THE EYE Anatomy
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Structure
Eyelids, Lacrimal Apparatus, and Orbital Cavity
Eyelids (A– C)
The eyeball is embedded in the orbit and is covered by the eyelids. The upper eyelid (A1) and the lower eyelid (A2) demarcate the palpebral fissure. The latter ends in the medial angle of the eye (A3) with a recess enclosing the lacrimal caruncle (A4).
In Oriental People, the upper eyelid continues medially
onto the side of the nose as a vertical fold of skin, the palpebronasal fold.
The fold can also be observed as a transitory formation in infants and is known
as epicanthus.
The eyelids are reinforced by firm plates of connective
tissue consisting of collagenous fibers, tarsus superior (B5) and
tarsus inferior (B6); the tarsal plates, or palpebral cartilages,
are attached to the margin of the orbit by the lateral palpebral ligament (B7)
and the medial palpebral ligament. The tarsal plates contain elongated tarsal
glands, the meibomian glands (C8), which spread over the entire
height of the eyelids. Their secretion prevents the tears from flowing over the
edge of the eyelids. They open at the posterior margin of the eyelid.
Several rows of eyelashes, the cilia (C10), emerge from the anterior
margin (AC9). The inner wall of the eyelids is lined by the conjunctiva
(C11), which extends to the anterior aspect of the eyeball at the conjunctival
fornix (C12). The smooth superior tarsal muscle (C13)
and inferior tarsal muscle (C14) (innervated by the sympathetic
nervous system), which control the size of the palpebral fissure, attach at the
tarsus. The eyelids are closed by the orbicular muscle of the eye (C15)
(facial nerve, p. 122). The upper lid is lifted by the levator muscle of the
upper eye- lid (BC16) (oculomotor nerve), which at-taches at the
upper margin of the optic canal. Its superficial tendinous membrane (C17)
penetrates into the subcutaneous connective tissue of the upper lid, while the
deep tendinous membrane (C18) attaches at the upper margin of the
tarsus.
Lacrimal Apparatus (B)
The lacrimal gland (B19) lies above the
lateral angle of the eye; it is divided by the tendon of the levator muscle of
the upper eyelid into an orbital part (B20) and a palpebral
part (B21). Its excretory ducts at the conjunctival fornix secrete
the lacrimal fluid (tears), which keeps the anterior aspect of the eyeball
continuously moist and collects in the lacrimal lake of the medial angle
of the eye. Here at the inner surface of each eyelid is a small opening, the lacrimal
point (B22), which leads to the lacrimal canaliculus (B23).
The canaliculi ascend and descend, respectively, and then turn at a right angle
to join and open into the lacrimal sac (tear sac) (B24),
from where the nasolacrimal duct (tear duct) (B25) leads
to the inferior meatus of the nose. Blinking not only results in uniform
moistening of the eyeball surface but also has a sucking effect on the flow of
tears by expanding and constricting the nasolacrimal duct.
Orbit (C)
The orbital cavity (eye socket) is lined by the
periosteum (periorbita) (C26) and is filled with fatty tissue,
the orbital fat body (C27), in which the eyeball (C28),
the optic nerve (C29), and the eye muscles (C30) are embedded. At
the anterior margin of the orbit, the fatty tissue is demarcated by the orbital
septum (BC31). The fatty tissue is separated from the eyeball by a
connective tissue capsule, the bulbar sheath (C32), which
encloses the sclera (C33). 35 Osseous wall of the orbit.
