Anatomy of Eye, adnexa in a Nutshell

Eyeball: Hollow sphere

• 3 coats

• • Fibrous outer coat
    • Sclera: tough, dense fibrous coat that covers 5/6 of eyeball
    • Cornea: continuous with sclera, transparent
  • Vascular middle coat—three components are continuous
    • Choroids: highly vascularzied, most richly vascularized structure in body
    • Ciliary body: donut shaped, surrounds lens
      • Ciliary zonule of Zinn: radial fibers from ciliary body to lens
        1. Suspends lens
        2. Works with zonular fibers—radial (suspensory ligament of lens)
        3. See more about function in accommodation of lens section below
    • Ciliary processes projects from chillier body: produces aqueous humor
      • Humor flows from posterior to anterior chamber thru pupil
      • Nourishes and bathes cornea and lens (avascular)
      • Liquid flows between iriscornea angle→venous channels (sinus venosus sclera, canal of schlemm)
        1. 1. Channels make circle around perimeter of cornea
      • Fluid is replenished every 90 minutes
    • Iris: surrounds pupil (hole=aperture of camera)
      • • Controls amount of light entering by changing shape and size of pupil
        • Contraction controlled by inferior division of oculomotor (parasymp)
          1. Goes thru ciliary ganglion
          2. Dependent on circular, concentric fibers
        • Dilation: sympathetic control (long ciliary nerves)
          1. Dependent on radial fibers

• Neural inner coat: retina—posterior 5/6
  • Retina has two different layers: developed separately
    • Pigmented layer: fused with choroids
    • Neural layer: picks up light rays
      1. Axons of ganglion cells travel towards optic disk
      2. Connect to bipolar cells→rods and cones
    • Detached retina: separation of two layers
      1. Happens to prizefighters
    • Continuous with optic nerve at optic disk/papilla:
      • In the middle: blind spot with no photoreceptors
      • With the optic nerve, find vessels (arteries are branches of ophthalmic artery)
        • Travels within dura
        • Central artery of retina pierces dura, arachnoid, pia to enter substance of optic nerve
      • Macula lutea (lateral to optic disk): yellow spot
        • Pit: Fovea centralis—only has cones
          1. Area of acute vision
• Vitreous humor: holds retina in place (primary function)
  • Vitreous body: 2/3-3/4 of back of eye
  • Transmits light to retina
  • Pushes against lens
  • 99% H₂O (not replenished) + collagen fibers (replenished)
  • Separates and dries out over life
  • Retina can peel away→spots in vision
• Lens, suspensory ligament (zonular fibers)
  • Lens is normally clear
    • Can get cloudy as you age (cataract)
    • Treatment: replace lens by sucking out old eye with needle
  • Fibers changes shape of lens by contraction (more info below in accommodation of lens section)
• Common lesions
  • Papilldema: swelling of optic disk due to increase in CSF pressure
    • Disruption of arachnoid granulations
    • Closes off ophthalmic veins→doesn’t allow blood to drain out→veins swell up (detectable)
  • Conjunctivitis: inflammation of conjunctiva
  • Glaucoma: elevated pressure of aqueous humor in anterior chamber
    • Block of iriscornea angle
    • Cornea bulges
    • Lens pushed backwards, pushes on vitreous body→retinal vessels obstructed→death of retina→blindness
• How image is made in the retina?
  • Image formed by refraction (bending) of light rays
    • Rays converge on retina
    • Parameters of refraction
      1. Refractive index: related to density
         1. Change in density across interface affects speed of light and angle of refraction
    • Angle of incidence of light rays
      1. Affected by curvature of interface
• 3 surfaces where refraction occurs
  • Cornea: air/cornea interface
    • Cornea: curvature is constant (therefore index is constant)
      1. Can use laser surgery to shave off cornea to change curvature to change refractive index
      2. Most refraction occurs here because biggest difference in indices
    • Lens (anterior): aqueous/lens interface
      1. Lens changes shape: different indices
    • Lens (posterior): lens/vitreous interface
    • Parameters of total refractive power
      1. Refraction at all 3 surfaces
      2. Distance between cornea and lens
      3. Refractive index of aqueous humor
      4. Not additive

Accommodation

• Lens: gel substance in capsule
• More rounded on back
• Changes in lens curvature allows rays to focus on retina
• Contraction of ciliary muscles under parasymp ctrl (CN III)
  • Circular ciliary muscles contract→zonular fibers are relaxed→lens capsule are relaxed→lens deform toward spherical shape: bend light more, see closer objects (parasymp)
  • Circular ciliary muscles relax→zonular fibers are taut→lens capsule stretches→lens is flattened (symp)
• Lens elasticity and accommodation decreases with age: presbyopia
  • Become near sighted and far sighted
  • Need gradual lenses

Iris

• Pupil contracts (sphincter pupillae) to limit light entrance to central of lens
  • Increases depth of field for near objects
  • Contraction: parasympathetic (CN III)
  • Dilation: sympathetics

Eye lids:

• Tarsal plate (superior / inferior tarsus):
  • The core of the eyelid
  • Held in place to Lateral and medial side of the orbit by lateral / medial palpebral ligaments.
  • Anywhere have an opening, the Orbital Septum fills in the space

1. 1. • Attaches eyelids to rib of orbit
      • Is made of fascia of tarsal plates

• • • Is continuous with periorbita

1. 1. 1. 1. Some muscles attach directly to upper / lower edge of tarsal plates

• Superiorly: Superior tarsal muscle (Miller’s muscle)
• Inferiorly: inferior tarsal muscle

• These are smooth muscle structures

• These raise upper eye lid / lower the lower eyelid.
• Increase the size of the palpebral fissure (open the eyelids)

• Levator Palpebrae Superioris
  • On superior aspect of the eye, passing deep to frontal bone
  • Tendons of attachment pass out to the skin of the eyelid
  • Interdigitates with circular muscle in upper eye lid, called Ovicularis Oculi (palpebral part)
  • Elevates the upper eyelid
  • Some fibers come off inferior aspect and attach to the superior tarsal plate

1. 1. • This defines the superior tarsal muscle
      • superior tarsal muscle originates from levator palpebrae superioris and inserts in the superior tarsal plate

• Tarsal plate: Contains tarsal glands
  • Secretion prevents lids from adhering to eachother
  • Is a sebaceous gland (modified meiobian gland)
• Orbital septum: allows levator palpebrae fibers to pass through, elsewhere it acts as a barrier

Conjunctiva:

• Serous membrane that lines the inner aspect of the eye and part of the eye itself
• Provides thin film of moisture that allows lids to slide over eye w/o irritation
• Has 2 parts
  • Palpebral portion: lines the inner aspect of the eyelids
  • Bulbar portion: on the eyeball from the fornices to the corneoscleral junction.
• Stops at cornea
  • At center of eye
  • Covers 1/6 of the eye
  • Not covered by the conjunctiva
• Remainder white portion of eye (non-cornea surface) is the sclera

1. • Where the two parts of the conjunctiva come together:

• Superior / inferior fornices
• This is where palpebral conjuntiva reflects onto eye to become bulbar conjunctiva

• Innervation: All from CN V (trigeminal)
  • Palpebral portion: upper lid from V1, lower lid from V2
  • Bulbar portion: from V1 (ophthalmic)

Fat: inside the orbital around the optic nerve is either intraconal fat or extraconal fat

Orbital Fascia:

• Muscles of the eye have fascia
  • Deep fascia thickens as approach the eye
  • Wraps onto the eye and wraps back onto optic nerve
• Tenon’s capsule
  • This is the fascial sheath derived from extraoccular muscles
  • Thin fascia that envelops eye
  • Extends from optic nerve to corneoscleral junction
  • Perforated by tendons of extraoccular muscles (EOMs)
  • Continuous with deep fascia of EOMs
• Recent studies:
  • Show that the deep fascia of the EOMs not only extends onto the eye, but it also attaches to the walls of the orbit
  • As a result, the pull (or functional origin) of these muscles is as much on the orbital walls as at the muscle origins
  • This is important for surgical procedures to correct amblyopia (lazy eye)
• Thick portion of fascia extends to medial / lateral walls
  • Called Medial / Lateral Check Ligaments
  • These limit inward / outward motion of the eye
  • They are strong expansions of the fascial sheaths of the horizontal recti that attach the bony orbit.
  • Not the same as palpebral ligaments
• Suspensory ligament (lockwood) of the eye:
  • Blended fascial sheaths of the inferior oblique and inferior rectus muscles
  • Is a sling-like ligament for inferior aspect of eye
  • Are continuous with Tenon’s capsule and thus attach to the check ligaments
  • This creates a continuous fascial hammock below the eye

Lacrimal Apparatus

• The lacrimal gland is in the lateral / anterior / superior aspect of the orbit (laterally behind the upper eyelid just inside the orbit)
• Flow of tears from lacrimal gland to nasal cavity: Pathway
  • Excretory ducts of lacrimal gland (about 12) in the superior conjunctival fornixà
  • Wash over bulbar conjunctiva / corneaà
  • Gather in the lacrimal lakeà
  • As you blink, flows into openings within the medial edges of upper / lower lid called lacrimal puncta (as they touch lacrimal lake)→
  • Fluid enters lacrimal canaliculi→ empty into lacrimal sac → leads to canal that contains the nasolacrimal duct→ 
  • Leads to the inferior nasal cavity- into the inferior nasal concha, which is continuous with the inferior nasal meatus which finally receives tears.
• Innervation of the lacrimal gland (3 types)
  • Parasympathetic:

• CN VII (Facial Nerve) contains preganglionic parasympathetic fibers that leave facial nerve as the Greater Petrosal Nerve
• Greater Petrosal nerve passes through pterygoid canal→ leads to pterygopalantine ganglion (a parasympathetic ganglion in floor of orbit)

• In this ganglion, preganglionic fibers in the greater petrosal nerve synapse and form post ganglionic parasympathetic fibers→ rejoin V2 (maxillary nerve) for short time→ jump onto zygomatic nerve

1. 1. • From zygomatic nerve→ travel on the lacrimal nerve→ to the lacrimal gland.
      • These are secretomotor for the gland- cause secretion of tears.

1. 1. 1. • Sympathetic innervation:

1. 1. 1. 1. 1. Preganglionic cells from upper thoracic spinal cord → use chain
            2. Post ganglionic cells are in superior cervical ganglion→ jump onto internal carotid artery to form sympathetic plexus→ leave carotid plexus on the deep petrosal nerve→ pass right through the pterygopalantine ganglion (no synapse)à take similar path as parasympathetic and end up on the vasculature of the gland.

1. 1. 1. • Sensory fibers

1. 1. 1. 1. 1. From capsule of gland (not parenchyma)
            2. Lacrimal nerve is a branch of V1à goes through the gland

1. 1. 1. 1. 1. Carries sensory info from lateral upper eyelid and conjunctiva (bulbar and upper palpebral)

1. 1. 1. 1. 1. Lacrimal nerve as it approaches the lacrimal gland is joined by sympathetic and parasympthatic fibers

Extrinsic Muscles of the Eye (EOMs) (7)

• Levator palpebrae superioris – raises the upper eyelid
• Vertical Rectus muscles:
  • Superior rectus
  • Inferior rectus
• Horizontal Rectus muscles:
  • Medial rectus
  • Lateral rectus
• Oblique muscles: fibers run obliquely to attach eye itself
  • Superior oblique–
• Fibers approach the trochlea
• Tendon passes through the trochlea (acts as a pulley), turns sharply and attaches onto eye
• • Inferior oblique
• Innervation reminder: LR6 SO4 AO3
  • LR6: lateral rectus by CN VI
  • SO4: Superior Oblique by CN IV
  • AO3: All Others by CN III
• All muscles penetrate the fascial capsule to reach the eye
• Origins:
  • All rectus muscles (4) take origin from the Common tendinous ring (Anulus of Zinn / Anulus fibrosis)
  • Superior Oblique: from the sphenoid bone (posterior roof of orbit)
  • Inferior Oblique: takes origin from the maxilla (anterior floor of orbit)
• Insertions:
  • All into the Sclera of the eye
  • Spiral of Tillaux: each of the four rectus muscles inserts into sclera at increasing distances from the edge of the cornea, forming a spiral.

• Medial rectus is closest to cornea
• Superior rectus is farthest from cornea

• Actions of the EOMs
  • General rule: with the exception of the horizontal rectus muscles (medial / lateral rectus) the actions of the other 4 muscles depends on which way the eye is directed
  • Adductors: look in

1. Primary: Medial rectus- innervated by CN III Oculomotor
2. Inferior / superior rectus also contribute

1. 1. Abductors: look out

1. Primary: Lateral rectus: -innervated by CN VI Abducens
2. Inferior / superior obliques also contribute

1. 1. Elevators: look up

1. When looking straight ahead: use
2. Inferior oblique

• Superior rectus

1. 1. Depressors: look downward

1. When looking straight ahead: use
2. Superior oblique

• Inferior rectus

1. 1. Cross-Pairs rule:

1. You need a pair of EOMs to look either straight up or straight down
2. Rule: change to opposite name to get pair

• Look up: superior rectus works with inferior oblique

1. Look down: inferior rectus works with superior oblique

1. 1. Rotators: eyes rotate slightly to adjust for slight head tilting

1. Is not a voluntary action
2. Eye rotation helps to maintain a visual horizon when head is tilted

• When neck (head) is flexed laterally to one side, the eyes rotate in opposite direction several degrees.

EOM Testing:

1. Some EOMs have multiple and complex actions on the eye
2. Their actions depend on direction of gaze
3. Superior rectus
   1. CN III
   2. Look laterally 23 degrees and upward
4. Inferior rectus
   1. CN III
   2. Look laterally 23 degrees and downward
5. Lateral rectus
   1. CN VI
   2. Look laterally
6. Medial rectus
   1. CN III
   2. Look medially
7. Inferior oblique
   1. CN III
   2. Look medially (as far as possible) and upward
8. Superior oblique
   1. CN IV
   2. Look medially (as far as possible) and downward

EOM innervation:

1. Cranial Nerve III comes through superior orbital fissure within common tendinous ring and divides
   1. Superior branch: supplies

1. Levator palpebrae superioris
2. Superior rectus

1. 1. Inferior branch: supplies

1. Medial rectus
2. Inferior oblique

• Inferior rectus

1. Trochlear nerve CN IV
   1. Goes through superior orbital fissure
   2. Passes into superior oblique muscle far back into orbital
2. Abducens nerve CN VI – innervates lateral rectus

Visual problems

• Emmetropia: image focuses on retina—20/20 vision
• Myopia (near sighted): image focuses in front of retina
  • Eyeball is too long
  • Correct by diverging light with concave/diverging lens
    • Reduce power of cornea
  • Hyperopia (far sighted): image focuses behind retina
    • Eyeball is shortened
    • Correct by converging light with convex/converging lens
      • Increase power of cornea
    • Astigmatism
      • Irregular curvature of lens or cornea→poor focus
    • Strabismus:
      • Non-parallel visual axes