THE FORM AND ACTION OF THE FOOT.
The foot of a horse consists of a variety of living structures,
differing in form and texture, and enclosed in a horny covering called
the hoof. Although the farrier's work is applied only to the hoof it is
necessary that he should know something of the whole foot, because it
is but too easy to injure the structures within by alterations of the
horny covering without.
The simplest way to understand the foot is to study separately the
different parts, and to apply that knowledge in obtaining a general
idea of the relations of all the parts to each other. There is not then
much difficulty in appreciating the functions of each part, and the
uses and action of the whole organ.
THE HOOF.
Everyone is familiar with the general appearance of the hoof. It is not
a regular geometrical figure. Each of the four feet of the horse shows
some peculiarity in form, by which a farrier can at once identify a
fore from a hind or a left from a right.
The fore feet should be similar in size and shape. Disease may be
suspected when any marked difference exists. But a healthy hoof which
has been broken, or much rasped, does not retain its proper form and
may thus confuse a novice.
The hind feet should be proportionate in size to the fore, and then it
is not of much practical consequence whether the whole are large or
small.
The front feet are rounder and less pointed at the toe than the hind;
they are also more sloping in front. The two fore feet and the two hind
should be in pairs. The right and left feet are distinguished from each
other by the inner side being more upright or, if examined on the under
surface, by the outer border being more prominent.
Although to a casual observer the hoof appears as one continuous horny
structure, it may easily be separated into three distinct parts by
prolonged soaking in water. The division takes place so as to leave the
sole, frog, and wall separate portions. These may now be considered.
[Illustration: Fig. 1.--A Fore Foot.]
[Illustration: Fig. 2.--A Hind Foot.]
=The Wall= is that portion of the hoof seen whilst the foot rests upon
the ground. It covers the front and sides of the foot. It extends
from the coronet downwards and slightly outwards so that its lower
circumference is greater than its upper. The front portion shows its
greatest height and obliquity, diminishing in these respects as it
passes backwards. At the heels the wall is turned in upon itself, and
passes forward towards the centre of the foot until it becomes lost in
the structure of the sole. These turned-in portions of the wall are
called _the bars_, and serve two purposes; they increase the bearing
surface of the wall, and by embracing a part of the sole on each side,
they afford an increased solidity to the union of the wall with the
rest of the hoof.
If we detach the wall its inner surface is seen to consist of a number
of thin horny projections running parallel to each other from above
downwards and forwards. These are called the horny laminæ. They number
from five to six hundred and correspond to similar processes on the
sensitive foot. (Fig. 3).
[Illustration: Fig. 3.--Half of a Hoof, showing the inside.]
Round the upper circumference on the inside of the wall is a depression
or groove presenting innumerable small pits or openings. This
corresponds to a part of the sensitive foot called the coronary band,
which will be noticed again.
A section of wall enables us to see variations in its thickness. (Fig.
4). It is thickest at the toe, becoming gradually thinner towards the
heels; thus affording strength and solidity to resist wear at one part,
as well as pliancy at another to ward off concussion.
[Illustration: Fig. 4.--Transverse Section of Wall showing variation in
thickness.]
The structure of the wall is fibrous--the fibres running parallel
to each other, and with the same obliquity as that presented by the
front of the wall. Although the wall varies in thickness from before
backwards, it does not from above downwards. It maintains the same
thickness from the coronet to its lower circumference.
The layers of the wall are hardest externally, becoming softer as they
approach the inner surface--a condition due to the outer layers being
exposed to friction and evaporation. This is a simple and valuable
provision of nature which should not be interfered with. The hard outer
layer is best adapted to withstand wear, and its density protects the
deeper layers from evaporation. This maintains the whole wall at the
degree of softness and toughness which best preserves elasticity and
strength of horn.
=The Sole= is that division of the hoof which forms the floor of the
foot. It is situated within the lower border of the wall, and is
slightly arched so that on a hard level surface its central part takes
no bearing. (Fig. 5). Posteriorly the sole is divided by a triangular
space into which the frog fits, and thus its continuation to the heels
consists of two angular portions embraced between the bars and the
wall. The unmutilated sole is throughout of nearly equal thickness,
but a slight excess round the circumference gives firmer attachment to
the wall.
[Illustration: Fig. 5.--The Sole with Frog removed.]
The inner surface presents a finely pitted appearance which is most
marked at the toe and round its border. The part immediately related
to the frog shows few pits, and we shall find that the whole surface
corresponds to the sensitive parts to which it is attached.
The structure of the sole is, like the wall, fibrous; but the fibres
are smaller. They run downwards and forwards in the same direction as
those of the wall. The outer layers are the hardest and protect the
deeper from injury.
=The Frog= is the smallest division of the hoof, and is a triangular
shaped body filling up the space left between the bars. (Fig. 6). Its
broad base is rounded and prominent, and is continued laterally by a
thin layer which binds together the heels and envelopes the back of the
foot. This thin layer is continuous with a horny band extending round
the upper part of the wall at its junction with the hair, and sometimes
prolonged downwards on the surface of the wall. (Fig. 7). It appears
to be a continuation of the outer layer of the skin, analogous to the
free border of skin at the root of the human nail. (Fig 8). It serves
the useful purpose of covering and protecting the young horn of the
wall at its source of growth.
[Illustration: Fig. 6.--The Frog, detached from the Sole.]
[Illustration: Fig. 7.--The Frog and frog-band.]
[Illustration: Fig. 8.--The frog band detached from wall by a small
wedge.]
The point of the frog, much the harder part, extends forward to the
centre of the sole. Though situated between the bars the frog is only
attached to their upper border--the sides remaining free and separate.
Thus on each side is formed a deep fissure which permits the frog to
expand laterally when compressed, without such force being continued to
the sides of the foot. The frog is elastic, and when pressed upon must
expand slightly. If these spaces between frog and bars did not exist,
the foot would be injured when the frog was compressed by the weight of
the horse--either the sensitive parts within would be bruised or the
heels would be forced apart.
The centre of the frog presents a depression or "cleft" caused by
the doubling in of the horn. Few shod feet exhibit it of natural
appearance, and the term cleft, by implying a narrow deep fissure,
keeps up the false notion. The cleft should be shallow and rounded.
It serves two purposes--it increases the mobility of the frog, and by
breaking the regularity of surface affords a secure foot-hold on level
ground.
The prominence of the frog might lead a superficial observer to
consider it a thick solid mass; and I believe this mistake is the cause
of its too frequent mutilation. It is merely a layer of horn following
the outline of the structures within, which are similarly prominent and
irregular in surface. (Figs. 9 and 10). The diagrams show a section
through the point and through the cleft of the frog.
[Illustration: Fig. 9.--Section of Foot at cleft.]
[Illustration: Fig. 10.--Section at point.]
The frog is fibrous, though not to such a marked degree as the other
portions of the hoof. Its chief qualities are elasticity and toughness.
THE SECRETING STRUCTURES.
If we macerate a dead foot in water for a week or two, the hoof may be
removed entire without injuring the tissues within. In this way the
sensitive foot or "quick" is exposed to view, and presents an exact
counterpart of the inside of the hoof. The sensitive foot consists of a
layer of fibrous tissue stretched over the bones and other structures
which form the centre of the foot. It is plentifully supplied with
blood-vessels and nerves necessary to its double function as the source
of horn growth and as the tactile organ of the foot. Horn is, of
course, not sensitive, although the slightest touch on a horse's hoof
is recognised by the animal, and this feeling is due to the impression
made upon the sensitive foot. In the living horse any injury to the
"quick" causes the greatest pain, and although this sensitiveness is
a serious disadvantage in disease it is a most valuable provision in
health, enabling the horse, even through a thick layer of horn, to
recognise the quality of the surface upon which he may be standing or
moving. It is this sense of touch--this tactile function--which demands
that the sensitive foot should be so bountifully supplied with nerves.
Every farrier knows how profusely blood flows from any wound of the
"quick"--evidence that the part is well supplied with blood-vessels.
This full supply of blood is not merely for the ordinary waste and
repair which takes place in every tissue; it is to meet a special
demand--to supply the material for the production of horn. The
sensitive foot is the secreting structure of the hoof, and the source
of the constant growth and reproduction of horn. It corresponds with
great exactness to the inside of the hoof, and as we have described the
hoof in sections it may be convenient to follow that course with this
structure, and to describe the _sensitive frog_, the _sensitive sole_,
and the _sensitive laminæ_. We shall begin with the last.
=The Sensitive Laminæ.= Corresponding to the horny leaves on the
inside of the wall, the sensitive foot presents an arrangement of
minute parallel folds which are called the sensitive laminæ. (Fig.
11). Between these the horny laminæ rest, so that there is a kind
of interleaved attachment which affords the very firmest connection
between the wall and the sensitive foot. If the laminæ be laid bare in
a living horse by removal of the wall, it is found that they have the
power to secrete a kind of horn, not a hard fibrous horn like that of
the wall, but a softer variety. This function is not very active in
health or we should find that the lower edge of the wall was thicker
than the upper; but it exists, and is very evident in some cases of
disease.
[Illustration: Fig. 11.--Foot with hoof removed showing at the upper
part the Coronary band, and below the Sensitive Laminæ.]
In laminitis the wall at the toe is often pushed forward out of
position by a horny mass formed by the laminæ, and so we have the
deformity of an excessive length of toe. In some cases of long
continued sandcrack the irritation of the laminæ causes excessive
secretion, and a horn tumour results. The sensitive laminæ, then,
fulfil two functions; they offer a firm connecting medium for the
wall, and they secrete horn. By the cruel experiment of removing the
horny sole and frog of a living horse and then forcing him to stand on
the maimed foot on a level surface, it has been shown that the laminæ
are capable of alone supporting the weight of the animal. It has been
argued from this that the laminæ always support the weight, and that
the horse's foot may be described as being slung by the connecting
laminæ. This is not true. The frog and sole help to support weight, and
the hoof acts as one continuous whole, each part taking its direct and
proportionate share of the weight placed upon the foot. The sensitive
laminæ are not elastic, they are unyielding, and, therefore, allow no
downward yielding which would impose excessive pressure on the sole.
=The Coronary Band.= (See Fig. 11). The sensitive laminæ do not cover
the whole of the upright portions of the sensitive foot. There is
between their upper extremity and the line which separates the skin
from the sensitive foot, a convex band which runs round the upper
border of the foot, and is turned downwards and inwards at the heels.
This is called the coronary band, and corresponds to the groove which
we noticed on the inner side of the upper border of the wall. On its
surface are innumerable small projections or papillæ which, in the
living animal, fit into the openings on the groove of the wall. From
each of these papillæ grows a horn fibre, and from the surface between
them is formed a softer horny matter--the two products forming together
the substance of the wall. The coronary band is, then, an important
structure, being the source from whence the wall is produced. Upon the
healthy condition of this band depends the soundness of the wall, and
any interference with its integrity must lead to defects or deformities
in the wall.
=The Sensitive Sole= (Fig. 12) is that portion of the "quick" to which
the sole is attached. Its surface is covered with papillæ, like those
on the coronary band but much smaller, giving an appearance somewhat
like the pile of velvet. From these the horn fibres of the sole are
formed, and a firm means of connection is afforded for the floor of the
hoof.
[Illustration: Fig. 12.--Under Surface of Foot showing Sensitive Frog
and Sole.]
=The Sensitive Frog= in structure resembles the sensitive sole, but its
papillæ are very much smaller, and the surface therefore is smoother.
The irregular prominent surface of the frog, with its cleft and the
space at each side of it, is exactly reproduced on the sensitive frog,
as might be expected, for the one is moulded on the other. There is
one difference between the sensitive frog and the other portions of
the sensitive foot which I may here mention. It is not attached to
the bones of the foot except by its point, but is situated behind the
bone, and has as a basis a mass of soft tissue which forms an important
cushion or pad, to be referred to later.
GROWTH OF HOOF.
Like every other part of an animal body, the hoof is constantly
changing. Wear and tear cause waste of the horn, which is replenished
by growth. When wear exceeds growth the foot becomes denuded of horn,
and lameness results. When growth exceeds wear the hoof becomes
disproportionately long, and some parts suffer by the overgrowth of
others--for instance, whenever the heels are unduly high the frog
becomes small and weak. In a state of nature the horse's foot keeps
itself of proportionate form. On hard ground the hoof is worn away
as quickly as it grows. On soft ground it may, for a time, become
overgrown, but this is rectified by the soft horn becoming fractured
and broken off. In enclosed cultivated grounds the movements of the
horse, even on grass land, are too limited to ensure a proportionate
form of hoof. When horses are turned out without shoes the feet should
not be left to take care of themselves, unless the pasture is of large
area and the time at grass extends for several months.
In a hoof which is overgrown--and all shod feet become overgrown in
four or five weeks--there is apparently a greater excess of horn at
the toe than elsewhere. This is due to the oblique direction of the
wall at the toe, and to the fact that the horn fibres of the hoof do
not grow down vertically, but obliquely forward. When the natural wear
of the hoof is prevented, the effect of growth is to lengthen the toe
and carry forward the bearing surface of the foot. Now this bearing
surface has a proper relative position to the limb above it. Therefore
a disproportionate foot must injuriously affect both the action and
position of the whole limb.
The rate at which the wall grows varies greatly in different horses,
and is affected by external conditions. The good average wall grows
nearly one inch in three months, and the whole hoof is replaced in from
ten to fifteen months. The hoof grows more rapidly when a horse is
actively exercised than when he is confined in a box. Febrile diseases
check growth, and irregularities of the system cause the formation
of ridges in the horn, each one commencing at the coronet and being
carried down with the growing horn until the hoof is marked by a series
of rings running transversely and parallel to each other. These rings
are of themselves no detriment to a horse, but they mark irregularities
of growth which may have been due to illness or lameness.
The growth of horn on a shod foot is affected by the bearing it takes.
When a part of the wall takes no bearing on the shoe it grows quicker
than that which does. We see this when a shoe is so fitted that the
heels take no direct pressure on the shoe, also when a portion of wall
is broken at the quarters, and again when, for any reason, a portion of
the edge of the wall has been rasped away to prevent bearing upon some
special spot. In all these cases, after the shoe has been worn a month,
it will be found that the horn has grown more rapidly at the part where
bearing did not take place, and, when the shoe is removed, the horn
which was relieved of pressure is found to have been in apposition with
the shoe.
The growth of horn cannot be accelerated by any application to its
surface. If we desire to hasten growth of the wall we can do so by
stimulating the part from which it is produced, _i.e._, the coronary
band. A mild blister to the coronet causes considerable increase in
the rapidity of growth, but no ointments applied to the surface of the
wall affect its production in the least, though they may modify its
condition and prevent dryness and brittleness.
The sole grows in much the same way as the wall, but it wears quite
differently. It never becomes overgrown to the extent seen in some
instances of the wall. The hard firm structure of the wall, if not worn
down by friction on roads or dry hard surfaces, may grow to a great
length. As a rule, when much overgrown, it splits in the direction of
its fibres and becomes detached in broken fragments. The sole, when
overgrown, has a tendency to become detached in flakes, and never very
much exceeds its normal thickness without becoming dry and brittle,
when the movements of the horse cause it to break up and fall off.
The frog when it takes a bearing on the ground wears off in shreds. A
frog which takes no bearing dries up, and sometimes a large superficial
layer is cast off. Though the softest of the horny divisions of the
hoof, the frog is able to withstand wear and tear as well as any of
the others. Being elastic and resting upon soft tissues, it is able to
yield to any undue pressure and leave the firmer horn of the wall and
bars to sustain the greater strain. The growth of the frog depends a
great deal upon the form of the back parts of the wall. If the heels
become overgrown, the frog is removed from bearing and consequently
wastes. High heels have always between them a small frog. On the other
hand low weak heels have always a large frog, and the explanation
is that the increased bearing thrown on the frog causes greater
development.
=Properties of Horn.= Horn is light, hard, tough, and elastic,
properties most essential to its usefulness as a protector of the foot.
Horn is porous, and absorbs moisture. Too much moisture in horn weakens
it, and therefore it must be remembered that the natural protection
against this is the hard outer layer of the hoof. When this layer is
rasped off moisture is more easily absorbed until the dry, hard surface
is restored by exposure and friction.
Horn is a bad conductor of heat, and thus an equally good protective
against the effects of snow in some countries, and of hot dry sands
in others. With a sound thick hoof the application of a red-hot shoe
produces very little effect on the internal structures, provided,
of course, it remain in contact only a reasonable time. With a foot
protected by a thin layer of horn, fitting a red-hot shoe must be done
quickly or it may damage the soft tissues.
DISSECTION OF THE FOOT.
So far we have only described the outer covering of the foot and the
structure from which it grows and by which it is connected to the parts
within. A little deeper examination is necessary to understand the
mechanism of the whole organ.
[Illustration: Fig. 11.--Section of Foot.]
If we divide into two lateral halves a foot cut off at the fetlock
joint, we have a section which should show the whole of the deeper
structures. In the centre we see the three lower bones of the limb--the
pastern, coronet, and pedal. (Fig. 11). On the front surface of these
bones we notice a tendon or sinew which comes from above the knee and
is fixed to the upper part of the pedal bone. At the back of the bones
two very large tendons run down and are fixed on the last two bones.
These tendons are the structures through which the movements of the
foot are made. They have in themselves no power of contraction but they
are connected above the knee, and in the hind leg above the hock, to
powerful muscles which possess the power of contraction. When these
muscles contract the tendons are drawn up towards knee or hock, and so
move the foot backwards or forwards.
To permit movement of one bone upon another the ends of the bones are
suitably shaped, and covered with a layer of gristle or cartilage. To
limit the movement and to hold the bones together the ends of each bone
are surrounded by ligaments, and thus we have joints formed.
[Illustration: Fig. 12.--Side view of Pedal Bone.]
The pastern bone is altogether above the level of the foot, the coronet
bone is partially within the hoof, and the joint between it and the
pedal bone is quite within. The pedal, often called the coffin bone,
(Fig. 12) is entirely within the hoof and fills the front part of
the horny envelope completely. It is a peculiarly shaped bone, being
continued backwards by two projections which follow the course of the
wall to a little beyond the quarters of the foot. (Fig. 13). From
this point to the extremity of the heels the wall is not supported by
bone but by strong plates of gristle, which are called the lateral
cartilages.
[Illustration: Fig. 13.--Under surface of Pedal Bone.]
[Illustration: Fig. 14.--The Lateral Cartilage.]
=The Lateral Cartilages= are situated one on either side of the foot
partly within and partly without the hoof. They form the basis upon
which the back part of the wall is moulded, and being elastic permit
a certain amount of movement in the posterior parts of the foot.
(Fig. 14). If the coffin bone filled the whole hoof, the foot would
be too rigid. With bone at the front portion we have a firm surface
for attachment, and with cartilage at the back we have an equally
firm attachment, but one that will yield to blows or pressure and
thus better protect the internal parts. These cartilages extend above
the level of the hoof, and may be easily felt in the living horse at
the back part of the coronet. (Fig. 15). Between them, and behind
the body of the coffin bone is a large space which is filled up by a
mass of soft tissue to which various names have been given, such as
plantar-cushion, frog-pad, etc.
[Illustration: Fig. 15.--Coffin Bone and Lateral Cartilages seen from
above.]
=The Frog-Pad= is the name under which we shall notice it. It forms
the bulbs of the heels and is the soft basis upon which is spread the
sensitive frog. It extends from side to side of the foot between the
two lateral cartilages, and fills up all the space within the hoof
behind the body of the coffin bone. The structure of this pad may be
described roughly as consisting of a network of fibrous bands, having
the interstices filled up with elastic tissue. (Fig. 16). Down the
centre of the pad runs a vertical partition of inelastic fibres; from
this strong fibrous bands pass to each cartilage, and so the whole of
the back part of the foot is tied together. The heels and quarters may
be pressed together to some extent, but they are prevented from being
forced asunder by the fibrous connections of the frog-pad. During
progression the downward movement of the coronet bone is provided
for by this soft pad, and so is an upward movement of the frog when
excessive bearing is placed upon it.
[Illustration: Fig. 16.--Section of foot showing the frog-pad and at
each side the cut edge of the lateral cartilage.]
The frog-pad serves other purposes besides those we have just referred
to. It is essentially a cushion or pad to prevent jar or concussion,
but it also plays an important part in the action of the foot, as we
shall see later on.
=The Coronary Cushion or Pad= is another mass of tissue of a similar
nature to the frog-pad. It is situated just above the upper border
of the hoof, and gives to the coronet its prominence and elasticity.
At this part of the foot there is an enormous number of small
blood-vessels and nerves, and the coronary pad forms not only a base
for these to rest on but a necessary protection for them. If instead of
this elastic bed they were placed merely between the skin and the hard
bones and tendons of the part, they would be injured by every slight
bruise. Even with this cushion, we have in practice very many serious
conditions following bruise of the coronet.
=Blood-vessels of the Foot.= It is not necessary to describe the
course of these vessels. All we need remember is that every part of
the tissues within the hoof is very plentifully supplied with blood,
and that the flow of blood is most rapid when the foot is in action.
In a dead foot from which the blood has escaped a certain amount of
movement of the bones within the hoof is easily effected. In the
living foot when every vessel is filled with blood no such movement
takes place. The blood in the vessels forms a sort of waterbed which
assists in preventing concussion and which distributes evenly over the
whole organ the pressure applied when weight is thrown on the foot.
In studying the dead foot with a view to understand its mechanism we
must not lose sight of the difference which results from having in one
case the blood-vessels empty, and in the other--the living animal--the
blood-vessels full.
THE FOOT AS A WHOLE.
The details I have given of the structure and uses of each separate
part of the foot will, I hope, be sufficient to enable us to understand
the form and action of the organ as a whole.
No one part of the foot is of greater importance than another, each is
dependent for its highest development and soundest condition upon the
integrity of neighbouring parts.
A weak wall allows of the flattening and spreading of the sole, whilst
a weak sole permits contraction of the wall. Overgrown heels cause
wasting of the frog, but low weak heels are usually accompanied by
excessive development of frog.
The special function of the foot is to sustain the weight of the
animal whilst standing or moving. The horse standing squarely on all
four feet rests his weight chiefly on the lower circumference of the
wall. On level ground the sole, on account of its arched form, takes
no direct bearing, but if sole and wall be sound a proportion of all
pressure applied to the wall is transmitted to the sole. So also must
all weight imposed on the arch of the sole be transmitted, through its
abutments or union with the wall, to the wall. If the sole be so thin
that it yields to pressure then its proper action is destroyed, and
instead of acting like an arch and supporting weight imposed on it, it
yields and injury results. The arched form of the sole indicates that
it was not intended to take a direct bearing on hard ground. On a soft
surface the edge of the wall sinks and the whole under surface of the
foot takes a direct bearing. Pressure of the sole on the soft surface
does no harm because it is diffused evenly over the whole of the sole.
We take advantage of this when the wall is diseased or injured, and
we desire to throw on the sole a larger share of weight. We turn such
animals out into a soft field or stable them on sand or saw-dust. Any
system of shoeing founded upon the true form and action of the foot
must recognise the arch, and not endeavour to force the sole to take a
bearing for which it is not adapted. There is only one part of the sole
which should act as a bearing surface, viz., that outer border which
is firmly joined to the wall. This part--the abutment of the arch--is
destined by nature to take a bearing and through it the whole of the
sole supports its share of weight.
The frog takes a bearing on the ground but it has a weight sustaining
function quite secondary to the harder and firmer parts of the hoof.
It is formed of a softer horn, and it has above it only soft tissues
which permit yielding. The frog then, when weight is placed upon it
by the standing horse, recedes from pressure, and leaves the heels
(wall and bars) to sustain the primary weight. Wall, sole, and frog,
each take their share in supporting weight, but this function is
distributed over them in different degrees, and it is fulfilled by each
in a varying manner. During progression the foot is repeatedly raised
from and replaced on the ground. It has not only to support weight but
to sustain the effects of contact with the ground at each step, and
the effects of being the point of resistance when the body is carried
forward and the foot is again raised from the ground.
What part of the foot comes first to the ground? Many different answers
have been given to this question. It has been said by some that the toe
first touches the ground, by others that the foot is laid flat down,
and by a few that the heel is the first part to come in contact with
the ground. Fortunately it is not now necessary to argue this question
on a purely theoretical basis. Instantaneous photography has shown that
on level ground, at all paces, the horse touches the ground first with
the heel. This fact gives significance to the structural differences
we find between the front and back portions of the foot. At the back
part of the foot we have the wall thinner than elsewhere, we have the
moveable and elastic frog, the lateral cartilages, and the frog-pad. We
have in fact a whole series of soft and elastic structures so arranged
as to provide a mechanism best adapted to meet shock and to avoid
concussion. Whilst drawing heavy loads, or ascending or descending
hills, the horse may vary his action to suit the circumstances,
and then we have the exception which proves the rule--then we have
sometimes the heel, sometimes the toe brought first to the ground.
At the time when the foot first touches the ground, the leg is extended
forward and the pastern is in the same oblique position to the shank
as when a horse is standing. This obliquity of the pastern is another
safeguard against concussion, and it renders impossible the first
contact with the ground at any point other than at the heel. As the
leg becomes straightened, the weight of the body is imposed upon the
foot, but the greatest strain arrives just before the toe leaves the
ground, for then there is not only weight to sustain, but the friction
to be borne which results from the toe being the fulcrum upon which
falls the whole effect of the muscular effort necessary to raise and
carry forward the body of the animal. The front part of the foot is
structurally well adapted for its use. It presents the thickest and
strongest part of the horny covering, and, as an inside basis, it
has the unyielding coffin bone. Thus we have at the toe strength and
rigidity--at the heels strength and elasticity.
Another important point in the action of the foot is implied by the
question--does it expand when weight is thrown on it? The principles of
horse-shoeing require that this question should be answered. There are
those who say that the foot does not alternately expand and retract as
weight is placed upon or removed from it. There are others who assert
that the expansion of the foot is an important natural function that
must be provided for in any system of shoeing. It is agreed by most
observers that at the upper border of the hoof, more particularly
at the heels, expansion does occur. It is when we come to the lower
border of the foot that the statements are most conflicting. Ordinary
measurements taken at this part with calipers or by tracings on paper
of the foot when raised from the ground and when resting upon it, show
no variations in the width of the foot. These methods of measurement
are not sufficiently delicate to be trustworthy. Experimentalists
in Germany and in this country have recently used an apparatus by
which the slightest variations are detected by electrical contact,
and the results are very interesting. These experiments show that
in a well-formed, healthy foot the hoof throughout its posterior
two-thirds does expand to pressure, and perhaps that the arch of the
sole is slightly flattened. This expansion is, however, comparatively
slight--about equal to the thickness of a sheet of writing paper--and
may practically be disregarded in considering the best methods of
shoeing sound feet.
One result of these experiments is to show what an important part the
frog plays in the foot, and also how the action of one part depends
upon the conditions of others. When the frog rests firmly on the ground
and weight is placed upon the foot expansion occurs, especially at the
upper or coronary border of the hoof. When the frog does not touch the
ground and weight is imposed upon the foot, contraction occurs. The
explanation of this difference seems to be as follows. When weight is
placed upon a foot, the coronet bone is depressed upon the soft mass of
the frog-pad. With a sound frog taking a bearing upon the ground, the
frog-pad cannot descend, and the compression to which it is therefore
submitted causes it to bulge laterally and so expand the back of the
foot. When the frog does not reach the ground and weight is placed
upon the frog-pad, there is nothing to prevent it yielding downwards,
and in so doing the fibrous bands connecting together the two lateral
cartilages of the foot are depressed and the cartilages drawn
together--hence the contraction of the foot. No better illustration
could be given of the unity of all parts of the foot, and how one or
many parts may suffer if the structure or function of one be defective.
There is one more movement of the hoof which is possible and which
must be referred to, as it has been made the basis of a grave error in
shoeing. I have said the back part of the foot is elastic and yielding.
If you examine a shoe, so applied to a foot that an inch or more of its
extremity has no contact with the hoof, you will find that when weight
is rested on that foot the horn yields downwards and comes in contact
with the shoe. This simply demonstrates that when there is nothing to
support it the horn at the heels may be forced downwards. It is not a
normal action, and in an unshod foot cannot occur on a level surface.
The effect of this downward movement of the heels is to put a strain on
the horn of the quarters. A shoe so fitted as to permit this evil is in
common use, and no fault is more serious than thus forcing an unnatural
action upon the hoof at every step. With unintentional irony this piece
of bad work has been called "easing the heels."
In concluding this chapter, I would just repeat that the natural
bearing surface of the horse's foot is the lower edge of the wall and
that portion of the sole immediately in union with it; that the arch of
the sole should not be in contact with the ground; that the frog ought
to have a bearing on the ground, but ought not to be so prominent as
to unduly share in sustaining weight. This natural bearing surface is
what we want to utilize in shoeing. We put on a shoe _merely to prevent
excessive wear of the hoof_. If we can protect the wall the frog can
take care of itself, and we have only so to apply our shoe that we do
not damage any useful structure or interfere with any natural function.
* * * * *
NOTE.--No person is expected to learn the structure of a foot
entirely from this description. He must obtain two feet cut off
at the fetlock joint. One he should soak in water till the hoof
can be pulled off. The sensitive foot is then visible and the
inside of the hoof; with these before him, the drawings and
descriptions in this chapter will be of great assistance. The
second foot he should have sawn vertically down the middle through
the point of the toe, and again across the quarters, so as to show
the inside of the foot from two different points of view; this
will afford a view of the relation of parts.