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