Eye Anatomy and Function — Week 3

Introduction

Eyeball housed in a bony orbit
Moved by extrinsic muscles
Protected by surrounding adipose tissue, lacrimal glands and eyelids
Responsible for sense of vision
- Binocular vision: occurs where visual fields of both eyes overlap; allows for focus on near objects and for depth perception; greater in $3$ carnivores than in their prey (i.e. herbivores)
Visual fields of cat and rabbit: 1 – binocular vision; 2 – monocular vision; 3 – blind area; 4 – visual axis (Dyce, Sack and Wensing’s Textbook of Veterinary Anatomy, 5th edition. Fig 9-1).

Eyeball structure

consists of:
- three $3$ thin tunics: - outer fibrous tunic - middle vascular tunic - inner nervous tunic
- lens for focusing light on receptors
- partly liquid (aqueous humour), partly gelatinous (vitreous body)
centre
Fibrous tunic
- dense collagenous tissue
- resists internal pressure
- consists of sclera (posteriorly) and cornea (anteriorly) which meet at limbus
Sclera
- opaque
- penetrated by fibres 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 which allows the passage of light into the eyeball
- covered by anterior and posterior epithelial layers
- avascular – nutrition by diffusion from lacrimal fluid (tears) and aqueous humour of anterior chamber
- very sensitive due to nerve endings (branches of ophthalmic division of CNV)
Sagittal section of a dog’s eye - Bacha & Bacha, 2000, p. 249, Fig. 19.1. Green – fibrous tunic; red – vascular tunic; blue – nervous tunic.

Vascular Tunic (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 by reflecting light back towards the retina
Tapetum lucidum note: not present in pigs or humans

Ciliary body

thickening of choroid anteriorly in radial ridges (ciliary processes)
provides anchoring point for zonular fibres (ciliary zonula) which suspend lens
ciliary muscle changes length of zonular fibres thus causes lens shape to change, required for focusing image on retina
produces aqueous humour

Iris

ring of tissue suspended between cornea and lens
opening in centre is pupil
pupil size controls how much light can pass through to the retina
consists of pigmented connective tissue covered with epithelium (pigmented posteriorly)
- connective tissue layer contains smooth muscle sphincter (circular – parasympathetic) and dilator (radial – sympathetic), which regulate pupillary size
separates the anterior chamber from the posterior chamber

Internal Tunic – Retina

contains light-sensitive receptor cells (posterior two-thirds of retina)
lines vascular tunic from pupillary margin posteriorly to optic nerve
thin outer layer (pigmented except over tapetum) and thick inner layer (neuroepithelial)
neuroepithelial layer (from outer to inner): photoreceptor layer; horizontal cells; bipolar cells; amacrine cells; ganglion cells

Layers and cells

Photoreceptor cells
- contain light-sensitive receptors in extended membrane of outer segment
- convert the light signal to a chemical signal then to an electrical signal to be transmitted at its synapse
- two types: $2$ types – rods (highly sensitive to light for night vision) and cones (colour perception)
- synapse with bipolar cells, which synapse with ganglion cells
Ganglion cells
- axons extend across inner surface of retina to optic disc where they meet to form optic nerve
- ganglion cells are cell bodies of CNII
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 to supply inner layers

Lens

soft, transparent, protein-rich biconvex structure
contains 2 epithelial layers:
- anterior cuboidal monolayer
- posterior epithelial monolayer differentiates into ‘lens fibres’ which run from anterior to posterior poles of lens and filled with crystallin proteins
surrounded by an elastic lens capsule (basement membrane)
avascular – nourished by aqueous and vitreous humour
tethered to the ciliary processes of the ciliary body by zonular fibres
function – focuses light into images on retina
Structure note (Cormack, 1987, p. 685, Fig. 25-6)

Aqueous Humour and Vitreous Body

Aqueous humour

fills space between cornea and lens (anterior and posterior chambers)
produced by epithelial cells on the surface of the ciliary body
passes from posterior chamber to anterior chamber
drains into venous sinuses in sclera at iridocorneal angle (filtration angle)
creates an intraocular pressure which can be measured through the cornea

Vitreous body

occupies space between lens and retina, generating much of the eyeball’s firm consistency
gel-like mass consisting of water and fine transparent collagen type II fibres filled with glycosaminoglycans
the fluid component of the vitreous body is referred to as the vitreous humour

Adnexa

Fasciae Periorbita

fibrous fascial sheath, which blends with 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 orbital fat (which provides cushioning of the contents of orbit against mechanical forces)

Extrinsic muscles

seven $7$ extrinsic muscles act together to move the eye within the 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 the visual axis
dorsal oblique:
- travels anteriorly along dorsomedial wall of orbit; tendon passes through trochlea (cartilaginous thickening in dorsomedial periorbita – acts as pulley), then inserts on dorsolateral aspect of fibrous tunic
- contraction pulls dorsolateral aspect of eye medially
ventral oblique:
- arises from ventromedial wall of bony orbit and passes laterally to insert on ventrolateral aspect of eyeball
- contraction pulls ventrolateral part of eyeball medially

Retractor bulbi

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

Orbicularis oculi

located within eyelids and around them
encircles the eye dorsally and ventrally
acts like a sphincter – closes eyelids when muscle contracts

Levator palpebrae superioris

originates on posterior wall of orbit and inserts in upper eyelid, within periorbita
raises eyelid

Eyelids

Upper and lower eyelids (palpebrae) – structures which when drawn across the eyeball protect the cornea, spread tears and stop light from entering the eyeball
meet at medial and lateral corners (commissures) which create medial and lateral angles (canthi)
surround palpebral fissure (opening through which eyeball is visible)
lacrimal caruncle – triangular prominence in medial canthus
formed from layers:
- skin
- musculofibrous layer – contains orbicularis oculi, fibrous periorbita, smooth muscle and tarsal plate
- tarsal plate – dense connective tissue which supports the eyelids
- tarsal glands (Meibomian glands) – modified sebaceous glands, open at free edge of lid
- conjunctiva – thin mucous membrane lining posterior surface of eyelids (palpebral conjunctiva) and reflected onto sclera (bulbar conjunctiva)
- conjunctival fornix – the point of conjunctival reflection
- conjunctival sac – the potential space between the palpebral conjunctiva and the cornea
cilia (eyelashes) – longer hairs found along the eyelid margin
lacrimal puncta (puncta lacrimalia) – tiny 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 (nictitating membrane)

between lower lid and eyeball
covered with conjunctiva on both surfaces
supported by a T-shaped piece of hyaline cartilage
gland of the third eyelid surrounds base of cartilage
secretes towards eyeball from posterior surface
while eyes open, normally held retracted by smooth muscle (sympathetic control)
provides additional protection and moisture for eyeball
Third eyelid and lacrimal apparatus (Dyce, Sack & Wensing, 1987, Fig. 9-15)

Lacrimation

lacrimal fluid distributed over anterior aspect of eye by blinking
required to moisten and nourish the cornea and flush away foreign objects
lacrimal secretion consists of three 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, i.e. space between palpebral and bulbar conjunctivae) - gland of third eyelid
- inner mucoid layer (from goblet cells in conjunctiva) – glycoprotein, binds tears to cornea
lacrimal fluid drains via lacrimal puncta and canaliculi to nasolacrimal duct

Vascular and Nerve Supply

Vascular supply

external ophthalmic artery – principal supply to eye
- branches from maxillary artery then penetrates apex of periorbita
internal ophthalmic artery – branches from rostral cerebral artery to supply CNII and spreads over retina from optic disc
venous drainage – primarily via several vorticose veins that emerge through sclera
- these drain into the ophthalmic plexus or into the facial vein or deep facial vein

Nerve supply to the eye and related structures

CN II (optic n.) – special somatic afferent nerve, carries perception of light from retina to brain
CN III (oculomotor n.) – general somatic efferent to many extrinsic eye muscles:
- dorsal, medial and ventral recti muscles
- ventral oblique muscle
- levator palpebrae superioris muscle
CN IV (trochlear n.) – general somatic efferent to dorsal oblique muscle only
CN V (ophthalmic and maxillary divisions of trigeminal n.) – ophthalmic branch primary sensory (general somatic afferent) to eye and orbit – including eyeball (especially cornea), eyelids and conjunctivae; maxillary branch innervates part of the superficial eyelids
CN VI (abducens n.) – general somatic efferent to lateral rectus muscle and retractor bulbi
CN VII (facial n.) – general somatic efferent to orbicularis oculi

Parasympathetic innervation

ciliary ganglion behind eyeball – site of most synapses for parasympathetic eye innervation
CN III (oculomotor n.) – general visceral efferent to smooth muscle of iris and ciliary body
CN V (ophthalmic branch) – a branch carries parasympathetic innervation to lacrimal glands
CN VII (facial n.) – also parasympathetic innervation lacrimal glands

Sympathetic innervation

exits cranial-thoracic spinal cord and ascends in vagosympathetic trunk to cranial cervical ganglion near tympanic bulla
postsynaptic neurons travel with CN V (ophthalmic branch) then join the ciliary nerve (branch of CN III) to supply smooth muscle of eye:
- periorbita (protrudes eyeball)
- pupillary dilator
- eyelids (keeps palpebral fissure open)
- third eyelid (retracts)

Eye Anatomy and Function — Week 3

Introduction

Eyeball structure

Vascular Tunic (uvea)

Choroid

Ciliary body

Iris

Internal Tunic – Retina

Layers and cells

Lens

Aqueous Humour and Vitreous Body

Aqueous humour

Vitreous body

Adnexa

Fasciae Periorbita

Extrinsic muscles

Rectus muscles

Oblique muscles

Retractor bulbi

Orbicularis oculi

Levator palpebrae superioris

Eyelids

Third eyelid (nictitating membrane)

Lacrimation

Vascular and Nerve Supply

Vascular supply

Nerve supply to the eye and related structures

Parasympathetic innervation

Sympathetic innervation

Further reading