548
UNIT 3
Regulation and Integration of the Body
15
sees two images instead of one. Tis condition is called
diplo-
pia
(dĭ-plo
9
pe-ah), or
double vision
. It can result from paralysis
or weakness of certain extrinsic muscles, neurological disor-
ders, or it may be a temporary consequence of acute alcohol
intoxication.
Congenital weakness of the external eye muscles may cause
strabismus
(strah-biz
9
mus; “cross-eyed”), in which the affected
eye rotates medially or laterally. ±o compensate, the eyes may
alternate in focusing on objects. In other cases, only the control-
lable eye is used, and the brain begins to disregard inputs from
Te extrinsic eye muscles are among the most precisely and
rapidly controlled skeletal muscles in the entire body. Tis pre-
cision reflects their high axon-to-muscle-fiber ratio: Te motor
units of these muscles contain only 8 to 12 muscle cells and in
some cases as few as two or three.
Homeostatic Imbalance
15.4
When movements of the external muscles of the two eyes are
not perfectly coordinated, a person cannot properly focus the
images of the same area of the visual field from each eye and so
Ora serrata
(a) Diagrammatic view.
The vitreous humor is illustrated only in the bottom part of the eyeball.
Ciliary body
Ciliary zonule
(suspensory
ligament)
Cornea
Iris
Anterior
pole
Pupil
Anterior
segment
(contains
aqueous humor)
Lens
Scleral venous sinus
Posterior segment
(contains vitreous humor)
Optic nerve
Posterior pole
Fovea centralis
Macula lutea
Retina
Choroid
Sclera
Central artery
and vein of
the retina
Optic disc
(blind spot)
(b) Photograph of the human eye.
Ciliary zonule
(suspensory
ligament)
Cornea
Lens
Anterior
segment
Margin
of pupil
Iris
Ciliary body
Vitreous humor
in posterior
segment
Choroid
Fovea centralis
Optic disc
Optic nerve
Ciliary
processes
Retina
Sclera
Figure 15.4
Internal structure of the eye (sagittal section).
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