Abstract
Purpose:
To document the anatomy and function of the avian mebrana nictitans (MN) using gross dissection, light and electron microscopy. The MN is located at the medial canthus and it is used to clean, moisten, and protect the cornea by making extremely rapid excursions over the surface. We document the special anatomic features involved in MN function in birds.
Methods:
Globes and attached nictitans were submitted to COPLOW from a Bald Eagle, Haliaeetus leucocephalus, and a Mallard duck, Anas platyrhynchos. The eagle globe was dissected to show the path of the pyramidalis tendon as it travels from the pyramidalis muscle anchored on the posterior sclera to the ventral MN where contraction pulls the MN over the corneal surface. The duck head was sectioned in the frontal plane making step sections to document the anatomy. The MN of the eagle was examined by light microscopy and transmission electron microscopy (TEM).
Results:
We show the anatomy of the pyramidalis muscle and tendon and the support of the tendon by a sling from the quadratus muscle as it swings around the optic nerve. The quadratus is also anchored on the posterior sclera superiorly in the vertical plane. Histology of the eagle MN shows a collagenous stroma with significant elastin and richly endowed with nerves. The bulbar epithelium of the MN undulates and mucin is entrapped in a unique cellular process called the feather epithelium. The duck MN does not have feather epithelium possibly because the tear film would be under water, precluding the need for other MN structures. On TEM the conjunctival epithelium on the bulbar surface of the third eyelid presented two layers of short columnar cells. The superficial cells presented a centrally located villous projection with multiple perpendicular secondary projections producing a “fish spine” arrangement. The core of the villous projections contained longitudinally arranged cytoskeletal elements. The remaining surface was rich with microvillar projections.
Conclusions:
The avian MN functions to clean, moisten, and distribute tears to the corneal surface. Understanding the anatomy and function of the MN helps to understand one aspect of enhanced visual function in birds.
Keywords: 419 anatomy •
486 cornea: tears/tear film/dry eye •
526 eyelid