Porth's Essentials of Pathophysiology, 4e

957

Disorders of Special Sensory Function: Vision, Hearing, and Vestibular Function

C h a p t e r 3 8

Epithelium

Bowman membrane

Cornea

Stroma

Descemet membrane

Endothelium

Dilator muscle Pigment layer Sphincter muscle

Iris

Pupil

Anterior chamber

Canal of Schlemm Trabecular meshwork

Conjunctiva

Lens

Ciliary body

Sclera

Lens

Ora serrata

Zonular fibers

Posterior chamber

B

Vitreous body

Retina

Choroid

Retinal blood vessel

Optic disk

Sclera

Lateral rectus muscle

Medial rectus muscle

Optic nerve

Fovea centralis

A

FIGURE 38-3. (A) Transverse section of the eyeball. (B) Enlargement of the anterior and posterior chambers of the eye, showing the layers of the cornea, the structures of the iris, the aqueous drainage system (trabecular meshwork, canal of Schlemm), and the ciliary process and ciliary muscle.

moves toward the retina (Fig. 38-3A). The cornea also contributes to the refraction (i.e., bending) of light rays and focusing of vision. Although transparent, the cornea consists of three cel- lular layers: an extremely thin outer epithelial layer, which is continuous with the bulbar conjunctiva; a middle layer called the substantia propria; and an inner endothelial layer, which lies next to the aqueous humor of the ante- rior chamber. 2 The substantia propria is composed of regularly arranged collagen bundles embedded in a muco- polysaccharide matrix. This organization of the collagen fibers, which makes the substantia propria transparent, is necessary for light transmission. Hydration within a limited range is necessary to maintain the spacing of the collagen fibers and transparency. The three layers of the cornea are separated by two important basement mem- branes: Bowman and Descemet membranes (Fig. 38-3B). Bowman membrane, which lies between the corneal epithelium and the underlying corneal stroma, acts as a barrier to infection. It does not regenerate; if damaged, an opaque scar forms that can impair vision. Descemet

membrane, which lies between the corneal endothelium and stroma, has a feltlike appearance and consists of interwoven fibers and pores. Unlike Bowman membrane, it regenerates readily after injury. The cornea is avascular and obtains its nutrient and oxygen supply by diffusion from the blood vessels of the adjacent sclera, the aqueous humor at its deep surface, and tears that bathe its surface. The corneal epithelium is heavily innervated by sensory neurons. Epithelial damage causes discomfort that ranges from a foreign body sensation and burning of the eyes to severe, incapacitating pain. Reflex lacrimation is also common. Conjunctivitis Conjunctivitis, or inflammation of the conjunctiva (i.e., red eye), is one of the most common eye disorders. 3–5 Most cases are due to viral or bacterial infections. Other causes include allergens, chemical agents, and physical irritants. The mode of transmission of infectious con- junctivitis is usually direct contact by way of the fingers,