Eye Opener: third eyelid

Ocular Adnexa

Orbit

Figure 1: Lateral view of skull.
Source: Evans H.E. and Lahunta A. (2010) Guide to the dissection of the dog, 7th edn., Missouri: Saunders Elsevier.

Cavity housing eyeball and its adnexa among orbital fat
Frontal, lacrimal, maxillary, presphenoid, palatine and zygomatic bones of skull
Periorbita

Fibrous sheath of connective tissue and smooth muscle
Surrounds eyeball, its muscles, vessels and nerves

Orbital fascia

Figure 2: Schematic transection of orbital fasciae and orbital structures at the level of the eyeball.
Source: Dyce K.M., Sack W.O. and Wensing C.J.G. (2010) Textbook of veterinary anatomy, 4th edn., Missouri: Saunders Elsevier.

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Superficial muscular fascia

Lies within priorbita
Abundant adipose tissue
Envelops levator palpebrae superioris and lacrimal gland

Deep muscular fascia

Fibrous sleeve
Extends from eyelids and limbus
Surrounds eyeball, ocular muscles and optic nerve
Vagina Bulbi

Surrounds eyeball
Narrow episcleral space between eyeball and vagina bulbi à movement of eyeball against retrobulbar fat

Eyelids (Palpebrae)

Figure 3: Sectioned eyelid viewed obliquely from behind.
Source: Dyce K.M., Sack W.O. and Wensing C.J.G. (2010) Textbook of veterinary anatomy, 4th edn., Missouri: Saunders Elsevier.

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Arise from bony margins of orbit
Moisten cornea by distributing tears or lacrimal fluid
Blinking for protection against trauma
Eyelashes (cilia)

Protects eye from particles
Small ciliary and sebaceous glands

Levator palprbrae superioris elevates upper eyelid
Orbicularis oculi closes eye
3 smooth muscle sheets

Maintain normal eye protrusion and retracted eyelid position

Tarsus

Thickened fibrous edge formed by orbital septum of periorbita
Tarsal ‘Meibomian’ glands

Secrete lipid

Palpebral conjunctiva

Mucous membrane with goblet cells
Continues as bulbar conjunctiva on sclera and anterior epithelium on cornea
Conjunctival sac & fornix

Space between lids and eyeball
Fun fact: This is a hiding place for grass seeds

Highly vascular

Third Eyelid

Figure 4: The eye and ocular adnexal structures.
Source: McCracken T.O., Kainer R.A. and Carlson D. (2008) Color atlas of small animal anatomy: the essentials, 1st edn., Iowa: Lippincott Williams & Wilkins.

Nictitating membrane
Conjunctiva fold
Dorsoventral orientation between lacrimal caruncle and eyeball
Supported by a T-bar of cartilage
Kept retracted by smooth muscle (m. orbitalis) and sympathetic nervous system
Gland of third eyelid
Eye protection and tear distribution

Tears enter eye on bulbar surface

References

Dyce K.M., Sack W.O. and Wensing C.J.G. (2010) Textbook of veterinary anatomy, 4th edn., Missouri: Saunders Elsevier.
Evans H.E. and Lahunta A. (2010) Guide to the dissection of the dog, 7th edn., Missouri: Saunders Elsevier.
McCracken T.O., Kainer R.A. and Carlson D. (2008) Color atlas of small animal anatomy: the essentials, 1st edn., Iowa: Lippincott Williams & Wilkins.

Glands of the eye

Figure 1: Superficial anatomy of glands and ducts of the canine eye.
Source: McCracken T.O., Kainer R.A. and Carlson D. (2008) Color atlas of small animal anatomy: the essentials, : Blackwell Publishings.

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Figure 2. Superficial anatomy of glands and ducts of the feline eye.
Source: McCracken T.O., Kainer R.A. and Carlson D. (2008) Color atlas of small animal anatomy: the essentials, : Blackwell Publishings.

Lacrimal gland

Small, flat lobular structure lying on the medial side of the orbital ligament within the periorbita
Small ducts that cannot be seen without a microscope empty their secretion into the conjunctival sac at the dorsal fornix
Tubuloacinar gland on the episclera in the superior temporal orbit. These glands produce approximately 60% of the serous portion of tears in dogs

Third eyelid gland

Tubuloacinar gland on the base of the third eyelid that produces approximately 40% of the serous portion of the tears

Lacrimal gland and third eyelid gland are mixed mucoserous tubuloacinar glands which are responsible for production of the aqueous portion of the tear film.

Meibomian (tarsal) gland

About 20 to 40 glands exist in the superior and inferior eyelid
Synonymous with tarsal gland. The eyelid glands produce the lipid layer of the tear film which prevent evaporation of tear film
Lipid secreting glands located within the superior and inferior eyelid that produces the outermost layer of the tear film on to the eyelid margin. The ducts exit the eyelid margin to deliver the lipid component of the tear film

Moll glands

Modified sweat glands

Zeis glands

Modified sebaceous glands associated with the cilia

Moll glands and Zeis glands are located within the eyelid and contribute minimally to the tear film formation.

Conjunctival goblet cells

Conjunctival goblet cells are mucin secreting glands located within the palpebral conjunctiva. Mucin produced by these glands contribute to the innermost layer of the tear film

Figure 3. Canine nasolacrimal duct

Source: McCracken T.O., Kainer R.A. and Carlson D. (2008) Color atlas of small animal anatomy: the essentials, : Blackwell Publishings.

Figure 4. Feline nasolacrimal duct