464
UNIT THREE
Tear glands secrete excessively when a person is upset or
when the conjunctiva is irritated. Tears spill over the edges
of the eyelids, and the nose F lls with fl uid. When a person
cries, parasympathetic nerve F bers carry motor impulses to
the lacrimal glands.
The
extrinsic muscles
of the eye arise from the bones
of the orbit and are inserted by broad tendons on the eye’s
tough outer surface. Six such muscles move the eye in vari-
ous directions
(f g. 12.24)
. Although any given eye move-
ment may use more than one muscle, each one is associated
with one primary action, as follows:
1.
Superior rectus
—rotates the eye upward and toward
the midline.
2.
Inferior rectus
—rotates the eye downward and toward
the midline.
3.
Medial rectus
—rotates the eye toward the midline.
4.
Lateral rectus
—rotates the eye away from the midline.
5.
Superior oblique
—rotates the eye downward and away
from the midline.
6.
Inferior oblique
—rotates the eye upward and away
from the midline.
The motor units of the extrinsic eye muscles have the
fewest muscle F
bers (F
ve to ten) of any muscles in the body,
so they can move the eyes with great precision. Also, the
eyes move together so that they align when looking at some-
thing. Such alignment is the result of complex motor adjust-
ments that contract certain eye muscles while relaxing their
Medial
rectus
Superior
rectus
Superior
oblique
Lateral
rectus
(cut)
Inferior rectus
Inferior oblique
FIGURE 12.24
Extrinsic muscles of the right eye (lateral view).
bone, and then into the
nasolacrimal duct,
which empties
into the nasal cavity.
Glandular cells of the conjunctiva also secrete a tearlike
liquid that, together with the secretion of the lacrimal gland,
moistens and lubricates the surface of the eye and the lining
of the lids. Tears contain an enzyme,
lysozyme,
that has anti-
bacterial properties, reducing the risk of eye infections.
Lacrimal gland
Superior and
inferior canaliculi
Lacrimal sac
Nasolacrimal
duct
FIGURE 12.23
The lacrimal apparatus consists of a tear secreting
gland and a series of ducts.
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