Cynology & Anatomy

Understanding the Nictitating Membrane in Dogs: Function and Prolapse

The nictitating membrane (membrana nictitans, third eyelid) is a specialized fold of mucous membrane in the medial canthus of the dog's eye. It is located in the inner corner of the eye and is usually barely visible in a healthy, awake dog. The nictitating membrane can be drawn horizontally over the eye – in dogs, unlike birds and reptiles, not actively by muscles, but passively by retraction of the eyeball.

The third eyelid in dogs: understanding its function and prolapse

What is the third eyelid in dogs?

The third eyelid (membrana nictitans, nictitating membrane) is a specialized fold of mucous membrane in the medial corner of the dog's eye. It lies in the inner corner of the eye and is usually barely visible in a healthy, awake dog. The third eyelid can be drawn horizontally across the eye — in dogs, unlike in birds and reptiles, not actively by muscles, but passively through retraction of the eyeball.

The third eyelid serves several protective functions: it distributes the tear film, mechanically protects the cornea, and contains lymphatic tissue that contributes to local immune defense. The embedded third eyelid gland (glandula nictitans) produces 30–35% of the tear film.

Background + scientific context

Gelatt et al. (2013, Veterinary Ophthalmology, 5th ed.) describe the anatomy and clinical significance of the third eyelid: The third eyelid consists of T-shaped cartilage (hyaline cartilage), a conjunctival lining, and the deeply embedded glandula nictitans. When the bulbus oculi retracts (e.g., during swallowing, in pain, or with muscle weakness), the third eyelid is drawn forward passively. Bilateral third eyelid protrusion is a clinically important sign: it may indicate dehydration, malnutrition, pain, systemic disease, or neurological disorders (Horner's syndrome). Prolapse of the third eyelid gland (cherry eye) is a common condition in breeds with loosely attached glandular tissue.

Maggs, Miller, and Ofri (2013, Slatter's Fundamentals of Veterinary Ophthalmology) describe the function of the third eyelid for the tear film and associated pathologies: prolapse of the glandula nictitans leads to exposure and inflammation of the gland — therefore, surgical repositioning (not removal) is essential to preserve tear production. Adenomas of the third eyelid (third eyelid gland adenomas) can occur in older dogs. The third eyelid tissue can also develop tumors such as mast cell tumors or melanomas. The third eyelid flap technique (suturing the third eyelid over the cornea) is used therapeutically for deep corneal ulcers.

Bjerk (2004, Veterinary Clinics of North America, https://pubmed.ncbi.nlm.nih.gov/15105008/) describes ocular signs in general practice: a visible third eyelid is a common clinical finding that must be assessed in a differentiated way. Unilateral third eyelid protrusion with ptosis and miosis = Horner’s syndrome (sympathetic lesion). Bilaterally protruding third eyelids with a reduced general condition at the same time = check for systemic disease. Third eyelid visible in relaxed dogs during sleep — physiological.

Vitomalia position

The third eyelid is normal and protective — in disease, it should be repositioned, not removed. A visible third eyelid while the dog is awake is always a finding that needs an underlying cause to be identified. Horner’s syndrome and cherry eye are the most common clinical scenarios.

When does the third eyelid become relevant?

  • Visible third eyelid while awake: identify the cause
  • Unilateral protrusion + ptosis + miosis: Horner’s syndrome (neurological diagnostics)
  • Bilateral protrusion: systemic disease, dehydration, pain
  • Red mass in the medial canthus: cherry eye (third eyelid gland prolapse → reposition)
  • Deep corneal ulcer: third eyelid flap technique as a therapeutic option

Practical application

Causes of a visible third eyelid — differential diagnoses:

Finding Side Possible cause
Visible bilaterally, dog asleep Bilateral Physiological
Visible bilaterally, dog awake Bilateral Dehydration, pain, systemic disease
Prolapse with ptosis, miosis Unilateral Horner’s syndrome
Red mass at the medial canthus Unilateral or bilateral Cherry eye (gland prolapse)
Tumor visible on the third eyelid Local Neoplasia (biopsy required)

Third eyelid gland — principle: Removing the glandula nictitans causes lasting damage to the tear film — risk of keratoconjunctivitis sicca (dry eye). Repositioning is always the method of choice.

Common mistakes & myths

  • “The third eyelid must be removed if it is visible.” Incorrect — the third eyelid and its gland are essential for tear production. Removal often leads to chronic dry eye. A prolapsed gland should be repositioned, not excised.
  • “Seeing the third eyelid = my dog is ill.” In relaxed, sleepy dogs, a slightly visible third eyelid is physiological. When the dog is awake, it appears on both sides and persists — then action is needed.
  • “Horner is always a brain tumor.” Horner syndrome results from a lesion of the sympathetic nerve pathway — from the hypothalamus via the spinal cord to the eye. Causes include middle ear infection, neck trauma, and spinal problems. A brain tumor is possible, but not the most common cause.

Scientific status 2026

The anatomy and physiology of the third eyelid in dogs are well researched. Surgical repositioning techniques (pocket technique, Kaswan method) for third eyelid gland prolapse are an established standard. The third eyelid flap technique for corneal ulcers remains a recognized treatment option. Current research is examining the immunological functions of third eyelid-associated lymphoid tissue (NALT) in ocular inflammatory diseases.

Frequently asked questions

Why can I suddenly see my dog’s third eyelid?

Possible causes: Horner syndrome (unilateral + ptosis + miosis), systemic disease, dehydration, pain (bilateral), or third eyelid gland prolapse (red mass). If it remains visible while the dog is awake → veterinary examination.

What is cherry eye in dogs?

Cherry eye is the prolapse of the third eyelid gland (glandula nictitans) — visible as a red, fleshy mass in the medial corner of the eye. Affected breeds: Bulldogs, Beagles, Cocker Spaniels. Treatment: surgical repositioning, not removal.

Can the third eyelid simply be removed?

No — the third eyelid gland produces 30–35% of the tear film. Removal leads in the long term to keratoconjunctivitis sicca (dry eye) with chronic pain. Repositioning is always the correct procedure.

Related terms

Sources & further reading

  1. Gelatt, K. N., Gilger, B. C., & Kern, T. J. (Eds.) (2013). Veterinary Ophthalmology (5th ed.). Wiley-Blackwell. ISBN 9780813800660.

  2. Maggs, D. J., Miller, P. E., & Ofri, R. (Eds.) (2013). Slatter's Fundamentals of Veterinary Ophthalmology (5th ed.). Saunders. ISBN 9781437723670.

  3. Bjerk, E. L. (2004). Ocular disease in general practice. Veterinary Clinics of North America: Small Animal Practice, 34(3), 811–830. https://pubmed.ncbi.nlm.nih.gov/15105008/

Wissenschaftliche Einordnung

Gelatt et al. (2013, Veterinary Ophthalmology, 5th ed.) describe the anatomy and clinical significance of the third eyelid: The third eyelid consists of a T-shaped cartilage (hyaline cartilage), a conjunctival lining, and the deeply embedded nictitating gland. Upon retraction of the eyeball (e.g., during swallowing, pain, or muscle weakness), the third eyelid is passively drawn forward. Bilateral third eyelid protrusion is a clinically important sign: it can indicate dehydration, malnutrition, pain, systemic diseases, or neurological disorders (Horner's syndrome). Prolapse of the nictitating gland (cherry eye) is a common condition in breeds with loosely fixed glandular tissue.

Maggs, Miller, and Ofri (2013, Slatter's Fundamentals of Veterinary Ophthalmology) describe the function of the third eyelid in the tear film and pathologies: Prolapse of the nictitating gland leads to exposure and inflammation of the gland—therefore, surgical repositioning (not removal) is essential to preserve tear production. Adenomas of the third eyelid (nictitating gland adenomas) are possible in older dogs. The third eyelid tissue can also develop tumors such as mast cell tumors or melanomas. The third eyelid flap technique (suturing the third eyelid over the cornea) is used therapeutically for deep corneal ulcers.

Bjerk (2004, Veterinary Clinics of North America, https://pubmed.ncbi.nlm.nih.gov/15105008/) describes ocular signs in general practice: A visible third eyelid is a common finding in practice that requires differentiated assessment. Unilateral third eyelid protrusion with ptosis and miosis = Horner's syndrome (sympathetic lesion). Bilateral prominent third eyelids with a simultaneously reduced general condition = check for systemic disease. The third eyelid is visible in relaxed dogs during sleep – physiological.