Seeing is believing...... sort of

So when starting this blog I hadn't really thought about how hard it is to deliver what I'm learning in an interesting way. I mean I find it interesting but that's not to say that anyone else does.

Soooo lets just start anyway.


The eye.. It's actually kinda a neat structure... and one that is surprisingly hard to cut on dissection.

The adult eye has three tunics. The inner layer is the retina, the middle layer is the uvea (vascular or choroid layer) and the outer is a fibrous layer.

At the back of the eye is a spot called the fovea centralis: this is the area that the visual axis meets directly (instead of at the optic nerve like one would think). This is the area of clearest sight and only has cones.

Dogs and Cats have both rods and cones, however the ratio is about 95% rods to 5% cones. This means that dogs and cats can see lots of light and shade but not much by the way of colour, though they do have some.

Now for the important bits:
The Retina: the light sensitive layer. This is where the photoreceptor cells are, essentially the key to sight. The ganglion cells lie here (which will run to the optic nerve allowing for the brain to recognize what it sees).

Clinically there can be a few issues in this location:

1) Detached retina: you may have heard of this before. It is pretty much what it sounds like, essentially the retinal layer has detached from the choroid/sclera. Animals with a detached retina depending on severity have little to no vision in the affected eye. There are a few different causes (ie a blow to the head, bleeding in the eye wall or damage to the vitreous humour) however it can also be caused in clinics. Sometimes seen in cats is a detached retina due to a spike in blood pressure (sometimes happening during surgery).

2) Another is Progressive Retinal Atrophy or "night blindness": this is sometimes seen in specific breeds (ie the Irish setter and the abyssinian cat). Basically the blood vessels progressively atrophy and pupils will dilate in order to increase the light into the eye (in order to see). This results in vision during the day, but lots of bumping around during the night (as there is too little light for even dilation of the pupil to compensate for).

Tapetum Lucidum. This I have to say is my favourite layer. On dissection it is a beautiful iridescent blue and turquoise and it's what allows animals to be able to see at night (it's also what gives the animals in your car headlights glowing eyes). This layer lies between the pigmented layer and the choroid and it's cells are light reflecting in order to allow more light to reflect against the photoreceptors at the back of the eye.

The eye to the right is from a cow, it is the back half of the dissected eye. The milky white portion at the bottom is the retina (still attached at the area of the optic nerve). The blue/green portion itself is the tapetum lucidum.

Now for the lens. This is a surprisingly strong and solid structure (at least surprising to me who was used to thinking of a lens as well... a contact). The lens is a kind of oval shape that is made of many layers closely wrapped (much like an onion). This is suspended from muscles on the sides of the inner eye (in the ciliary body) by suspensory ligaments. The ligaments and muscles of the ciliary body are what allow you to focus on what you are seeing. For distance vision the ciliary body relaxes and the ligaments are taut allowing the lens to lie flat. For close vision, the ciliary body tenses, the fibres slacken and the lens is round.

So what can go wrong here? well most commonly are cataracts: the clouding of the lens due to age (that old dog eye look), genetics or diabetes (the side effect of sorbitol accumulation). Essentially these are irreversible without surgery. Cataracts can also be accompanied by lens luxation (displacement of the lens to the anterior chamber). Basically with cataracts, transparency is reduced and vision is affected to some degree. The change however is usually gradual over time and the reduction in vision may or may not be noticeable.

The Aqueous humour: this is that liquid just inside the cornea of the eye (in the anterior chamber). It is formed by the filtration of blood plasma by the ciliary processes and is what provides the iris with nutrients (it feeds it). The humour is formed near the ciliary body, moves towards the pupil and out towards where the iris meets the cornea (there is a venous plexus there that drains it from the anterior chamber). It is constantly formed and removed (keeping the meal going!).

The most common issue here is glaucoma, a raise in intraocular pressure. Usually caused when the drainage of the aqueous humour is blocked at the limbus. It can cause blindness indirectly (the raised pressure impedes circulation, killing off retinal cells in the eye) and can compress or even damage the optic nerve. Of note, in cats this can be a symptom of chronic renal failure (because of hypertension). This can be treatable by drugs (those that will decrease humour secretion or increase the drainage of it)

The cornea! Which most of us are familiar with at some point in time. It has many nerve endings and is very sensitive (hence why it hurts so much when you have a microscopic speck in your eye!). This is where light focuses. It's surface is a single layer of squamous epithelia and thankfully (in those times that you get a scratch on your eye) it repairs itself very quickly (cell turnover within 24 hours).

Quite a few things can go wrong here, some dangerous, some not and some just plain gross (ie epibulbar dermoid: hairs growing on the cornea surface.. Thankfully not really a problem and can be dissected off but ewww...)

For those that have horses a corneal ulceration may be familiar. Especially if the ulcer allows further entry of bacteria into the chamber of the eye (yes I am very familiar with this one.. my own little pony pulled out this trick also known as a stromal abscess).  It can be caused by scratches (infected ones) or even more chronic conditions like dry eye (or keratoconjunctivitis sicca, an autoimmune disorder causing failure of tear production) if this happens to your pet better get some artificial tears for it before you get an ulcer!

Just for the record this horse here doesn't have a green infection. That's the flourescent dye marking the area of the ulceration (in order to diagnose the problem).

Well I think I'll call it a day here. I know for you keeners I missed out on discussing conjunctivitis but perhaps next time...