Cornea

The cornea forms the fibrous layer of the anterior portion of the eye. It functions to refract light entering the eye. 

Summary

• location: anterior one-sixth of the eyeball
• blood supply: avascular
• innervation: long ciliary nerves 
• relations: continuous with the sclera posteriorly and covered by the conjunctiva anteriorly

Gross anatomy

Anteriorly, the cornea is a convex structure covered by bulbar conjunctiva. The curvature is greater than that of the rest of the eyeball covered by the sclera. Posteriorly, the cornea is concave and round. It is made up of five layers. 

Transparency 

Transparency of the cornea is due to the regular arrangement of the collagen fibrils in lamellae in the stroma. Blood vessels and lymphatics are absent from the cornea to permit pellucidity.

Arterial supply 

The cornea is largely avascular. The peripheral parts are related to capillaries from the anterior ciliary arteries. It is nourished by the aqueous humor and gaseous exchange occurs through the tear film. 

Venous drainage

The cornea is largely avascular. 

Lymphatic drainage

The cornea is an alymphatic organ. 

Innervation

Densely innervated by the long ciliary nerves, branch of the nasociliary nerve, which is derived from the ophthalmic division of the trigeminal nerve.

Histology

Histologically, the cornea is made up of 5 layers (from superficial to deep):

1. epithelium
2. Bowman's membrane
3. stroma (substantia propria)
4. Descemet's membrane
5. endothelium 

Radiographic features

Imaging of the cornea can be performed by a number of techniques, including:

• corneal topography: characterizes the shape of the cornea qualitatively and quantitatively
• anterior segment optical coherence tomography (AS-OCT): produces a cross-section image of the cornea and other anterior segment structures
• ultrasound biomicroscopy: high-frequency and high resolutions are required

In general, diagnostic radiologists are not involved in corneal imaging outside the research setting. Most of these techniques lie outside the domain of a radiology department and are usually found in the ophthalmology department.

CT and MRI are not performed for imaging of the cornea.

History and etymology

Derived from the Latin cornea tela (“horny tissue”), itself from the root cornu (“horn”).

Related pathology

• astigmatism: refractive error due to aspheric or uneven cornea curvature
• keratoconus: most common corneal ectasia, is a progressive corneal condition characterized by central thinning and steepening of the cornea
• Fuchs endothelial dystrophy: non-inflammatory dystrophy of the cornea
• keratitis