- Ciliary body (muscle) and suspensory ligaments—
change shape of lens, which is made of a transpar-
ent, elastic protein and which refracts light. - Iris—two sets of smooth muscle fibers regulate
diameter of pupil, that is, how much light strikes
the retina. - Retina—innermost layer of eyeball; contains rods
and cones.- Rods—detect light; abundant toward periphery
of retina. - Cones—detect color; abundant in center of
retina. - Fovea—in the center of the macula lutea; con-
tains only cones; area of best color vision. - Optic disc—no rods or cones; optic nerve
passes through eyeball.
- Rods—detect light; abundant toward periphery
- Posterior cavity contains vitreous humor (semi-
solid) that keeps the retina in place. - Anterior cavity contains aqueous humor that
nourishes the lens and cornea; made by capillaries
of the ciliary body, flows through pupil, is reab-
sorbed to blood at the canal of Schlemm.
Physiology of Vision
- Refraction (bending and focusing) pathway of
light: cornea, aqueous humor, lens, vitreous
humor. - Lens is adjustable; ciliary muscle relaxes for distant
vision, and lens is thin. Ciliary muscle contracts for
near vision, and elastic lens thickens and has
greater refractive power. - Light strikes retina and stimulates chemical reac-
tions in the rods and cones. - In rods: rhodopsin breaks down to scotopsin and
retinal (from vitamin A), and an electrical impulse
is generated. In cones: specific wavelengths of light
are absorbed (red, blue, green); chemical reactions
generate nerve impulses. - Ganglion neurons from the rods and cones form
the optic nerve, which passes through the eyeball at
the optic disc. - Optic chiasma—site of the crossover of medial
fibers of both optic nerves, permitting binocular
vision. - Visual areas in occipital lobes—each area receives
impulses from both eyes; both areas create one
image from the two slightly different images of
each eye; both areas right the upside-down retinal
image.
The Ear (see Figs. 9–7 through 9–10)- Outer ear—auricle or pinna has no real function
for people; ear canal curves forward and down into
temporal bone. - Middle ear—eardrum at end of ear canal vibrates
when sound waves strike it. Auditory bones:
malleus, incus, stapes; transmit vibrations to inner
ear at oval window.
- Eustachian tube—extends from middle ear to
nasopharynx; allows air in and out of middle
ear to permit eardrum to vibrate; air pressure
in middle ear should equal atmospheric pres-
sure.
- Inner ear—bony labyrinth in temporal bone, lined
with membranous labyrinth. Perilymph is fluid
between bone and membrane; endolymph is fluid
within membrane. Membranous structures are the
cochlea, utricle and saccule, and semicircular
canals. - Cochlea—snail-shell shaped; three internal canals;
cochlear duct contains receptors for hearing: hair
cells in the organ of Corti; these cells contain
endings of the cochlear branch of the 8th cranial
nerve. - Physiology of hearing—sound waves stimulate
vibration of eardrum, malleus, incus, stapes, oval
window of inner ear, perilymph and endolymph of
cochlea, and hair cells of organ of Corti. When hair
cells bend, impulses are generated and carried by
the 8th cranial nerve to the auditory areas in the
temporal lobes. Round window prevents pressure
damage to the hair cells. - Utricle and saccule—membranous sacs in the
vestibule; each contains hair cells that are affected
by gravity. When position of the head changes,
otoliths bend the hair cells, which generate
impulses along the vestibular branch of the 8th cra-
nial nerve to the cerebellum, midbrain, and cere-
brum. Impulses are interpreted as position of the
head at rest. - Semicircular canals—three membranous ovals in
three planes; enlarged base is the ampulla, which
contains hair cells (crista) that are affected by
movement. As body moves, hair cells bend in oppo-
site direction, generate impulses along vestibular
branch of 8th cranial nerve to cerebellum, mid-
brain, and cerebrum. Impulses are interpreted as
movement of the body, changing speed, stopping
or starting.
218 The Senses