the extremity of the
fang, being situated within the substance of the ivory; and the double
teeth are known to have as many cavities of this kind as there are roots
to the part. The same has been assumed to be the case also with the
horse, but I cannot say that investigation enables me to corroborate
this opinion. I find no cavity, that is no empty space, constantly
present in the fangs of the horse's teeth, and no separate or defined
cavities at the roots of the molars. The bone or ivory may be deficient,
more or less, towards the centre of the fang; but this space, not yet
occupied by the osseous deposit, is filled by a membraneous substance
which is continuous with the crusta petrosa of the root, and will
ultimately be converted into bone. I cannot separate the tissues forming
the pulp, from the crusta petrosa itself; and I assume the two to be
continuous. The only difference I can observe is, that the tissue of the
pulp is the finest; but as the vessels of the one primarily pierce
through and ramify upon the other, I cannot perceive the necessity for
their separation. It will be understood, that I am speaking of these
parts in a fresh subject, and not alluding to a mascerated and dried
specimen, the appearance presented by the last being calculated to
mislead. With age, the pulp diminishes; and in a tooth extracted from a
very old animal, no positive remains of it will be found--osseous matter
filling up the space which the pulp once occupied.
23. _Nature provides the horse with two sets of teeth._ The first are
small, or of a size proportioned to the diminutive stature of the foal,
and are called milk teeth--colt's teeth--or temporary teeth--either of
which terms equally well characterizes them. The temporary teeth are
twenty-four in number--six incisors in either jaw, and three molars on
both sides of the upper and also of the lower jaw. The incisors
sometimes begin to appear in the mouth before birth, though generally
the gums are unbroken when the foal is dropped; they commence to be shed
when the colt is two years and a half old--none remaining in the jaws
after the fifth year. The molars, which are the first, second, and
third, or the anterior three, begin to be cast off at two years and a
half, being entirely removed between the third and fourth year. It used
to be asserted that the last temporary molar was shed between the fourth
and fifth year of the animal's life; but while I was connected with the
Veterinary College, I inspected several heads and found none that
corroborated the doctrine, which, for half a century, had been
inculcated at that Institution. My observations were at first violently
contested, and opportunities were sought to uphold the received and old
manner of teaching. By slow degrees, however, the truth prevailed; and
I have now the pleasure of knowing, that what I pointed out to the
professors, those gentlemen at present communicate to their pupils.
Several parties, however, have been mean enough to pretend they either
are ignorant of what I effected, or believe what I accomplished was the
work of another person; while I am sorry to add, that a professor has
even gone so far as to lay claim to a discovery, which at the time cost
me some pains to fully make out, and more to firmly establish. My
observations were, in the first instance, made alone, and the only
person who eventually assisted me was Mr. I. K. Lord, of Tavistock. That
gentleman did afford me much help, and to him I beg publicly to record
my sense of obligation. No other individual was in any manner concerned
in the investigation, and I regret that I should be necessitated to make
such an assertion. The point, however, is now fully established; and it
will be seen, that it may be of practical importance. The molars, as
truthfully as the incisors, denote the age; and, as corroborators, they
should, in every doubtful case, be inspected.
24. _To know the temporary from the permanent teeth_, or to be able to
distinguish one from the other, is very essential; but such knowledge is
not so common as may be generally imagined. Dealers have blundered, and
many a gentleman having purchased a yearling for a horse, has afterwards
been surprised to discover that the animal was losing some of its front
teeth. These kind of mistakes are mostly confined to the smaller
breeds--with ponies the incident is common enough, but even with horses
it has occurred. I may therefore be excused if I enter rather minutely
into those points of difference which distinguish the one from the
other. The reader must be content to study well this portion of the
treatise, for the age is best marked by the number of milk teeth
retained in the jaw, and easiest told during those years when these
teeth are in the course of being shed.
25. _To recognize a milk incisor tooth_ the reader must bear in mind
that it is of a temporary nature, and intended only to exist during the
few first years of the animal's life, or during its colthood; for when
all the permanent teeth are fairly up, the creature becomes a horse.
The recollection of their temporary character will cause them to be the
more easily distinguished: they look less solid and less firm; they are
smaller, more white, and if ideas of beauty and sentiment befit such a
subject, have a prettier and more innocent appearance. This is perfectly
true as a general rule, but nevertheless must not be absolutely applied;
for I have seen rare instances in which the milk teeth were, from
constitutional or other causes, so discoloured, as altogether to lose
their infantine and prepossessing character--the permanent incisors
appearing cleanly by the contrast. On account of so singular a
circumstance occasionally happening, if for no stronger reason, it is
imperative therefore, even at the risk of being tedious, to enter
minutely into this part of the subject. In colour these teeth are white,
because the coating of crusta petrosa which originally invests them is
particularly thin, and soon, for the most part, disappears, exposing the
enamel; their crowns are shorter, and the neck is well developed,
because their fangs are narrow and of less length. In the annexed
wood-cut, for the purpose of making the dissimilarity the more
conspicuous, a temporary and a permanent incisor are contrasted.
_Figure_ 1 is the horse, _Figure_ 2 the milk tooth. The fang of the milk
tooth is properly represented of considerable less diameter than the
crown; and just where it arises will be observed a little shading, which
indicates a sinking in, or nipping up, at that particular spot: the
place so narrowed is the neck, which in the temporary teeth of the colt
is not badly developed.
[Illustration: _Fig._ 1.]
[Illustration: _Fig._ 2.]
However, while the milk tooth is retained in the mouth the neck is not
particularly well shown, because the gum encircling the crown in a great
measure conceals that part. But the form of the outer surface of the
crown can hardly be mistaken. In the foal's tooth it more or less
approaches to a semi-circle, from which the permanent teeth greatly
differ, as will be seen by comparing the representations of the two as
given in the plates. The outer surface likewise is characterized by
peculiar indentations: a number of small channels run along it, taking a
course from the neck towards the margin of the table, and giving an
irregular fluted appearance to the enamel. There will be seen, on the
crown of _Figure_ 2, lines intended to convey an idea of the situation
and direction of these little grooves. The table is always oblong, but
the infundibula are generally absent after the second year, and when
they are present, the cavities are narrow, and obviously of little
depth. These numerous signs enable the milk tooth to be recognized with
facility, at a single glance, for it is seldom that one of the various
indications is absent; and after all have been impressed by observation,
the milk tooth can hardly be mistaken.
26. _The permanent incisors are indicated by their greater size_, a
circumstance which the foregoing wood-cut does not exaggerate. The gum
is much further retracted, and this gives to these organs an irregular
oblong figure, which is very different from the semi-circular shape of
the milk teeth. The incisors, however, are not of equal length in all
horses, and in some the gum may be so prominent as to give the permanent
somewhat the appearance of a temporary incisor; other indications,
however, even then, enable the different nature of each to be with
certainty distinguished. The enamel is only partially exposed in the
horse tooth; the shaded portion of _Figure_ 1, in the preceding
wood-cut, denotes the extent to which it is generally covered during the
period of dentition; and when the teeth first appear, the enamel is
wholly concealed by a covering of crusta petrosa. That covering is never
entirely removed--a small portion of it, even in the oldest mouths, is
seen near to the gums. It looks like an accumulation of tartar, and is
most conspicuous in the tooth that has been newly cut. The presence or
absence of this substance, therefore, should be noted, and moreover, the
enamel surface should be further inspected. The numerous channels that
give so peculiar a fluted appearance to the milk tooth, are not to be
seen upon that of a permanent description; but in the stead are to be
detected only one or two broad grooves, which extend the entire length
of the crown. _Figure_ 1, at page 48, indicates the shape and direction
of the grooves upon the surface of the permanent tooth, though probably
a better idea of them is to be derived from an inspection of the
coloured engravings, in which they are more prominently exhibited. By
means of such guides, the opinion is rendered positive, and the
inspection of the table is hardly needed to confirm it; but the table
affords corroborative evidence, which may be of service. The table is
broader, and the infundibulum is deeper. In this latter cavity, food
accumulates in the permanent, but rarely in the temporary incisor. The
gum may so contract as to give to the tooth some appearance of
possessing a neck, but in reality no such part is developed in the horse
incisors, as will be seen by referring to the previous cut, which may,
with profit, be compared with the coloured engravings, which depict
these members as they are exhibited in the mouth of the living animal.
27. _The temporary cannot be well distinguished from the permanent
molars, while the horse is alive._ It is true that the table of the
first is somewhat narrower than, though of equal length, with that of
the last; but the difference is not so marked as to justify an opinion
being pronounced upon the imperfect inspection which, under the most
favourable circumstances, can be made of these organs. The number of the
molars may, however, be counted, and from this the age of the animal
deduced with correctness. The operation can be at all times performed,
and the fact ascertained. When, however, removed from the jaw, the
character of the molar is easily recognized, for the difference in the
length and shape of the fang renders it then impossible to confound the
temporary with the permanent tooth.
28. _The various points which denote youth_ must now be noticed. The
remarks on this subject, however, must be confined to the mouth, to
which this treatise is devoted. In the foal, the membrane of the mouth
is of a delicate pink colour, suggestive of its high vascularity. The
gums are prominent, for as the permanent are cut posteriorly to the milk
incisors, and the germs of the horse teeth are developed even at the
time of birth, the anterior part of the jaw is pushed forward to make
room for their growth. This gives the little animal somewhat the
appearance of being parrot-mouthed, as will be observed by referring to
the coloured plate of nine months. Then the palate is low in its
position, and the bars appear almost on a level with the tables of the
upper teeth, and may even descend beyond them. The lower jaw, also, if
felt, will be found to be quite round; and the face, if observed at that
part under which the molar teeth are situated, will seem full, or the
cheek will look a little blown out. These last signs, more or less,
remain till the process of dentition is completed.
29. _The indications of age_ are no less deserving of notice. The edges
of the lower jaw become sharp, and ultimately retracted; the cheeks grow
flat; the membrane of the mouth loses its vascularity, and puts on a
dull yellowish colour; the gums look hard, and the front of the jaw
becomes narrower, while at the same time it inclines more backward,
causing the teeth to project horizontally. The inclination which age
gives to the teeth, will be easily perceived by comparing the coloured
engravings with each other; and the peculiarity of the upper and lower
incisors not being perceptible at the same time in the mouth of a
thirty-years old, entirely springs from the acuteness of the angle which
they form at that period. The bars, as years accumulate, also change
their positions, being drawn upwards. From the space between the corner
incisors and the first molars, the tongue on either side protrudes, as
if the narrowed cavity left not sufficient room for the organ to repose
in. The extent to which the tongue will project may be conjectured, by
regarding the representation of it given in the coloured plate of the
thirty-year old mouth. At that age, the protrusion is too strongly
marked to escape the observation of any one whose attention has once
been directed to the circumstance. Accompanying the inability to retain
the tongue within the jaws, is a constant flow of saliva, which
continually falls from the mouth of the old horse when the lips be held
apart. The latter fact, the author, when he made the drawings for the
present work, had ample experience of, as in some instances it
occasioned serious delay.