Regulation and Integration of the Body
sees two images instead of one. Tis condition is called
. It can result from paralysis
or weakness of certain extrinsic muscles, neurological disor-
ders, or it may be a temporary consequence of acute alcohol
Congenital weakness of the external eye muscles may cause
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.
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
(a) Diagrammatic view.
The vitreous humor is illustrated only in the bottom part of the eyeball.
Scleral venous sinus
(contains vitreous humor)
and vein of
(b) Photograph of the human eye.
Internal structure of the eye (sagittal section).