Anatomy of the Eye

What is the equator of the eye?

An imaginary line encircling the globe of the eye equidistant from the anterior and posterior poles (divides eye into anterior and posterior halves).

Is the anterior or posterior chamber largest?

Anterior

Unlike other structures in the head 'rostral' and 'caudal' are not used when referring to the eye, what terms are used instead?

Anterior and posterior

Which chamber/s contain aqueous humour?

Anterior and posterior chambers

Small blood vessels are visible on the conjunctiva, what are these branches of?

Anterior ciliary arteries

What is the difference between aqueous and vitreous humour?

Aqueous humour: - Watery - Produced by ciliary processes. - Provides nutrients for lens and cornea. - Maintains intraocular pressure (25mm.Hg) - Replaced several times a day (2µl/min) Vitreous humour: - Gelatinous - Produced by the ciliary body - Supports the lens anteriorly and the retina posteriorly. - Pressure from the vitreous humour prevents retinal detachment. - Not continuously replaced

Are predators eyes set forward or on the side of the head?

Eyes are set forward

Are prey animals eyes set forward or on the side of the head?

Eyes set on side of head

What does the position of the eyes in the head relate to?

Feeding method/habitat/environment

What can be used to highlight the extent of a corneal ulcer?

Fluorescein eye stain

What is the anterior pole?

Highest point on the cornea

What is the posterior pole?

Highest point on the sclera

What runs in the vitreous chamber, from the back of the lens to the optic disc?

Hyaloid canal Present in the ox, pig and carnivores.

What is the fundus?

Interior surface of the back of the eye, the part of the eye you see when looking in the eye with an ophthalmoscope. From Latin, literally 'bottom'. Each sp. has a characteristic fundus and can be used for identification purposes.

What is the meeting point of the sclera and cornea called?

Limbus or corneosclera The cornea bulges forward in all species, more so in cattle and horses, and less so in cats and dogs.

Which nerve causes dilation of the pupil (target iris) and relaxation allowing accommodation for distant vision (target ciliary muscle)?

Sympathetic fibres from the cranial cervical ganglion follow V1 (ophthalmic division of trigeminal) to orbit

What is the sympathetic innervation of the eye?

Sympathetic nerve fibers arising from the cranial cervical ganglion follow arteries or the ophthalmic nerve to the orbit, where they innervate the orbital muscle and dilator of the pupil. Tonus in the orbital muscle keeps the eyeball protruded, the third eyelid retracted, and the palpebral fissure open. Loss of sympathetic innervation results in a sunken eye, protrusion of the third eyelid, and constriction of the pupil (Horner's syndrome). Dilation of the pupil (mydriasis) is initiated by fear, excitement, or pain.

In the dorsal part of the fundus the choroid forms a variously colored, light-reflecting area, what is this known as?

Tapetum lucidum Some species have modified cells that split light into its component parts --> iridescence, this is a nocturnal adaptation to cause double stimulation if the retina to cause a brighter, although slightly blurred, image.

What does the iris divide the space between the lens and cornea into?

The anterior and posterior chamber

Which chamber/s are located anterior to the lens?

The anterior and posterior chambers are located in the anterior cavity, anterior to the lens.

Is the cornea transparent or opaque?

Transparent It is transparent due to its structure (highly ordered collagen and no blood vessels) and physiology (pumps remove excess corneal tissue fluid as water would separate the collagen bundles).

Vitreous humour is present at birth and does not change much over the course of aging. True or false?

True

Aqueous humour is continually produced and reabsorbed. True or false?

True - it is a delicate balance

What is the function of iridic granules?

UV protection and movement sensors. They are found in ungulates (particularly prominent in lamas) and may offer UV protection or act as a movement sensor (like having a grid of front of the visual apparatus so any slight movement is enhanced - horses that spook more easily may have larger iridic granules). They are found dorsally and ventrally and can be used to help orientate the eye as they are more dominant dorsally. They look similar in structures found in birds eyes know as pecten (comb-like).

What is the vascular tunic also know as?

Uveal tract (uvea) "Grape; uvula", the vascular tunic is the main blood supplying layer and it looks dark in histological sections.

The choroid is part of which tunic?

Vascular

Which layer of the eye is the tapetum lucidum part of?

Vascular tunic

Where does the optic nerve emerge?

Ventrolateral to the posterior pole in domestic species Ventromedial in primates.

What is monocular and binocular vision?

Vision is either coming from one eye (monocular) or both eyes (binocular)

Where is the retina almost transparent?

Where is overlies the tapetal region.

What causes the outer capsule of the lens to be under constant tension?

Zonular fibers, extend from ciliary processes

Are the anterior and posterior chambers separate or continuous?

Continuous The ciliary processes produce aqueous humour that flows from the posterior to the anterior chamber.

What is more important in refracting light, the lens or the cornea, in domestic species?

Cornea The cornea refracts more light in domestic species, where as the accommodating power of the lens is more important in humans.

What tissue comprises the fibrous tunic?

Dense collagenous tissue

What is the posterior corneal epithelium also known as?

Descemet's membrane

What structures form the outer fibrous tunic layer?

Sclera and cornea

Which chamber/s contain vitreous humour?

The vitreous chamber

Which chamber/s are located posterior to the lens?

The vitreous chamber is located in the posterior cavity, posterior to the lens.

What is the sieve-like area on the sclera called where fibres of the optic nerve pass?

Cribriform area It forms a weak spot and an indent can form here if an animal has glaucoma.

Which blood vessels supply the sclera?

- Anterior ciliary arteries Penetrate near the limbus and also supply the conjunctiva, anterior portion of the choroid, the ciliary body, and the iris. - Long posterior ciliary arteries Pass through the sclera close to the equator. Superficial, pass 3 'o' clock and 9 'o' clock either side of optic nerve. - Short posterior ciliary arteries Penetrate near the optic nerve, also supply adjacent choroid and optic nerve. Superficial, found around circumference of optic nerve.

What is the structure of the tapetum lucidum?

- Avascular layer - Between the capillaries and the network of larger vessels of the choroid. - The tapetal cells contain crystalline rods arranged in such a way that light striking them is split into its components, which results in the characteristic iridescence.

Which structures form the vascular tunic?

- Choroid - lines the sclera from the optic nerve almost to the limbus - Ciliary body - thickened zone opposite the limbus - Iris (including iridic granules) - projects into the cavity of the eyeball posterior to the cornea (They are all continuous)

What are the functions of the vascular tunic?

- Ciliary body - accommodation of the lens (ciliary muscle); aqueous humor production (ciliary processes); and anchoring the lens in place (zonular fibres). - Choroid - supplies blood and forms tapetum lucidum in the dorsal fundus in many species (nocturnal adaptation) - Iris - gap in the iris creates the pupil, it is responsible for controlling the diameter and size of the pupil via the iris sphincter and dilator muscle.

What conditions can develop due to problems during eye development?

- Collie eye anomaly - deficiency in any of the tunic layers. - Coloboma - gap in part of the structures of the eye - Persistent pupillary membranes - remnants of a fetal membrane that persist as strands of tissue crossing the pupil.

Which blood vessels pierce the sclera?

- Larger vorticose veins (principal venous return for the eyeball that emerge from the sclera posterior to the equator at regular intervals) - Smaller ciliary arteries

What is the function of the choroid?

- Main blood supplying layer - Some sp. have a light reflecting layer - tapetum lucidum

What are the 3 tunics/layers of the eye?

- Outer fibrous tunic (the dura mater fuses with the sclera) - Middle vascular tunic (arachnoid and pia mater merge with the choroid) - Inner nervous tunic, retina (continuation of nervous tissue) => reflects eyeball's origin as an outgrowth of the embryonic brain (diencephalon). This is important since superficial eye infections could potentially cause meningitis via CSF and the subarachnoid space.

What are the two layers of the retina?

- Outer pigmented layer (pars ceca retina) Patches from optic nerve to pupillary margin. Absorbs reflected light and prevents blurred images. From caecum 'blind', non-visual. Fused to the choroid and attaches to the posterior iris and ciliary body. - Inner non-pigmented layer (pars optica retina) Snotty/viscous layer from optic nerve to ora serrata (the retina is without pigment where it overlies the tapetum lucidum). From optos 'seen', visual part, cells involved in visual pathway. Easily removed in dissection.

What is the function of the ciliary processes?

- Produce aqueous humour - Provide attachment for the zonular fibres (suspensory ligaments for the lens)

What is the nervous tunic composed of?

- Retina (same as neural tissue) - Optic disc

Describe what happens when light hits the pars optica retina.

1. Light stimulates the rods and cones (photoreceptors), found in the outer most layer of the pars optic retina 2. bipolar cells (1st neurone of visual pathway) 3. Retinal ganglion cells 4. Axons of retinal ganglion cells form optic nerve.

Describe the embryonic development of the mammalian eye.

1. Optic sulci (little dips) form in neuroectoderm, either side of the neural groove. 2. The notochord stimulates the formation of the neural tube (pinches off) and surface ectoderm from the neuroectoderm. 3. The sulci on the neural tube expand laterally to form the optic vesicles (little projections of neural tube from diencephalon), the ends swell and they remain attached via the optic stalk. 4. Neuroectoderm contacts surface ectoderm and stimulates it to thicken via cytokines into a "lens placode". 5. Distal part of optic vesicle invaginates ventrally to form optic cup invagination, which allows hyaloid a. to supply posterior part of the lens, the fissure created here eventually closes. 6. Lens placode invaginates to form the lens vesicle: nourished by hyaloid artery. The lens placode falls into the mouth of the cup and the vesicle is pinched off. 7. Mesenchyme condenses around the optic cup and will form the vascular and fibrous tunics. 8. Two layers of optic cup develop into pigmented (pars ceca) and neural (pars optica) parts of retina.

Describe the flow of aqueous humour in the eye.

1. Produced by the ciliary processes 2. Posterior chamber 3. Anterior chamber 4. Through the iridocorneal angle (consists of sponge-like trabecular tissue + pectinate ligaments) 5. Drains into scleral venous sinus 6. Venous system

How many chambers are there in the eye?

3 - anterior, posterior and vitreous

Do predators have a large field of binocular or monocular vision?

Binocular They can see near objects clearly and have good depth perception, which is good for hunting. They have small monocular vision.

What functions in the accommodation of the lens (regulates curvature)?

Ciliary muscle (ciliary body) Between the ciliary body (5) and the sclera (6) is the smooth ciliary muscle (7), which functions in accommodation (the ability of the eye to focus on near or distant objects by changing the shape of the lens). Remember - domestic species don't have much ability to manipulate the shape of the lens, most of the light refraction is due to the corneal.

What produces aqueous humour?

Ciliary processes They form a 'frill' around the lens.

Is the tapetum lucidum located in the dorsal or ventral fundus?

Dorsal

The optic nerve has a connective tissue sheath, which two structures does it connect?

Dura mater and sclera Remember - eyes are a direct outgrowth of the brain.

What is the main blood supply to the eye?

External ophthalmic artery, a branch detached from the maxillary The blood supply to the human eye enters the orbit with the optic nerve. This route is represented in the domestic mammals by the rudimentary internal ophthalmic artery.

What is the angle of filtration also known as?

Iridocorneal angle - it is between the iris and the cornea. If it is too small (like it can be in some breeds of dogs, such as Basset Hounds) or there is trauma or even parasites, it can become blocked. This leads to inadequate drainage of aqueous humour, increased intraocular pressure, glaucoma and degeneration of the optic nerve.

Why is pigment in the posterior of the eye important?

It absorbs reflected light (light will stimulate the pars optic retina and pass through to the pigmented part where it is absorbed). It prevents blurred vision (as the light is absorbed there is no scattering of light, which would stimulate the retina a second time).

What is the function of the tapetum lucidum?

It helps animals to see in dim light conditions by reflecting incident light and is a nocturnal adaptation. Light hits the rods of the retina then continues through and hits the light reflecting layer in the dorsal fundus, where it is reflected back onto the light-sensitive receptor cells of the retina for a second time. The two images are superimposed, they are brighter but blurred.

What structures for the border of the anterior chamber?

It is between the Descemet's membrane (posterior corneal epithelium) and anterior lens.

What structures for the border of the posterior chamber?

It is between the posterior iris and anterior part of the lens.

What is the lens composed of?

Lens fibres within an elastic capsule

Do prey animals have a large field of binocular or monocular vision?

Monocular They are aware of a large proportion of their surroundings, but can only see clearly just in front of their nose (the have small binocular vision)

What drugs would you give an animal to allow you to view deeper structures of the eye more clearly?

Mydriatic --> dilation of the pupil

What blood vessels travel in the sclera?

None, it is avascular.

What is the function of the hyaloid artery?

Nourishes the developing lens Can sometimes find remnants of the hyaloid artery and canal if the animal is young, as a black stringy line from the posterior lens to optic nerve.

Which nerve causes contraction of the pupil (target iris) and contraction allowing accommodation for near vision (target ciliary muscle)?

Oculomotor n. via ciliary ganglion Post-synaptic fibres (short ciliary nerves) innervate ciliary muscle and constrictor of pupil.

Is the sclera transparent or opaque?

Opaque - the collagen fibres are a meshwork

The cornea is very sensitive due to the free nerve endings of which nerve?

Ophthalmic division of the trigeminal

Which tunic is the only complete layer?

Outer fibrous tunic

What is the parasympathetic innervation of the eye?

Parasympathetic presynaptic nerve fibers enter the orbit within the oculomotor nerve. They synapse in the ciliary ganglion, and the postsynaptic fibers (which form the short ciliary nerves) innervate the ciliary muscle and the constrictor of the pupil. They control both the accommodation of the lens and the pupillary contraction (miosis) response to light.

What causes the posterior of the eye to be dark?

Pigment of the retina (outer layer, pars ceca) and choroid

What does shining a light into an animals eye test?

Pupillary light reflex - CN II - sensory - CN III (PS component) - motor There should be a direct and consensual pupillary light reflex - both pupils should constrict at the same time.

How can you tell if an animal has conjunctivitis or if the infection has travelled deeper?

The blood vessels of the bulbar conjunctiva are freely moveable. The swollen conjunctiva can be manipulated and if the vessels/colouring move it is a superficial infection. If it does not move the infection is deeper and the treatment strategy must be determined accordingly.

Which part of the vascular tunic is important in supplying blood?

The choroid It contains a dense network of blood vessels embedded in heavily pigmented connective tissue. The network is supplied by the posterior ciliary arteries and is drained by the vorticose veins.

What does the hyaloid canal contain?

The hyaloid artery in the embryo Usually degenerates after birth and then the lens is nourished by diffusion.

What controls the size of the pupil?

The iris Smooth muscles; a circular group called the sphincter pupillae constrict (parasympathetic - oculomotor n.), and a radial group called the dilator pupillae dilate (sympathetic - ophthalmic division of trigeminal).

What does the colour of the iris depend on?

The number of pigmented cells and amount of pigment (melanin) within cells. Lots --> brown Medium --> yellow Little --> blue (subalbinotic - sensitive to light as little pigment to protect internal structures). None --> pink (no pigment thorough out body - albino- , eyes very sensitive, look because the blood in the capillaries is not obscured).

What is the optic disc?

The point at which nerve fibres of the retina (retinal ganglion cells) come together to form the optic nerve and travel through to the sclera.

What suspends the lens?

The radial ridges of the ciliary body, known as the ciliary processes, extend zonular fibres to the equator of the lens, suspending it around its periphery.

There are 1000's of lens fibres, describe their structure.

The substance of the lens consists of very regularly arranged fibers. These form concentric sheets that can be peeled off like the layers of an onion. There are too many to meet at a central point so the fibres join at 'Y' shaped sutures, 'lens stars'. These can become visible if an animal has cataracts and the lens fibres start to split apart, later on they completely split and 'stars' are no longer visible. In the peripheral part of the lens the fibers are relatively soft; they are firmer and thinner toward the center where they form a harder nucleus. Note - horse and cattle have multiple 'Y' shaped sutures.