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13 Cards in this Set

  • Front
  • Back
What is binocular vision?
-occurs where visual fields of both eyes overlap
-allows for focus on near objects and for depth perception
-greater in carnivores than in their prey (ie. herbivores)
What does the eyeball consist of?
-consists of:
3 thin tunics:
*outer fibrous tunic
*middle vascular tunic
*internal nervous tunic
-lens for focusing light on receptors
-partly liquid, partly gelatinous centers
Describe the fibrous tunic
-dense collagneous tissues
-resists internal pressure
-consists of sclera (posteriorly) and cornea (anteriorly) which meet at limbus

✩SCLERA
-opaque
-penetrated by fibers of optic nerve
-continuous with dura mater of optic nerve
-provides attachment for tendons of extrinsic muscles of eye

✩CORNEA
-specialized transparent, dense connective tissue
-covered by anterior and posterior epithelial layers
-transparency dependent on:
→highly organized layers of collagen fibers
→continuous pumping out of interstitial fluid by posterior epithelium
-avascular → nutrition by diffusion from lacrimal fluid (tears) and aqueous fluid of anterior chamber
-very sensitive due to nerve endings (branches of ophthalmic division of CN V)
Describe the vascular tunic
-also known as uvea
-consists of choroid, ciliary body, and iris (from posterior to anterior)

✩CHOROID
-lines sclera from optic nerve almost to limbus
-contains blood vessels in pigmented connective tissue
-provides nutrients for outer layers of retina
-contains avascular tapetum lucidum:
→area of dorsal choroid
→reflective iridescent
→not present in pigs (or humans)
-probably assists in nocturnal vision

✩CILIARY BODY
-thickening of choroid anteriorly in radial ridges (ciliary processes)
-provides anchoring point for zonular fibers, which suspend lens
-ciliary muscles (smooth) at base of ciliary body:
→contraction causes relaxation of zonular fibers and rounding of lens → focus on near objects (accomadation)
-produces aqueous humour

✩IRIS
-ring of tissue suspended between cornea and lens
-opening in center is pupil
-consists of pigmented connective tissue with epithelium (pigmented posteriorly)
→connective tissue layer contains smooth muscle sphincter (circular - parasympathetic) and dilator (sympathetic) which regulate pupillary size
-separates anterior from posterior chamber
-colour dependent on amount of pigment
-species variation in shape of pupil when constricted
→ DOG = round, cat = vertical slit, horse, ox = oval
Describe the internal tunic
-contains light-sensitive receptor cells (posterior 2/3rds of retina)
-lines vascular tunic from pupillary margin posteriorly to optic nerve
-thin outer layer (pigmented except over tapetum) and thick inner layer (neuroepithelium)
♦photoreceptor layer = rods & cones
♦horizontal cells
♦bipolar cells
♦amacrine cells
♦ganglion cells - non-myelinated axons extend across inner surface of retina to optic disc, where they meet to form optic nerve ie. ganglion cells are cell bodies of CN II
-photoreceptors synapse with bipolar cells which synapse with ganglion cells
-horizontal cells modify transmission between photoreceptors and bipolar cells
-amacrine cells modify transmission between bipolar cells and ganglion cells
-blood vessels enter retina at optic disc with optic nerve and spread across retina
→supply inner layer of retina

✩PHOTORECEPTOR CELLS
-outer segment contains an array of stacked membranous discs with photopigments in membranes
-when light hits photoreceptor, photopigment is transformed, leading to hyperpolarization and inhibition of transmission of action potential to bipolar cells
-rods highly sensitive → adapted for night vision
-cones less sensitive to light, but responsible for colour perception
→dogs & horses have fewer rods and limited colour perception compared to humans
Describe the lens
-soft, transparent, biconvex structure
-composed of 'lens fibers' - epithelial cells running in layers from anterior to posterior poles of lens
-avascular - nourished by aqueous and vitreous humour
-focuses images on retina
-surrounded by an elastic capsule (basement membrane)
Describe the aqueous humour and vitreous body
✩AQUEOUS HUMOUR
-fills space between cornea and lens (anterior and posterior chambers)
-produced by cells of ciliary body
-passes from posterior chamber to anterior chamber
-drains into venous sinuses in sclera at iridocorneal angle

✩VITREOUS BODY
-occupies space between lens and retina
-gel-like mass consisting of stroma of fine transparent fibers filled with glycosaminoglycans and water
Describe the structure of the periorbita
-fibrous fascial sheath, which blends with the periosteum medially and dorsally
-inserts in eyelids and surrounds eyeball and extrinsic muscles of eye
-attached to skull near optic foramen
-contains smooth muscle which keeps eyeball slightly protruded with normal tone (under sympathetic control)
-contains and surrounded by periorbital fat (cushions contents of orbit)
Describe the extrinsic muscles of the eye
-seven extrinsic muscles act together to move eye within orbit
-all except ventral oblique originate on orbital wall near optic foramen
-insert on fibrous tunic

✩RECTUS MUSCLES
-dorsal, ventral, lateral, and medial rectus muscles

✩OBLIQUE MUSCLES
-rotate eye about visual axis

-dorsal oblique:
→travels anteriorly along dorsomedial wall of orbit; tendon passes through trochlea (cartilaginous thickening in dorsomedial periorbita - acts as pully), then inserts on dorsolateral aspect of fibrous tunic
→contraction pulls dorsolateral aspect of eye medially (ie. medial rotation)

-ventral oblique:
→arises from ventromedial wall of bony orbit and passes laterally to insert on ventrolateral aspect of eyevall
→contraction pulls ventrolateral part of eyeball medially

✩Retractor bulbi
-several slips of muscle which insert on posterior aspect of eye
-surrounds optic nerve
-retracts globe in socket
-absent in humans

✩LEVATOR PALPEBRAE SUPERIORIS
-originates on posterior wall of orbit and inserts in upper eyelid, with periorbita
-raises eyelid
Describe the eyelids
✩UPPER AND LOWER EYELIDS (palpebrae)
-meet at medial and lateral angles (canthi)
-surrounded by palpebral fissures
-formed by the following layers
♦skin
♦musculofibrous layer:
→contains orbicularis oculi, fibrous periorbita and smooth muscle
→tarsal glands - open at free edge of lid; secrete fatty material (contribution to lacrimal fluid)
♦conjunctiva
→thin mucous membrane lining posterior surface of eyelids (palpebral conjunctiva) and reflected onto sclera (bulbar conjunctiva)
-lacrimal caruncle → mucosal elevation at medial canthus
-puncta lacrimalia:
→minute slits on upper and lower lids adjacent to lacrimal caruncle
→openings to canaliculi leading to nasolacrimal duct (in lateral wall of nasal cavity)

✩THIRD EYELID
-between lower lid and eyeball
-covered with conjunctiva on both surfaces
-supported by T shaped piece of cartilage
-gland of third eyelid surrounds stem of T:
→secretes towards eyeball from posterior surface
-while eyes open, normally held retracted by smooth muscle (sympathetic control)
-provides additional protection and moisture for eyeball
Describe lacrimation
-lacrimal fluid distributed over anterior aspect of eye by blinking
-required to moisten and nourish cornea and flush away foreign objects
-secretion stimulated by conjunctival, corneal or nasal irritation
-lacrimal secretion consists of 3 layers:
♦outermost lipid layer (from tarsal glands) spreads tear film evenly and prevents evaporation
♦aqueous layer from:
→lacrimal gland (located on dorsolateral aspect of eye within periorbita; secretes through several fine openings into palpebral sac ie. space between palpebral and bulbar conjunctivae)
→gland from third eyelid
♦inner mucoid layer (from goblet cells in conjuctiva) - binds tears to cornea
-lacrimal fluid drains via lacrimal puncta and canaliculi to nasolacrimal duct
Describe the vascular supply to the eye
♦external ophthalmic artery - principle supply to eye
→branches from maxillary artery then penetrates the apex of periorbita
→gives off 3 groups of branches which penetrate the sclera at different levels to rach the vascular tunic and retina
*short posterior ciliary arteries
*long posterior ciliary arteries
*anterior ciliary arteries

♦internal opthalimic artery - supplies CN II and spreads over retina from optic disc

-venous drainage via several vorticose veins that emerge through sclera
Describe the nerve supply to the eye and related structures
CNII - perception of light

CNIII - somatic efferent:
- dorsal, medial and ventral recti
- ventral oblique
- levator palpebrae superioris
– visceral efferent:
-parasympathetic innervation to smooth muscle of iris and ciliary body

CNIV - dorsal oblique

CNV (ophthalmic and maxillary divisions) – sensory to eyeball (especially cornea), eyelids
and conjunctivae

CNVI - lateral rectus, retractor bulbi

CNVII – motor to orbicularis oculi

Sympathetic innervation
• exits cranial thoracic spinal cord and ascends in vagosympathetic trunk to cranial
cervical ganglion near tympanic bulla
• postsynaptic neurons travel with CNIII to supply smooth muscle of eye:
- periorbita (protrudes eyeball)
- pupillary dilator
- eyelids (keeps palpebral fissure open)
- third eyelid (retracts)