Notes:
I. Eyelids and associated structures
A. Palpebrae
1. Superior and Inferior palpebrae - eyelids proper
2. Palpebral fissure - opening between the upper and lower lids
3. Medial and lateral palpebral commisures - “corners” of the eye
4. Medial and lateral palpebral ligament - attaches commisures to the adjacent bone
a. Medial palpebral ligament is much more firmly attached than lateral one
5. Orbital ligament – fibrous connective tissue forming the lateral border of the orbit
B. Conjunctiva - mucous membrane lining the eyelids and covering parts of the sclera
1. Palpebral conjunctiva- Special mucous membrane lining the eyelids
2. Bulbar conjunctiva - special mucous membrane covering the globe of the eye
3. Fornix - point of reflection of the palpebral to bulbar conjuctiva
4. Conjunctival sac - space between the lid and the globe
C. Lacrimal gland and associated structures
1. Lacrimal gland - located deep to the periorbita on the dorsolateral aspect of the eyeball
a. Usually located beneath the orbital ligament in canine
2. Lacrimal caruncle - small triangular fleshy elevation at the medial commisure
a. Small and insignificant in dogs
b. Protrudes in equine
c. Just deep to medial commisure in bovine
3. Lacrimal puncta - tiny openings located 2-5 mm from the medial commisure on each eyelid
4. Lacrimal duct run medially parallel to the lid margins then enter the lacrimal sac
5. Lacrimal sac - located ventromedial to the medial commisure
a. The dilated origin of the nasolacrimal duct
6. Nasolacrimal duct - drains into the rostral nasal cavity near the nostril
D. Third eyelid - a fold of conjunctiva reinforced by a t-shaped cartilage plate located between the medial commisure and the globe of the eye
1. The third eyelid is usually hidden within the orbit, but it is large enough to cover the cornea
a. Superficial gland of the third eyelid - surrounds the base of the T-shaped cartilage plate
i. An accessory lacrimal gland
ii. Prolapse of the base of the gland produces “cherry eye”
iii. A deeper gland is also present in bovine
E. Muscles of the eyelid
1. Obicularis oculi - surrounds the eye and closes it
a. Innervated by the facial nerve (VII)
2. Retractor anguli oculi - covers the lateral palpebral ligament and helps close the lateral commisure
3. Levator palpebrae superioris - elevates the upper eyelid and is not attached to the globe
a. Innervated by the oculomotor nerve (III)
II. Eye and associated structures
A. Orbit - conical cavity containing the eyeball and ocular adnexa
B. Ocular adnexa - all the structures surrounding and pertaining to the eyeball
C. Periorbita - a cone-shaped sheath of connective tissue that encloses the eyeball and its muscles, vessels, and nerves
D. Extrinsic muscles of the eyeball
1. Two obliques- rotate the globe around the optic axis
a. Dorsal oblique - bends around a cartilage plate, the trochlea, before inserting on the eyeball beneath the tendon of the dorsal rectus
i Rotates the dorsal surface of the eyeball medially
ii. Innervated by the trochlear nerve (IV)
b. Ventral oblique - Originates from the pterygopalatine fossa
i. rotates the ventral surface of the eyeball medially
ii. Innervated by the Oculomotor nerve (III)
2. Four rectus muscles –insert around the equator of the eyeball
a. Dorsal rectus
i. Innervated by the Oculomotor nerve (III)
b. Ventral rectus
i. Innervated by the Oculomotor nerve (III)
c. Medial rectus
i. Innervated by the Oculomotor nerve (III)
d. Lateral rectus
i. Innervated by the Abducens nerve (VI)
3. One retractor bulbi (four fascicles)
a. Retractor bulbi - four small fascicles arranged in a cone around the optic nerve.
i. Withdraws the eyeball into the orbit
ii. Innervated by the abducens nerve (VI)
E. Bulbous oculi (Eyeball)
1. External fibrous coat - most superficial layer of the eyeball proper, except where it is covered by conjunctiva. The only complete tunic
a. Cornea - Anterior one fourth of the fibrous coat
i. Covers the iris and the anterior chamber
ii. Smaller radius than sclera
iii. Normally transparent and avascular
iv. Richly innervated
v. Refracts incoming light rays
b. Sclera - Posterior three fourths of the fibrous coat
i. Opaque
ii. Rich in elastic fibers
iii. Maintains the shape of the eyeball
iv. Cribriform area posteriorly allows passage of the optic nerve fibers from the retina
c. Limbus (corneoscleral junction) - area where anterior corneal epithelium is continuous with the bulbar conjuctiva
2. Middle vascular coat (uvea) - thick middle layer of the eyeball between retina and sclera
a. Contains blood vessels and smooth muscle
b. Nourishes globe
c. Controls shape of the lens and size of the pupil
i. Iris - most anterior part of the vascular coat. Colored part of the eye
1. Pupil - central opening in the iris
a. Two layers of smooth muscle in the iris (pupillary sphincter, pupillary dilator) act to change the size of the pupil
b. In the horse, both the upper and lower margins of the pupil (most evident on the upper margin) have an irregular excrescence (granula iridis) which has been interpreted as “shades” to limit the entry of light
ii. Choroid - thin dark-pigmented membrane inside sclera which lines the posterior portion of the eyeball
1. Orra serrata - junction of the choroid and the ciliary body
a. Seen as an undulating line in the overlying retina
2. Tapetum lucidum - avascular light-reflective layer of choroids superior to the optic disc
a. Causes eyeshine in headlights, etc.
b. Reflects low levels of light to improve night vision
iii. Ciliary body - thickest part of the vascular coat, between choroids and iris
1. Thick circular mound at the level of the limbus
a. Ciliary processes - radially arranged folds on the inner surface of the ciliary body
i. Epithelium secretes aqueous humor
3. Internal coat (retina) - deepest layer of the eyeball which functions in image formation and light management
a. Retina
i. Pars optica retinae - visual portion
1. Sensitive to light rays
2. Lines the caudal part of the eye (fundus), ending at the ora serrata
ii. Pars ciliaris retinae Epithelium covering the ciliary body
1. Nonvisual
2. Produces aqueous humor
iii. Pars iridica retinae - epithelium covering the posterior surface of the iris
1. Nonvisual
iv. Dilator pupillae - myoepithelial cells lining the retina
v. Optic disc - area where axons from the retina leave the eye as the optic nerve
1. No rods or cones present
2. May be in the tapetum lucidum or the tapetum nigrum ventral to it
vi. Fundus of the eyeball - the posterior portion of the eyeball which includes the area of the optic disc, tapetum lucidum, and adjacent nontapetal nigrum
4. Lens and associated structures
a. Lens - solid, but elastic enough to change shape
i. Transparent
ii. Biconvex
iii. Caudal to the iris
iv. Cataracts are opacities of the lens
b. Zonule - separates the posterior chamber from the vitreous body
i. Zonular fibers - suspensory ligaments which extend from the pars ciliaris retinae and attach the lens to the ciliary body
c. Vitreous body - fills the vitreous body
d. Vitreous chamber - posterior to the lens
i. 60% of the volume of the eyeball
ii. contains jelly-like vitreous humor which helps maintain the shape of the eyeball and holds the retina in place
e. Aqueous humor - fills the space between the cornea and the lens
f. Anterior chamber - between the cornea and the iris or pupil
i. Contains aqueous humor
ii. Deep in cats
g. Posterior chamber - narrow, located between the posterior surface of the iris and the lens
i. Contains aqueous humor
h. Iridocorneal angle - trabecular meshwork through which aqueous humor drains
i. Venous scleral sinus - the place where aqueous humor enters the venous system
j. Pectinate ligaments - meshwork of fibers which traverse the iridocorneal angle
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