Sense of Sight
A number of accessory organs assist the visual receptors in
the eyes. These include the eyelids and lacrimal apparatus
that help protect the eyes and a set of extrinsic muscles that
Visual Accessory Organs
Each eye, lacrimal gland, and associated extrinsic muscles
are housed in the orbital cavity of the skull. The orbit, lined
with the periosteums of various bones, also contains fat,
blood vessels, nerves, and connective tissues.
(palpebra) is composed of four layers—skin,
muscle, connective tissue, and conjunctiva. The skin of the
eyelid, the thinnest of the body, covers the lid’s outer sur-
face and fuses with its inner lining near the margin of the lid
(f g. 12.22)
The muscles that move the eyelids include the
levator palpebrae superioris.
Fibers of the
orbicularis oculi encircle the opening between the lids and
spread out onto the cheek and forehead. This muscle acts as
a sphincter that closes the lids when it contracts.
Sensory nerve fibers
(a) Head in still position
(b) Head rotating
) When the head is stationary, the
cupula of the crista ampullaris remains upright. (
) When the head is
moving rapidly, (
) the cupula bends opposite the motion of the head,
stimulating sensory receptors.
A crista ampullaris is in the ampulla of each