Eye Flashcards
What are the components of the visual apparatus?
Globe – eyeball itself, housed in orbit with lots of fat around the outside, which gives body to the eyes and allows it to move and offers protection.
Adnexa – structures which surround or attach to the globe.
What is the adnexa comprised of?
Eyelids
Orbital fascia
Lacrimal apparatus
Extrinsic muscles
Nerves and blood vessels
What are the functions of the adnexa?
- Protection from light, heat and trauma
- Movement in 3 dimensions
- Keep exposed parts moist and by producing tears to create film over the eye for moisture, as well as tears having antibacterial properties.
What are the lateral and medial canthi?
Canthi - angle where lower and upper lids meet
Medial is larger and has multiple perforations and has the lacrimal caruncle within it.
What is the lacrimal caruncle?
Raised prominence with sebaceous gland to secrete oil
What are the roles of the upper and lower lids?
Have many sebaceous gland to produce of oil, which allows tears to adhere to the surface of the eye in a film
What is the palpebral/blepharon fissure?
Space between the 2 lids
What is the orbit?
Cone shaped cavity on lateral surface of the skull. Houses eyeball and adnexa.
What is the structure of the orbit and the species differences of this?
Delimited externally by bony ring or bony ring completed by orbital ligament between the 2 processes.
Complete bony rim = sheep
Incomplete = pigs and dogs
What issues do brachycephalic dogs have with their orbits?
Brachiocephalic dogs have shallow orbits and so have eye problems when lids do not entirely close over the eyes.
Describe the structure of the orbit walls.
- Entirely bony in human/primate
- Only osseous medially and dorsally in domestic species
- Affects diseases and surgical approaches in this area
What are the 3 layers of orbital fascia surrounding the eyeball?
Periorbita
Superficial muscular fascia
Deep muscular fascia
Describe the structure of the periorbita.
- Cone of thick fibrous tissue
- Blends with periosteum at orbital margin and at medial/dorsal orbit
- Forms wall of orbit ventrolaterally
- Attached near optic foramen
- Trochlear cartilage embedded in dorsomedial wall
What is the orbital septum?
Section of tissue that forms part of the eyelid and gives it structure. Within the periorbita.
What is the embryology of the eyelids?
Originate embryologically from surface ectoderm
Fused at birth in carnivores – physiological ankyloblepharon
What are the 3 layers of the eyelids?
External layer – skin
Middle layer – musculofibrous
Inner layer – mucous membrane double layer, palpebral conjunctiva on eyelid and bulbous conjunctiva on the globe surface.
Describe the structure of the musculofibrous layer of the eyelids.
- Interdigitation of muscles and fibrous tissues
- Orbicularis oculi muscle
- Orbital septum
- Aponeurosis of levator palpebrae superioris, which upon contraction allows upper eyelid to be pulled upwards
- Same structure for lower lid, except does not have levator palpebrae superioris
Describe the structure of the conjunctiva inner layer.
- Posterior lid lined with palpebral conjunctiva
- Continuous with bulbar conjunctiva – conjunctival sac and fornices between 2 layers, at the bend where tissue wraps round to form 2 layers.
Describe the structure and function of the palpebral ligaments.
Medial and lateral. Suspends eye in the socket, tethering it to each respective canthus
Only medial is well defined:
- Originates from periosteum of frontal bone
- Blends with eyelids at medial canthus
What is the structure of the 3rd eyelid?
- Nictitating membrane
- Fold of conjunctiva
- Supported by T shaped piece of cartilage
- Comes across eye medially
What are the functions of the 3rd eyelid?
- Glandular, so has oil secreting properties
- Smooth muscles keeps it retracted
- Involves in protection as well
What is ‘cherry eye’?
Inflamed and enlarged third eyelid
What are the 4 key symptoms of Horner’s syndrome?
- Sunken eye – enophthalmos
- Constricted pupil – miosis
- Droopy upper eyelid – ptosis. Involves the tarsal muscle that assist the levator palpebrae superioris
- Prominent 3rd eyelid
What are the lacrimal apparatus and their functions?
Lacrimal gland – produce tears
Lacrimal ducts – tears to the nasal cavity via the lacrimal duct (why nose runs when you cry)
Conjunctival sac – tears drain from medial canthus and into lacrimal duct
2 lacrimal puncta – drain stagnant tears
Naso-lacrimal duct – to nasal cavity
What is the position and function of the extrinsic muscles of the eye?
- Posterior to eyeball
- Insert on sclera
- Allow eye to be pulled in different directions upon contraction
- Originate near optic foramen – ventral oblique
Name the extrinsic muscles of the eye.
4 straight: ventral rectus, dorsal rectus, lateral rectus, medial rectus
2 oblique: dorsal oblique and ventral oblique, are diagonal to rotate the eye
Retractor bulbi - cuff around the eye to pull the eye back in the socket, protective mechanism
What is the innervation of the extrinsic muscles of the eye?
Most are supplied by oculomotor, except that lateral rectus and retractor bulbi supplied by the abducent nerve and dorsal oblique supplied by trochlear nerve.
What movement around perpendicular axis do each muscles allow?
Movement around dorsoventral axis = medial and lateral rectus muscles
Movement around mediolateral axis = dorsal and ventral rectus muscles
Movement around anterior posterior axis = dorsal and ventral oblique muscles
Describe the blood supply to the eyes and associated structures.
Maxillary artery > external ophthalmic > internal ophthalmic
External artery > muscular branches + lacrimal (to lacrimal apparatus) + supraorbital + palpebral
What does the position of the eye relate to and the species differences in eye position?
Position in the head relates to feeding method, habitat and environment.
- Predators – eyes set forward, large field of binocular vision
- Prey – eyes set on side of head, large set of monocular vision
What is the orientation of the eye?
Rostral = anterior pole (front of eye)
Caudal = posterior pole (back of eye)
What are the 3 tunics of the eye?
Outer fibrous tunic – ‘white of the eye’, sclera, cornea, outer fibrous layer
Middle vascular tunic – called uvea. Ciliary body produces aqueous humor and changes size and shape of the lens.
Inner nervous tunic (retina) – reflects eyeball’s origin as an outgrowth of the embryonic brain.
Describe structure of the fibrous tunic.
- Cornea and sclera
- Continuous
- Meet at limbus
- Only complete layer
Describe the structure and properties of the cornea.
- Bulges forward as it is filled with aqueous humor
- Transparent due to structure and physiology
- No blood vessels/avascular – nutrients from adjacent structures
- Refracts light more in humans than lens in domestic species
- Very sensitive due to free nerve endings
- Supplied by ciliary nerves so very sensitive
What is the limbus?
Where the structure the white of the sclera and translucent of the cornea meet, as if the eye was all white, no light could pass.
Describe corneal ulcers.
Can start superficially and penetrate to deeper layers (descemets membrane, forming small little swellings). Detect corneal ulcers with flouroxene in UV light, eye drops or injected to supply veins at back of the eye. Can also get penetrating eye injuries.
What makes up the sclera?
- White of eye – dense collagen fibres
- Pierced by vorticose veins and small ciliary arteries
- Has cribriform area where fibres of optic nerve pass – this is a weak spot, and the optic nerve has connective tissue sheath that continues dura mater to sclera.
What blood vessels supply the sclera?
Anterior ciliary arteries across ocular surface
Long and short posterior ciliary arteries
What is the conjunctiva comprised of?
- Small blood vessels visible
- Those of bulbar conjunctiva move freely. There is also palpebral part
Distinguish the sclera and cornea.
So while the sclera is pierced by blood vessels, the cornea is avascular. Cornea bulges forward and is sensitive due to free nerve endings (V1). Sclera is made of collagen and has a cribriform area for passage of optic nerve fibres.
Describe the structure and function of the vascular tunic/uveal tract.
- Controls size of pupil
- Suspends lens and regulates its curvature
- Produces aqueous humor
- Supplies blood
What are the functions of the iris?
- Divides space between lens and cornea into anterior and posterior chambers.
- Colour depends on number of pigmented cells
- Iridic granules in ungulates for UV protection or movement sensor
Describe the structure and function of the ciliary body of the eye.
- Smooth muscle within it accommodates lens in primates – not as much in domestic species
- Ciliary processes extend from it
- Have zonular fibres that attach to lens equator
Where is aqueous humor produced form and circulated to?
- Ciliary body produce aqueous humor, which circulates through anterior and posterior chambers and leaves the eye through spaces in pectinate ligaments and drains into scleral venous sinus.
- Small angle of filtration (iridocorneal angle) affects drainage
What is the innervation of the vascular tunic?
Sympathetic fibres form the CCSG – follow V1 to orbit and innervate dilator of pupil
Parasympathetic fibres – pre-synaptic fibres run to orbit in oculomotor nerve. Post-synaptic fibres (short ciliary nerves) innervate ciliary muscle and constrictor of pupil.
What are the cranial nerves involved in the pupillary light reflex?
Shining light into eye in pupillary light reflex.
Sensory CNII
Motor CNIII
Describe the choroid layer.
- Supplies blood
- Has light reflecting layer in some species
- Forms light reflecting layer in dorsal fundus – tapetum lucidum
What is the function of the tapetum lucidum?
Tapetum lucidum for reflecting stray photons that did not pass to a rod or cone photoreceptor on first pass. For higher visual acuity in the dark.
Describe the dog fundus.
- Tapetum lucidum
- Complete vascular ring
- Non-tapetal choroid
- Optic disc
Describe the tapetum lucidum of silver dappled species.
Have a stippled tapetum lucidum.
Describe the puppy fundus.
Vessels are much more prominent
Describe the border collie colour diluted/subalbinotic eye.
- Has no tapetum so fundus appears red due to vessels.
- Blue iris - minimal pigment
Describe the pig fundus.
- No fundus
- Large veins
Describe the fundus of a small ruminant.
- Tapetum lucidum
- Twisted vessels
- Optic disc
Describe the fundus of a calf.
Ring of myelinated fibres around periphery of optic disc
Describe an adult ruminant fundus.
- Optic disc
- Remains of retina
- Tapetal choroid
Describe a horse fundus with a subalbinotic wall.
- Choroidal vein show through
- Poor retinal vasculature
- Fine retinal vessels form a ‘brush border’
- Tapetum lucidum
Summarise the properties of the vascular tunic.
- Deep to sclera
- Composed of iris, ciliary body and choroid
- Supplies blood
- Regulates size of pupil and suspends lens
- Ciliary processes produce aqueous humour
- Choroid forms tapetum lucidum in the dorsal fundus in many species (nocturnal adaptation)
What are the 2 layers of the nervous tunic?
- Outer pigmented layer (pars ceca retina) – from optic nerve to pupillary margin
- Inner non-pigmented layer (pars optica retina) – from optic nerve to ora serrata
Pigment of retina and choroid make posterior eye dark – absorbs reflected light and prevents blurred vision
Which cells are in the pars optica retina layer?
Ganglion cells
Amacrine cells
Horizontal cells
Muller cells
Bipolar cells
Rods and cones
Pigment layer
What is the optic disc?
Optic disc is point at which nerve fibres converge to form the optic nerve.
What is the vitreous chamber?
- Posterior to lens
- Contains gel-like vitreous body
- Hyaloid canal runs form back of lens to optic disc. Contains hyaloid artery and primary vitreous in embryo.
Describe the structure of lens.
- Outer capsule under constant tension by zonular fibres
- Hard nucleus composed of many fibres
- Fibres join at Y shaped sutures – site of cataracts
How does the eye develop embryologically?
- Optic vesicles develop laterally from the diencephalon
- Neuroectoderm contacts surface ectoderm and stimulates it to thicken into a ‘lens placode’
- Distal part of the optic vesicle invaginates to form optic cup
- Lens placode invaginates to form the lens vesicle and is nourished by hyaloid artery.
- Mesenchyme condenses around the optic cup and will form the vascular and fibrous tunics.
- 2 layers of optic cup develop into pigmented and neural parts of the retina.
What is the embryological feature of nesting animals?
Dogs, cats and rabbits, for example – the lids are fused together at birth.
What are 3 embryological conditions of the eye?
Collie eye anomaly – slight degenerative with an area of the retina lateral to the optical disc.
Coloboma – dysfunction between iris and pupil.
Persistent pupillary membranes – membranes can develop over the surface of the pupil, can’t see through the pupil. Can predispose infection.
What is keratoconjunctivitis sicca?
More commonly known as “dry eye” occurs in dogs, cats and horses. It is caused by deficiency in the aqueous part of tears. It can result in conjunctivitis and corneal ulceration and scarring.
What are iridic granules?
Movement sensor. UV protection? A fringe on the border of the iris.
Compare the eye of a horse with wall eye and a normal brown-eyed horse.
Weak almost absent tapetum lucidum, with very thin pigmented part of the retina on inner surface in wall eye horse
What happens when the T shaped cartilage of the eye scrolls up?
Bloodhounds and great Danes can develop a scrolling a the T shaped cartilage, where the mucosal fold rolls up. Result in poor tear film distribution across the eye. a solution is to remove the affected/scrolled section, but not the entire structure, since this would remove the protective effect of the 3rd eyelid.
How can cherry eye be treated?
Fixed surgically, because it is a major tear gland, the structure should be preserved where possible. Gland should be replaced and anchored with sutures to the orbital rim, periorbital fascia or nictitans cartilage. Or covered with adjacent mucosa. Partial excision should be avoided.
Complete excision may predispose to KCS in 30-40% of dogs in later life.
When might an enucleation be performed?
Cancer of the eye
Glaucoma
Infection resistant to antibiotics
Painful eye, causing intractable symptoms
Blind eye causing complications
What damage could occur to neurones to result in Horner’s syndrome?
- Blunt force trauma to the thorax, head or neck
- Animal bites
- Inflammatory disorders of the eye which damage the nerve branches
- Tumours of the head, neck and thorax that erode/compress the nerves
- Infections of the middle ear, near to which the nerves pass en-route
Define dacryocystitis.
Inflammation of the nasolacrimal ducts.
What are the possible causes of dacryocystitis?
- It commonly occurs in rabbits as a result of dental disease as the ducts run over the roots of the molar teeth and are therefore susceptible to damage if the roots overgrow.
- Lack of fibre in the diet and subsequent reduced tooth wear can lead to overgrowth of the molar roots.
- Other causes include infection and blockage by foreign bodies. Rabbits with very short noses may also be predisposed as the tear ducts become curved and obstructed by the shape of the skull.
- Blockage or damage to the lacrimal ducts lead to epiphora, an overflow of tears down the face which can irritate the skin and predispose to infection.
Which structure in the eye produce aqueous humour?
Ciliary process
What runs through the internal acoustic meatus?
Facial and vestibulocochlear nerves
Which layer of the eye is the tapetum lucidum part of?
Vascular tunic
Which structure joins the middle ear to nasopharynx?
Auditory tube
What is found inside the membranous labyrinth?
Endolymph
What is the innervation of the tympanic membrane?
Vagus and V3
What structure in the eye regulates the curvature of the lens in some species?
Ciliary body
What is keratitis?
Inflammation of the cornea, which may arise from many different insults: traumatic injury to the eye, infections, such as herpes virus, inflammatory/autoimmune causes and structural causes. Can be divided into 2 separate categories – ulcerative and non-ulcerative.
Describe keratitis in brachycephalic dogs.
Orbits are shallower, meaning the eyes are less sheltered by the bony construct and bulge forwards. Eyelids may not extent in their entirety over the whole eye to close the palpebral fissure. This may mean that the eyelids fail to protect the cornea, bathing it with the tear film and removing foreign bodies. So, can develop corneal ulcerative disease form trauma/irritation as a result of these structural abnormalities.
What are the 3 chambers of the eye?
Anterior – between cornea and iris. Contains aqueous humour and is a site of drainage.
Posterior – located between the iris and lens. Contains aqueous humour and is a site of production.
Vitreous – between lens and retina. Largest chamber and contains vitreous humour.
