The Eye Flashcards
Label the components of this cross-section of the eye:
Label the anatomy of the eyelid:
Label the anatomy of the third eyelid:
Identify the labels of the anterior segment of the eye:
What is exophthalmos?
Abnormal protrusion of the eye from the orbit
(globe is protruding but normal in size)
side note: can be identified by looking from above or assessing retropulsion (push on globe via upper eyelid (doesn’t work well on animals with shallow orbits))
What is enophthalmos?
Abnormal recession of the eye within the orbit
(globe is sunken but remains normal size)
What is hydrophthalmos?
Enlargement of the globe but maintains normal position within the orbit
What is microphthalmos?
Congenitally* abnormal (small) eye but remains in the normal position within the orbit
*from birth
What is the orbit?
What is the purpose of the orbit?
Why are there foramina within the walls of orbit?
Cavity within the skull that the eye sits in
It protects and separates the eye from the cranial cavity
The foramina provide pathways for blood vessels and nerves to reach the eye
What is the difference between open and closed orbit and which do carnivores and herbivores have?
Open = Incomplete
- has a lateral orbital ligament
- found in carnivores (and pigs)
Closed = Incomplete
- fusion of zygomatic and frontal bones
- found in herbivores
What felid of vision do herbivores and carnivores have?
Herbivores = monocular vision
Carnivores = binocular vision
What is the orbit composed of on a basic level?
Bony cone and soft tissue floor
Why do carnivores have incomplete orbits?
Allows their jaw to be opened wider
What are the 3 different canine skulls shapes/sized?
(mesaticephalic or mesocephalic)
How many bones in total make up the orbit?
What 3 bones make up the orbital rim?
What is the other structure that helps make the orbit?
5-7, species dependant
The frontal, lacrimal and zygomatic bones
Soft tissue structures
Identify and name the bones 1-4?
Bones in the orbit:
Medial limit -
Dorsal limit -
Rostral and lateral limits -
Caudal limit -
Medial limit - frontal lobe (thin, operates obit + nasal cavity)
Dorsal limit - frontal sinus
Rostral and lateral limits - zygomatic, lacrimal and maxillary bones (make up orbital rim)
Caudal limit - sphenoid bone (optical canal + orbital fissure pass through)
Soft tissues in the orbit:
Dorsolateral limit -
Rostral and lateral limit -
Ventral floor -
Dorsolateral limit - temporal muscle and orbital ligament
Rostral and lateral limit - masseter muscle
Ventral floor - pterygoid muscles
How many foramina are in the orbital?
8 different foramina which creates interspecies variation
What nerves/vessels pass through the optic foramen?
Optic nerve
Internal ophthalmic artery
What nerves/vessels pass through the orbital fissure (dogs/cats – elongated)/orbital foramen (horses/ruminants)?
Oculomotor nerve
Abducens nerve
Trochlear nerve
Ophthalmic nerve
What (cranial) nerves are the following extraocular muscles innervated by?
Dorsal rectus
Medial rectus
Ventral rectus
Lateral rectus
Retractor bulbi
Dorsal oblique
Ventral oblique
Oculomotor (lll) - Dorsal rectus, Medial rectus, Ventral rectus, Ventral oblique
Abducens (Vl) - Lateral rectus, Retractor bulbi
Trochlear (lV) - Dorsal oblique
What soft tissues are considered the intraconal space in orbit and what are considered the extraconal?
Intraconal - 4 rectus muscles enveloped in a periorbital fascial sheath (also content nerves vessels, smooth muscle, fat and the orbital lacrimal gland)
Extraconal - any remaining soft tissue structures
How is the eye supplied with arterial blood?
The eye has high metabolic activity so has a rich blood supply from branches of the ophthalmic artery which in most mammals comes form the internal carotid artery. They supply the highly vascular uveal tract.
How does venous drainage in the eye work?
Most is through the vortex veins and orbital venous plexus
Small alternative route in the ophthalmic vein
All eventually drain to the external jugular vein
What are some clinical issues that come with high ocular blood supplies and venous drainage?
BS
- ocular damage common with systematic hypertension (high bp)
- systematic disease causing uveitis (inflammation of uveal tract)
VD
- orbital venous plexus well formed in rabbits so must be cautious when enucleating (removing entire globe)
- excess restraint around neck affecting intraocular pressure measurements
The following other structures if diseased can then lead to orbital disease as they are adjacent to each other, what are these structures?
- Nasal cavity + Paranasal sinuses
- Caudal roots of 4th premolar + 1st/2nd molar teeth
- Brain
- Temporal + Massester muscles
- Zygomatic salivary glands
What is…
Anisocoria
Miosis
Mydriasis
Strabismus
Nystagus
How can the ramus mandible cause issues?
When open it moves towards the globe, the there is orbital disease this can cause the dog pain when trying to open its jaw
What does palpebral mean and what is the palpebral fissure?
Eyelids
Opening between the eyelids
How many layers is the eyelid composed of and what are they?
3 layers
1. haired eyelid, skin on outer surface (outside)
2. muscle extending to a fibrous tarsal plate containing meibomian glands (middle)
3. palpebral conjunctiva on inner surface (inside)
What muscle controls the closing of the eyelids and what nerve is it innovated by?
The orbicular oculi - facial nerve (Vll)
What muscle controls the opening of the eyelids and what nerves are they innovated by?
Levatator palpebral superiors - oculomotor nerve (lll)
Muller muscle (smooth muscle) - sympathetic innervation
Several others with innervation from facial nerve (Vll)
What nerves supply sensation to the eyelid?
Branches of ophthalmic and maxillary nerves from trigeminal (V)
What are meibomian glands?
Modified sebaceous glands that are white or yellow columnar structures, sit at right angles to the eyelid margins
Where are meibomian gland openings found, how many are on each eyelid and what do they produce?
Grey line
10-40
Meibum - lipid part of tear film that stops tears evaporating too quickly
What is the main function of the eyelids?
Protects the eye from trauma
When blinking:
- distributes tear film which is nutritious and hydrating
- drains tear film into nasolacrimal canaliculi
- removes debris from ocular surface
How does the eyelid contribute to the tear film?
Meibomian glands produce lipids
Conjuctiva that lines the eyelids produces mucin
What provides structure to the third eyelid and what surrounds it?
What is fund at the base of the 3rd eyelid and what does it produce?
central T-shaped cartilage, cartilage surrounded by connective tissue
Nictitans glands, produces 1/3rd aqueous (water) part of tear film
What is found on the bulbar (facing globe) and palpebral (facing eyelid) surface of the 3rd eyelid?
conjunctiva
Where on the third eyelid can lymphoid follicles be found?
Posterior surface
Are dog and cat 3rd eyelids muscle dependant?
Dog: no muscle involved, when dog blinks globe retracts causing it to come over
Cat: smooth muscle retracts it
What is the main functions of the 3rd eyelid?
Protects the ocular surface, leading edge distributes tear film, physically removed debris from corneal surface
What secreting cells does the 3rd eyelid have and why is this important?
Immunoglobin (IgA) secreting plasma cells
- role in defence of ocular surface
What is conjunctiva?
Conjunctiva of the eye is found in one continuous sheet, what does it cover?
Mucous membrane (secrets mucous)
Covers: both eyelids, 3rd eyelid, anterior portion of the globe
In the eye, where are foreign bodies often found?
At the conjunctival fornix’s
Give the name of the conductive, the location, and the colour:
What is found in the conjunctiva epithelium?
What lies under the conjunctivas epithelium?
How are the conjunctiva and tear film related?
- Conjunctiva nourished by the tear film
- Conjunctiva helps with tear film production (goblet cells producing mucin)
What CN supplies sensory innervation to the conjunctiva?
Ophthalmic branch
What does the bulbar conjunctiva lie on top of?
The sclera
What are these fine branching vessels?
Conjunctival vessels found on the most surface layer
What large straighter vessels can be seen in the image?
What would cause these to be visible?
The episcleral vessels
Inflammation and/or high pressure inside the eye
How does the conductive allow free movement of ocular structures whilst being protective?
Closely attached at eyelid margin and limbus but loose everywhere else
Why mist conjunctiva respond quickly to pathogens?
What stimulates the lymphocytes in the conjunctiva?
Why might conjunctiva be referred to as “everted lymph nodes”?
Most exposed mucous membrane
Antigens activate the follicles
Contains lots of layered, spread out (conjunctival-associated) lymphoid tissue, not in the usual solid round structure
How many lacrimal puncta do the following species have?
Cats - 2
Dogs - 2
Ruminants - 2
Rabbits - 1
Horses - 2
(2 = upper and lower)
How does the nasolacrimal system work (animal with 2 puncta)?
Tears drain into lacrimal puncta
They go into lacrimal canaliculi
These meet at the lacriminal sac
The sac drains into the nasolacriminal duct
This connects to the nasal punctum
What is blepharospasm?
Uncontrolled eyelid blinking, twitching or movement
What test during an eye examination allows you to view the third eyelid?
Retropulsion of the globe
–> pull down lower eyelid and gently push on upper part of globe
What tests/techniques can be used to examine the nasolacrimal system?
- Flush the nasolacrimal ducts by inserting a canal into the upper or lower nasolacrimal puncta
- Jones test - Flush die to see if it is draining out of the nose (drainage = positive result)
What can abnormal colour to the conjunctiva indicate as its a mucous membrane?
Anemia, jaundice, cyanosis, toxaemia …. (many other conditions)
What is entropion?
What is ectropion (what does it commonly lead to)?
Inwards turning of all or part of the eyelid, due to excessive skin around the eye
Outwards turning of the eyelid*, due to eyelids being too long (commonly leads to conjunctivitus)
*can sometimes also roll in due to excessive length
What is diamond eye?
Condition common in large breeds and bulldogs
Eyelid is so long it doesn’t fit eye at all
Can cause conjunctivitis or lead to entropion
Ligaments around the eye super loose
What is a macropalpebral fissure?
Palpebral fissure (gap between eyelids) is too big
(diamond eye also associated with this)
Identified by too much of the white of the eye being visible
Common in brachycephalic breeds
What is distichiasis?
What is ectopic cilia?
What is trichiasis?
Distichiasis – extra eyelashes emerging from meibomian gland orifices (edge of eyelid margin)
Ectopic cilia - arise from follicle inside or near meibomian gland (emerge through conjunctival surface of eyelid)
Trichiasis - normally located but abnormally directed hair contacting ocular surfaces (located correctly but still turning in)
What is cherry eye?
When the nictitans gland at the bottom of the 3rd eyelid prolapses
Common in dogs with shallow orbits
Treatment involves repositioning, NOT, removal
What is scrolled cartilage?
The T-shaped cartilage abnormally grows too long and will evert (or sometimes invert)
Surgery required to correct as 3rd eyelid can’t function properly causing debris build up
What conditions cause 3rd eyelid protrusion?
Exophthalmos
Enophthalmos
Microphthalmus
Ocular pain
Horners syndrome
(sedation/anaesthesia)
What is conjunctivitis?
Redness, swelling and discharge of the conjunctiva
Chemosis is severe conjunctivitis and causes extreme the swelling of the conjunctiva
(not all red and inflamed conjunctiva is conjunctivitis)
What is epiphora?
What is dacrocystitis?
Tear overflow (issue with nasolacrimal system)
Infection in the tear drainage system (common with rabbits due to molars being too long and pushing on the system)
What are the 3 layers of the eye and what are the composed of?
Fibrous (outer) layer
- sclera and cornea
Vascular (middle) layer
- uveal tract
Neuroectodermal (inner) layer
- retina and optic nerve
How thick is the tear film?
What pneumonic can be used to remember the layers of the tear film?
What are the layers of the tear film and what are their purposes?
7-8 um
L.A.M.E
Lipid - limits evaporation
Aqueous - most of volume
Mucin - helps spread and adherence of tear film on ocular surface
Epithelium - projections stick into tear film stabilising it
Where does the lipid layer of tear film come from?
Where does the aqueous layer of tear film come from?
Where does the mucin layer of tear film come from?
Meibomian glands
70% Orbital glands, 30% nictitans glands
Conjunctival goblet cells
What is the function of the tear film?
Provides nutrients and oxygen to ocular surface, protects against bacteria (mucin), desiccation, lubricates, and helps with the transparency and smoothness of the ocular surface
What are the layers of the sclera?
What is the sclera main composed of?
Lamina fusca - elastic fibres
Sclera proper/stroma - white fibrous CT
Episclea - loose CT, outer most layer, dense, fibrous, vascularised
Collagen and fibroblasts
What is the sclera and what covers it’s anterior portion?
Largest portion of the outer coat of the eye, its white or opaque
The bulbar conjunctiva
What is the limbus?
Where cornea, sclera and bulbar conjunctiva merge
Source of stem cells for the cornea
What is the tenons capsule?
How is it related to the episclera?
Connective tissue sheath that connects the bulbar conjunctiva to the sclera
They blend together anteriorly
How thick is the cornea and what would cause its thickness to change?
What is the basic structure of the cornea?
Very thin, 0.5-0.8mm, slightly thicker in middle
Age, breed, size of animal (larger = thicker), disease
Epithelium - 10% of thickness
Stoma - 90% of thickness
Endothelium - 1 cell thick
What cells can be found in the epithelium of the cornea?
How thick is the cornea?
Stratified (multiple layers), squamous (thin, flat), non-keratinised (no keratin)
50-60um (5-7 cell layers in cats and dogs, 12-15 in horses)
What is the function of the basement layer of the epithelium?
How thick is this membrane?
Maintain tissue architecture
40- 60nm thick
What is the basement membrane that separator the stroma and endothelium called?
What is its significance?
The descent membrane
Deep ulcers can reach it and it can then be seen
The cells in the epithelium of the cornea have tight junctions, why is this important?
Act as a permeability barrier against the tear film keep the cornea dehydrated
In the epithelium of the cornea how does cell turnover work?
Continuous cell turnover:
- new cells produced at limbus and move towards eye centre
- Bottom cells are differentiated, move towards surface and are lost into tear film
What is the purpose of the corneal endothelium and how does it do this?
Is the corneal endothelium self-renewing?
Stops water getting into stroma above from aqueous humour
Has an Na/K ATPase pump which pumps ions from the stroma into thew aqueous humour, water will therefore also follow this gradient
No, so number of cells decreases with age
What is the stroma in the endothelium composed of?
What happens if water gets into the stroma?
Collagen fibres (so few cells) which are same size, uniform, regularly spaced and dehydrated which provides the clear look of the cornea
The collagen fibres will no longer be dehydrated causing them to become cloudy
What secrets the descents membrane in the cornea?
Endothelium cells throughput life so it becomes slowly thicker throughout life
Why is the cornea clear and what is the reason for this?
- smooth surface
- no keratin, blood vessels or melanin
- dehydrated
- low cell density
- collagens regular arrangement
for light transmission and refraction
What are the two ways in which the tear film can be checked?
- corneal reflection should have sharp signs and reflect back exactly
- schirmer tear test, measures tear production
What is fluorescein staining?
Orange dye that turns green in alkaline tears
It sticks to and stains hydrophilic structures like exposed corneal stroma which allows the diagnosis of corneal ulcers
What is keratoconjunctivitus sicca (KCS)?
Inadequate tear production, often know as dry eye. Caused by bodies immune system atatcking the orbital and nictitan glands which leads to an over production of mucin (can then lead to secondary infection due to lack of antimicrobials in tear film).
STT = <15mm/min
What are clinical signs of KCS?
- conjunctivitis = mucoid discharge
- blepharospasm
- corneal ulceration
- corneal vascularisation (trying to get nutrients the tear film is no longer providing) and pigmentation
- reduced vision
What is a corneal ulcer?
What are the causes of corneal ulcers?
What are the 3 depths of corneal ulcers?
Break in continuity of corneal epithelium exposing underlying stroma
Trauma, eyelid abnormality, eyelash lesion, infection, KCS, degeneration/dystrophy
- Superficial (stroma exposed)
- Stromal (can see physical indent and blood cells trying to help)
- Descemetocoele (goes down to basement membrane which is strong but thin and doesn’t take the dye, can lead to membrane rupture if fluid starts to push on it (causing other structures to move forward and block the hole))
How does epithelium corneal wound healing occur?
Epithelium is self renewing due to constant cell turnover
Basal epithelium cells made at limbus - travel towards centre of cornea - turn into squamous cells - move towards surface and then lost in tear film
How does stroma corneal wound healing occur?
Epithelium regenerates, migrating across, into the ulcer
Fibroblasts then move in and produce new collagen fibre to thicken stroma
This requires vascularisation so blood vessels move in to help
Scar tissue left (collagen fibres not uniform) and remodelled over time until the cornea is clear again
What can cause blue corneal opacity?
Corneal oedema - fluid enters stroma disrupting collagen fibres (cobble stone appearance)
—-> Epithelium dysfunction - corneal ulcer
—-> Endothelium dysfunction - endothelial dystrophy, intraocular inflammation (more opaque)
What causes red corneal opacity?
Blood vessels entering the eye
- seen with ulcers and dry eye
What causes white corneal opacity?
Infection of the ulcer (cellular infiltration) or scarring
What causes black/brown corneal opacity?
Post chronic irritation
- common in pugs
When a horse blinks during the menace response what nerve is being checked?
The facial CN (Vll)
How does the pupillary light reflex work?
Optic nerve takes information from the eye to the brainstem then the oculomotor nerve takes action causing the pupil to dilate
What muscles may be affected if an animal has a strabismus?
Superior rectus muscle
Medial rectus muscle
Inferior rectus muscle
Inferior oblique muscle
What controls eye movement?
The oculomotor nerve
What has been affected if an animal has mydriasis (dilated pupils)?
What has been affected if an animal has ptosis (drooping upper eyelid)?
The Hillary body and sphincter pupillae, their opening regulated by sympathetic fibres
Levator palperbrae muscle
In the photo there are 3 types of strabismus, name the CN nerve responsible for each:
- Oculomotor
- Trochlear
- Abducens
