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Small Farmer's Journal
PO Box 1627
Sisters, Oregon 97759
800-876-2893
541-549-2064
agrarian@smallfarmersjournal.com
Mon - Thu, 8am - 4pm PDT

The Equine Eye

The Equine Eye

by Heather Smith Thomas of Salmon, ID

Horses don’t see like we do. Equine vision is better than ours in that the horse has a much broader field of vision, and better night vision. But in other ways the horse’s vision is poorer; he can’t focus as easily and clearly on close objects.

The horse’s head is large, with eyes set wide apart at the sides of his head; he seldom sees an object with both eyes at the same time and generally sees a different picture with each eye. In the wild, this double vision was a big advantage, making it difficult for a predator to sneak up on him. He can see movement on both sides and to the rear, and be prepared to flee from danger. He can focus both eyes to the front to watch something, but it takes more effort. Only when making a concentrated effort to look straight ahead does the horse have depth perception as we know it.

Wide angle vision is easy with the horse’s large and prominent eye; when grazing with his head down he can see to the front, both sides, and behind (except for the small areas blocked from view by his legs). Using both eyes, he has a visual field of nearly 340 degrees, almost a complete circle. The average horse sees a 215 degree range with each eye. Humans, by contrast, have a field of vision encompassing only 180 degrees (half circle) using both eyes.

The horse sees a fairly wide view with his head lifted, but there is a blank spot behind his body, and the lower part of his visual field is a blind zone. He can’t see anything closer than about 3 feet in front of his forefeet. With his head up, he can’t see where his feet are landing, but can see the ground where they are about to land.

Because the eyes are set at the sides of his head, they would seem likely to suffer injury from bumping. Yet the horse’s eye is seldom injured, partly because he has a long flexible neck and is very conscious of his eyes, swinging his head out of the way of most obstacles and dangers. A cow, by contrast, with her thick, tough eyelids, automatically closes her eye to protect it from bumping. The horse prefers to keep his eyes open, so he slings his head up or to the side to avoid a bump.

Also the eye is encircled by bone which forms a strong, rigid arch (the supraorbital process) above and in front of the eyeball, to protect it. Behind and above the eyeball is a fat-filled cavity which acts as a cushion. This is the supraorbital fossa, the dent between the eye and ear, which moves in and out as he chews. This dent contains a pad of fat continuous with the fat behind the eyeball. If the eye is bumped, the pressure on the eyeball pushes it back into this cavity, sheltering the eye in the bony orbit. A hard blow will cause the eye to retreat backward instead of bursting, and the fat bulges out into the supraorbital fossa temporarily as it is displaced by the retreating eyeball.

This cavity also serves another useful purpose. If a fly or piece of dirt touches the eye surface, the eye can draw back a little into the fatty cavity and a third eyelid membrane quickly wipes over the front of the eyeball.

The Equine Eye

This third eyelid, the membrane nictans, is a firm sheet of elastic cartilage which can move easily over the round eyeball. It also keeps the eye moist by spreading tears over its surface. The third eyelid is attached to the pad of fat behind the eyeball. Any action which causes the muscles of the eye to pull the eye back into this fatty cavity also causes the third eyelid to move upward and flick across the eye. This happens when the horse feels an insect or foreign object touch his eye, or when he yawns. It also happens if he has tetanus, because this disease causes the retractor muscle at the back of the eyeball to contract in spasms, shooting the third eyelid across the eye.

The horse’s eye is an irregular ball, covered at the front by a thin mucous membrane called the conjunctiva. This pink membrane forms a continuous covering over the inside surface of the eyelids, the third eyelid and the cornea’s edges. It covers the cornea as a single layer of transparent cells. Light enters at the cornea, which is a thick, tough, clear tissue over the front of the eye. Surrounding the cornea is the sclera, or white of the eye. Not much of this is visible, except when the horse rolls his eyes. The sclera is where ocular muscles that move the eyeball are attached.

A salty solution, called the aqueous humor, fills the front chamber of the eye, between the cornea and the lens behind it. This salty fluid flows freely through the pupil — the hole in the muscular diaphragm or iris (colored part). The iris surrounds the pupil and limits the amount of light that reaches the retina at the back. If the light is too bright, the iris constricts and makes the pupil smaller. The foal or young horse has a round pupil, but it becomes more elongated horizontally as he grows older. By the time the horse is 5 or 6 the pupil is elliptical.

The lens directs light rays onto the retina at the back of the eye. The retina is the back lining, containing the light-receiving cells. The image is transmitted to the brain by the optic nerve; when the brain receives the nerve impulses generated by the light receiving cells, it translates them into visual images.

The horse can see quite well in the dark because of a special triangular shaped layer (tapetum lucidum) which lies behind the retina at the upper part of the back of the eye. This mirror-like structure is an iridescent pigment layer which reflects light back onto the retina. It concentrates the light and intensifies the image, giving the horse better vision when light is dim. The glistening tapetum lucidum is what makes an animal’s eyes shine in the dark when a light is directed at them. Light beamed at a horse’s eye in the dark will give back a green reflection.

Because his eyes are so sensitive to light, the horse must keep out extra glare during bright sunlight. The horizontally shaped pupil keeps out more daytime glare than the round pupil of a human eye, but is not as effective as the vertical slit in a cat’s eye. But the horse compensates by having built-in sunglasses. These consist of several tiny, round appendages hanging from the upper edge of the pupil, like an awning or sunshade. These black or chocolate colored pea-like appendages are the corpora nigra and appear on the edge of the iris when the horse is 5 or 6 years old, increasing in size for many years and shading the eye. Sometimes a few smaller ones project upward from the lower edge of the pupil, but most hang from the top. They partially block the pupil opening when it is constricted, absorbing excessive light.

Most animals and humans focus their eyes by changing the shape of the lens, using the muscles that surround it. These muscles can make the lens rounder for close-up vision, or flatter for looking far away, to properly bend the light rays to give the clearest image. By contrast the horse doesn’t have highly developed lens muscles and generally changes the position of his eye by raising or lowering his head to get the best focus.

Often the flighty reactions of a horse to harmless objects is due to the way he sees things. He may react strongly to any movement at the outer edges of his vision because his instinct tells him it may be a predator. And because his eyes may not focus rapidly when shifting his gaze to different distances, he may be easily startled by something that suddenly looms up in front of him or comes up behind him. Many horses spook at something unusual along the path until they have a chance to examine it more closely and satisfy themselves it is harmless.

Spotlight On: How-To & Plans

Horseshoeing Part 2B

Horseshoeing Part 2B

If we observe horses moving unrestrained over level ground, we will notice differences in the carriage of the feet. Many deviations in the line of flight of hoofs and in the manner in which they are set to the ground occur; for example, horses heavily burdened or pulling heavy loads, and, therefore, not having free use of their limbs, project their limbs irregularly and meet the ground first with the toe; however, careful observation will detect the presence of one or the other of these lines of flight of the foot.

How To Prune

From Dusty Shelves: Pruning Guide from 1917

Horse Powered Snow Scoop

Horse Powered Snow Scoop

by:
from issue:

The scoop has two steel sides about 5 feet apart sitting on steel runners made out of heavy 2 X 2 angle iron, there is a blade that is lowered and raised by use of a foot release which allows the weight of the blade to lower it and then lock in the down position and the forward motion of the horses to raise it and lock it in the up position. This is accomplished by a clever pivoting action where the tongue attaches to the snow scoop.

Plans for Hog Houses

Plans for Hog Houses

by: ,
from issue:

Missouri Sunlit Hog House: This is an east and west type of house lighted by windows in the south roof. A single stack ventilation system with distributed inlets provides ventilation. Pen partitions may be of wood or metal. This plan takes the place of the original Missouri sunlit house since many farmers had difficulty in building it.

Haying With Horses

Hitching Horses To A Mower

When hitching to the mower, first make sure it’s on level ground and out of gear. The cutter bar should be fastened up in the vertical or carrier position. This is for safety of all people in attendance during hitching.

New Idea Mower

New Idea Mower

from issue:

For proper operation the outer end of the cutter bar should lead the inner end when the machine is not in operation. After long use the cutter bar may lag back and if this happens it can be corrected by making adjustments on the cutter bar eccentric bushing as follows: First making sure that the pin and bolt in the hinge casting “A” Fig. 5 are tight and in good condition.

Fencing for Horses

Fencing for Horses

by:
from issue:

The first wire we tried was a small gauge steel wire which was not terribly satisfactory with horses. Half the time they wouldn’t see it and would charge on through. And the other half of the time they would remember getting shocked by something they hadn’t seen there and would refuse to come through when we were standing there with gate wide open. We realized that visibility was an important consideration when working with horses.

Farmrun On the Anatomy of Thrift

On the Anatomy of Thrift: Side Butchery

On the Anatomy of Thrift is an instructional series Farmrun created with Farmstead Meatsmith. Their principal intention is instruction in the matters of traditional pork processing. In a broader and more honest context, OAT is a deeply philosophical manifesto on the subject of eating animals.

Blacksmithing with Pete Cecil Building a Fire

Farm Drum #29: Blacksmithing with Pete Cecil – Building a Fire

Lynn Miller & Pete Cecil talk about Blacksmithing basics, and Pete demonstrates building a fire in the forge.

Pulling A Load With Oxen

an excerpt from Oxen: A Teamster’s Guide

Permanent Corncribs

A short piece on the construction of corncribs.

Shed and Barn Plans

Below is a short piece from Starting Your Farm, by SFJ editor and publisher Lynn R. Miller. Click the links below to see Chapter One of Starting Your Farm and to view the book in our online bookstore. “You may have purchased a farm with a fantastic set of old barns and sheds. You, on […]

Sleds

Sleds

by:
from issue:

The remainder of this section on Agricultural Implements is about homemade equipment for use with draft animals. These implements are all proven and serviceable. They are easily worked by a single animal weighing 1,000 pounds, and probably a good deal less. Sleds rate high on our homestead. They can be pulled over rough terrain. They do well traversing slopes. Being low to the ground, they are very easy to load up.

How To Prune a Formal Hedge

How To Prune A Formal Hedge

This guide to hedge-trimming comes from The Pruning Answer Book by Lewis Hill and Penelope O’Sullivan. Q: What’s the correct way to shear a formal hedge? A: The amount of shearing depends upon the specific plant and whether the hedge is formal or informal. You’ll need to trim an informal hedge only once or twice a year, although more vigorous growers, such as privet and ninebark, may need additional clippings.

Blacksmithing

Blacksmithing

from issue:

Modern farm machinery is largely of iron and steel construction, making an equipment of metal working tools necessary if satisfactory repairs are to be made. Forging operations consist of bending, upsetting, drawing out, welding, punching, drilling, riveting, thread-cutting, hardening, tempering, and annealing. Heat makes iron soft and ductile. Practically all forging operations on iron can be done more rapidly when it is at a high heat. Steel will not stand as high a temperature.

Log Arch

Log Arch

by:
from issue:

The arch was built on a small trailer axle that I cut down to 3 feet wide and tacked back together. This was done so that I could keep the wheels parallel. I cut the middle out after construction was complete. I used heavy wall pipe from my scrounge pile for the various frame parts. It is topped off with an angle iron bar for added strength and to provide a mount for the winch and some slots for extra chains.

Portable A-Frame

Portable A-Frame

by:
from issue:

These portable A-frames can be used for lots of lifting projects. Decades ago, when I was horselogging on the coast I used something similar to this to load my short logger truck. Great homemade tool.

How To Get Into Farming With No Money

How To Get Into Farming With No Money

by:
from issue:

Let’s assume the beginning ‘farmer’ has absolutely nothing. Nothing but a will to farm and a reasonably normal body. The very first thing you must do is search out a farmer, preferably a farmer who farms close to the way that you want to farm. You must watch him, ask questions, do as you are told and learn everything you can. Very shortly you will be on your own and you will find that the more you learn now, the better you will be when you have only yourself to rely on.

Small Farmer's Journal

Small Farmer's Journal
PO Box 1627
Sisters, Oregon 97759
800-876-2893
541-549-2064
agrarian@smallfarmersjournal.com
Mon - Thu, 8am - 4pm PDT