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.

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.