Friday, 2 October 2015

All About .... The Eyes and Sight.

Following on from last weeks' blog 'All About The Ears' this week I thought I would look at the eyes.  As with the other senses the eye takes in information and this is then changed into a form of information that can be sent via the nerves to the brain for interpretation.

A horse's sight is quite different from a human's and this means they often react in a different way. As a prey animal a horse's eyes are positioned to allow them to see as much as possible and to spot predators early.  The eyes are positioned near the top of the head to allow the horse to see over the grass whilst he is grazing.  As the eyes are positioned at the side of the head a horse has a panoramic view which means that they can see us when we are sitting on board!  However, the disadvantage of the eye position is that the horse also has 2 blind spots, 1 directly in front of their nose and 1 directly behind them.   It is important not to approach a horse from behind for this reason - they won't see you and will be frightened into flight.  Horse's (and humans) will turn their heads to ensure they can see what is behind them.

Individual horses also have slightly differently positioned eyes and so each individual will have a different field of vision.  Horses have better vision at a distance than for closer objects, they also have much better night vision than humans.  However, their eyes take longer to adjust to changes in light.

Horses also perceive colour differently to humans, horses see less colours than we do!  They are said to have 'red-blue vision' which means that greens and yellows will look very similar although they may appear lighter and darker.

Horses have another advantage over humans because they have binocular and monocular vision.  They are able to view something with both eyes (as humans do) which is binocular vision.  However, horses are also able to see with each eye independently, monocular vision.  So they can be looking at something to their right at the same time as seeing something different to their left!

The Structure of the Eye

The eyes are situated inside cavities called orbits.  These enclose and protect most of the eye.  The front part, which is not enclosed, is protected by a thin membrane called the conjunctiva and by the eyelids, eyelashes and lachrymal (tear) glands.  The conjunctiva is moist and lines the inner eyelids and continues over the surface of the eyeball to keep it moist.

When a horse blinks the eyelashes act like windscreen wipers and keep the front surface of the eye clear from dust and dirt. The tears which are secreted from the lachrymal glands assist with this.  The tears also prevent the eyes from drying out and contain a bactericide  which reduces the risk of infection.  The eyelids help keep the eyeball moist.  The third eyelid moves horizontally across the eye, again to remove any debris (it is a nictitating membrane). The eyes are never still, they are constantly moving due to light changes and to direct attention to any object.

The Eyeball is roughly spherical and is composed of 3 layers.  The outer layer, sclera, is a protective coating, the middle layer, choroid, is a vascular layer and the inner layer, the retina contains the photoreceptors which are the light sensitive cells.

Light enters through the cornea which is a transparent layer continuously attached to the sclera.  The cornea is tough, it is a 5 layered membrane that bends the light waves as they pass through it, this focuses the light onto the photoreceptors in the retina.

Behind the cornea is a chamber which contains aqueous humour, which is a clear watery fluid, this separates the cornea from the lens.

The lens is a flattened sphere made up of lots of transparent fibrous cells which are arranged in layers.  The lens can fine tune the focusing on the back of the eye.  Small muscles help the lens change shape for focusing on near and distant objects.

The iris gives colour to the eye and consists of circular muscles around a central opening called the pupil.  These muscles control the size of the pupil and cause it to contract and dilate to change the amount of light entering the eye.

The main part of the eye behind the lens is filled with vitreous humour.  This is a transparent jelly like substance which gives shape and firmness to the back of the eye.

The cornea, aqueous humour, lens, and vitreous humour are all involved in focusing an image onto the retina. 

The Retina contains the photoreceptor cells which respond to light (rods and cones). It also contains the optic nerves which transmit the information to the brain.

So what does all this mean.

As horses' see differently to us it perhaps isn't such a surprise that they react differently to objects and situations:-

  • A horse can't see a jump just under their nose so on approach allow them to turn or raise their head to ensure they can have a good look at it. 
  • If a jump is a similar colour to the field they may have difficulty distinguishing it. 
  • Horses can be nervous going from a bright day to a dark stable or trailer. 
  • It is also important to remember if you are jumping from light to dark eg: into or out of a wooded area, the horse may not see the jump! 
  • As a horse cannot use his monocular vision at the same time as binocular vision he will need to reposition his head and eyes to swap between them.  This is why a horse will try to spin around if he spots something behind him, he wants to use his binocular vision to have a better look!

The understanding of how a horse sees and how it differs from us is relatively new.  I remember being told that horses' only see in black and white - we now know this is not true.  It is fascinating and worth taking into consideration when your horse spooks or refuses at a jump!

Have you seen this week's video yet 'How to ... Measure a horse for a rug' ?
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Until next time!

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