AESTHESIOLOGY.
The study of organs of special sense, the ear, eye and organs of
special sensation, skin, hair, foot, etc.
THE EAR.
The apparatus of hearing is composed of three parts the outer, middle,
and inner ear; the two first being accessory for the collection and
transmission of sounds, and the latter the essential organ which
receives the impressions thus conveyed.
The inner part, or drum, of the ear, is situated in the hardest bone
of the body, called the petrosal. The nerve which passes into the drum
of the ear and gives the sense of hearing, is called the auditory
nerve. From the drum a small opening passes out into the outer part of
the ear; this is the portion which is seen on top of the head. It is
made up of a membrane known as the cartilage, which gives the ear its
stiffness. This cartilage is covered by a fine, delicate skin, covered
on the outside by fine, short hair. Situated on the inner side of the
outer ear are numerous long hairs projecting outward, the use of which
is to keep foreign bodies from dropping into the ear. The ear is moved
backward and forward by small muscles which are attached around it.
THE EYE.
The apparatus of vision comprises the essential organ, the globe of
the eye or eyeball, and its accessory parts or appendages. The eyeball
is situated in the orbital fossa, mentioned in chapter on the bones of
the head. It is chiefly made up of several coats around the outside,
and in the center by the humours of the eye. On the inner side of these
coats is a thin membrane called the retina, which contains the branches
of the optic nerve. This receives the reflections of objects as they
pass through the humours of the eye and from which the sensation
passes along the optic nerve to the brain. The oblong opening seen
in the middle of the eye is known as the pupil. If a horse be led
from a dark stall into the light and the pupils of the eyes watched
closely, it will be noticed that they get smaller, but on returning
it to the stall the pupils will be noticed to dilate or get larger;
thus it is seen that the pupils do not always remain the same size.
The chief use of the pupil is to gauge the sight. At the back part of
the eye are several muscles attached from around the eye to the bones
in the fossa. These muscles move the eye and assist in holding it to
its place. Around the front part are two movable curtains, one above
and the other below, called eyelids, the use of which is to open and
close the eye, and also to protect it from injuries. Around the free
border of the eyelids are what is known as the eyelashes, which keep
foreign substances from falling into the eye. Situated in the inner
angle is what is known as the haw of the eye; this membrane also helps
to protect it. In the corner of this angle is a small duct or opening,
through which a fluid called the tears passes down into the nasal
tubes, from whence it is carried down through the bones of the head and
emptied into the under part of the nostril or nose. A small gland is
situated on the upper part of the eye. This gland secretes the tears
which lubricates the eyes. The color of the eye is generally brown, but
in some cases it is white. It is then called a moon eye.
THE SKIN.
The skin is a membrane or external casing of the body. The skin itself
consists of two layers covered with hair, fine or coarse, long or
short, according to its position or purpose which nature intended it to
serve. The outer layer is called the epidermis, the inner the dermis.
THE EPIDERMIS.--The epidermis is the outer layer. It is not supplied
with nerves and blood vessels, its purpose being to protect the inner
layer. This layer undergoes a continual process of being made up and
passing away in dandruff.
THE DERMIS.--The dermis or true skin lies under the epidermis. It is
well supplied with nerves and blood vessels, part of the nerves being
the nerves of touch. This fact accounts for its becoming so very
sensitive and painful when through injury of any kind the outer layer
is scraped off. It is attached to the body by a layer of white tissue
known as the areolor tissue, this being that which is cut through
when the animal is being skinned. The thickness of the skin varies in
different parts of the body, being thinnest in the under parts. The
sweat glands are situated in the dermis.
THE HAIR.
There are three kinds of hair on the horse--the common, the finest of
the three, covers most of the body; that of the mane and tail, coarse
and long; and that growing on the muzzle or nose and lips, long and
usually black, known as tactile or cat hairs.
On the inside of the front legs, just above the knee, and on the inside
of the hind legs, above the hock, are rough, horny spots. These are
called chestnuts.
EXPLANATION OF PLATE VII
ANATOMY OF THE FOOT.--This illustration represents the foot of a horse
sawed from above the fetlock down through the center of the foot. It
shows the structure of the foot, the name of each part being given
according to number.
1. Lower end of large metacarpal, or cannon bone.
2. Bursa, which secretes the joint oil that lubricates the place where
the tendon, or cord, on the front of the leg passes down over the front
of the fetlock joint. This is important as it sometimes gets injured
and becomes enlarged. It is then called a bursal enlargement, and is of
the same nature as a wind gall.
3. Fetlock joint.
4. Os suffraginis, or large pastern bone.
5. Pastern joint. This joint is important; when diseased it is the seat
of a high ringbone.
6. Os coronae or small pastern bone.
7. Coffin joint. This joint is important, for when it is diseased it is
known as a low ringbone.
8. Wall of the hoof.
9. Os pedis, or coffin bone.
10. Sensitive wall, or quick of the foot.
11. Sensitive sole, or quick of the foot.
12. Frog of the foot, or horney frog.
13. Plantar cushion, or fatty frog.
14. Navicular bone. This is also important, for when diseased it is the
seat of navicular, or coffin joint lameness.
15. Back tendons below the fetlock.
16. Sesamoid, or fetlock bones.
17. Skin.
18. Back tendons above the fetlock.
[Illustration: PLATE VII.
CROSS SECTION OF THE FOOT OF THE HORSE.]
FOOT.--In equine anatomy the word “foot” implies the hoof, together
with the bones and soft structures contained therein. Many of these
objects have already been described, so that our description here will
be confined almost to the hoof and the structures with which it comes
into immediate contact internally.
WALL OF THE FOOT.--The wall is that part of the hoof seen when the
foot is resting flat on the ground. It is divided into the toe, the
quarters, the heels and the bars. The toe forms the front, and is the
thickest and strongest part of the wall. The quarters are situated at
the side. The walls are not nearly so thick here as at the toe, but are
almost straight up and down. The heels are situated at the back part
of the foot. From the heel is a process of hoof, which looks like a
bar, passing forward between the frog and the sole of the foot; this
can be seen plainly by raising up the foot. There is one of these at
each side of the frog. They act as braces to the heel and the quarters
of the wall; these are called the bars. Covering the outside of the
wall is a fine membrane called the periople, which gives the hoof its
polished appearance. This can be seen best when the hoof is well washed
off, as it is after traveling through wet grass. This membrane keeps
the moisture in the hoof and protects it from water. This is a point
of importance in shoeing horses, as it is very injurious to file the
wall too much. Around the top part of the wall, where it unites with
the skin, is a groove which contains a white band, called the coronary
substance, or band. This nourishes the wall of the hoof, or, in other
words, it is from this that the wall of the hoof grows. The under part
of the wall, or that which rests on the ground in the unshod animal,
is called the spread of the foot. On the inside of the wall, attaching
it to the bone of the foot called the os pedis, is the part called the
quick, or sensitive laminae. It is important to note this when driving
nails in shoeing. The nail should not be driven into this membrane, nor
should it be even pressed upon, for it is very sensitive. When a nail
has been driven so as to injure the membrane it is a common expression
to say, “You have pricked that horse’s foot.”
SOLE OF THE FOOT.--The sole is a thick plate of horn which helps to
form the under part of the hoof. It is situated between the inner
border of the under part of the wall already mentioned and the front of
the frog. The under part of the sole is concave, or hollowed out. The
upper part of the sole is attached to the under surface of the os pedis
bone, or bone of the foot, by a membrane called the quick, or sensitive
sole--this membrane is merely a continuation of the sensitive laminae.
The outer part of the sole is attached to the inner part of the wall.
When pared down a white ring is seen where the sole and the wall unite.
At the back part of the sole there is a notch the shape of the letter
V; in this notch the frog is situated. It is important to remember when
shoeing never to let the shoe rest on any part of the sole; neither
is it well to pare off too much of the barky-looking substance of the
sole, as this helps to keep the moisture in the foot. When this is
taken off it allows the moisture to escape and the hoof becomes dry and
contracted.
FROG OF THE FOOT.--The frog is the important spongy horn found in
the V-shaped notch in the back of the sole. It is wide at the back
and helps to form the heel of the foot; the pointed part in front is
called the apex of the frog. The under part of the frog is triangular
in shape and has a hollow in it called the cleft of the frog. There is
a hollow at each side of the frog, between it and the bars, called the
commissures of the frog. On the upper part is a membrane, known as the
sensitive frog, which attaches it to the under part of the os pedis, or
foot bone. This membrane is simply a continuation of the sensitive sole
spoken of in connection with the sole. The back part of the frog is the
widest part and spreads out to form the heel.
The study of the foot of the horse is of the greatest practical
importance, owing to the many diseases and injuries to which it is
liable. It resolves itself here into the consideration of the hoof or
horny case, and the parts contained within it.