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Explain this

This text is from a veterinary surgical anatomy textbook, describing the nictitating membrane (third eyelid) - a structure found in animals but largely vestigial in humans.
Here's a breakdown of the key points:

Surgical Anatomy of the Nictitating Membrane (Third Eyelid)

What it is

  • A roughly triangular fold of mucosa located at the medial canthus (inner corner of the eye)
  • The base of the triangle forms its free/leading margin - the visible edge that sweeps across the eye

Surfaces

  • Bulbar (posterior) surface - faces the eyeball
  • Palpebral (anterior) surface - faces the eyelid
  • Both surfaces are continuous with the conjunctival mucosa

Internal Cartilage

  • Contains a T-shaped piece of cartilage inside the membrane
  • The "arms" of the T run along the leading margin
  • This cartilage gives the membrane structural support and helps maintain corneal contour

The Nictitans Gland (Gland of the Third Eyelid)

  • Also called the superficial gland of the nictitating membrane
  • Wraps around the base of the cartilage
  • Produces seromucoid tears
  • Its excretory ducts open onto the bulbar mucosal surface
  • This gland can be a major or minor contributor to the aqueous tear film layer - meaning it plays a significant role in keeping the eye lubricated

Lymphoid Follicles

  • Found on the bulbar surface
  • Appear as raised translucent spots
  • Part of the immune defense of the eye

Blood Supply

  • Comes from branches of the internal maxillary artery

Functions of the Nictitating Membrane

  1. Corneal protection - sweeps across and shields the cornea
  2. Tear film spreading - distributes tears evenly across the eye surface
  3. Tear production - contributes the aqueous layer to the preocular tear film

Clinical/Surgical Relevance

The last section notes positioning: the animal is placed in ventral recumbency (lying face-down) for surgical procedures involving the third eyelid. This is relevant for conditions like:
  • "Cherry eye" - prolapse of the nictitans gland
  • Follicular conjunctivitis
  • Trauma repair
This anatomy is most relevant in dogs, cats, rabbits, birds, and other animals where the third eyelid is prominent and clinically significant.
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