C h a p t e r 3 8
Disorders of Special Sensory Function: Vision, Hearing, and Vestibular Function
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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,
Pupil
Conjunctiva
Posterior chamber
Retina
B
A
Choroid
Sclera
Lateral rectus
muscle
Fovea
centralis
Optic nerve
Medial rectus
muscle
Retinal
blood vessel
Vitreous body
Sclera
Lens
Optic
disk
Anterior chamber
Zonular fibers
Ciliary body
Ora serrata
Canal of Schlemm
Trabecular meshwork
Epithelium
Stroma
Cornea
Endothelium
Dilator muscle
Pigment layer
Sphincter muscle
Descemet membrane
Bowman membrane
Iris
Lens
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.
