Our most important sensory receptors are the eyes and the ears. The eye is the primary organ for sight, and the ear is the primary organ for sound and equilibrium. Obviously, any impairment of either of these sensory receptors can be a traumatic experience and can cause serious disability.
The eyes contain the receptors for light stimuli and are the organs of vision. The eyes are protected within the eye orbits by surrounding bones, the eyelids, the eyelashes, the eyebrows, and the conjunctiva, or inner mucous membrane surface of the eyelids. The meibomian glands on the inner surface of the upper and lower eyelids produce lipidlike secretions to help keep the eye moist. When the eye blinks, the upper lid presses on the oil, pulling a sheet of oil upwards to coat the tear layer and keep it from evaporating. The lacrimal apparatus produces and removes tears. Tears help to keep the outer part of the eye and the conjunctiva moist. The eye is a hollow, spherical organ composed of three layers. The outer layer consists of the sclera, an opaque, white portion of the eye; and the cornea, an anterior window of the eye. The cornea bends the light rays as they pass through its convex curvature.
FIGURE. (A) The human eye and (B) internal anatomy of the eyeball.
The middle layer of the eye consists of the choroid coat, the ciliary body, and the iris. The choroid coat contains the blood vessels of the eye and melanin. The melanin absorbs light within the eyeball and prevents glare. The ciliary body contains the ciliary muscles, which contract and relax to change the shape of the lens of the eye and form a ring around it.
Ligaments hold the lens in place. The lens focuses the light rays precisely on the retina. The colored portion of the eye is the iris, and it controls the amount of light entering the eye by controlling the size of the pupil. The pupil is the opening in the center of the iris through which light passes to the lens.
The inner layer of the eye contains the retina, which lines the interior of the eye. It contains the rods (for light detection) and cones (for color detecting) and neurons. The macula is a yellow disk on the retina directly behind the lens. In the center of the macula is a small depression called the fovea centralis that contains only densely packed cones. The fovea is the area of the sharpest, bright-light vision. The sight impulses formed by the rods and cones are transmitted to ganglion neurons that converge at the optic disk forming the optic nerve. Blood vessels nourish the eye.
The space between the cornea and the lens is called the anterior cavity. The anterior cavity is filled with aqueous humor. The aqueous humor helps to maintain the shape of the cornea and is responsible for the pressure of the eye. Behind the lens is the posterior cavity, which is filled with a clear gel-like substance called vitreous humor. The vitreous humor helps to maintain the shape of the eye by pressing firmly against the wall of the eye. Refraction is the process of bending the light rays. Refraction is produced by the cornea and lens. There is further bending by the lens (accommodation) to provide fine adjustments focusing the image on the retina. Once the light stimulus is on the retina, it must convert into impulses that are sent to the brain via the optic nerve. In the brain, the impulses are interpreted as visual images.
FIGURE. Outer, middle, and inner ear structures in a frontal section through the right temporal bone.
The ears are the organs of hearing and equilibrium. The ear is divided into three parts: the external, the middle, and the inner parts. The external ear is the outer, funnel-like structure called the auricle or pinna, and the external auditory meatus is called the external auditory canal. Hearing begins in the external ear, where sound waves are carried through the auricle and canal. The middle ear consists of the tympanic cavity, the tympanic membrane, and three small bones called the ossicles (maleus, incus, and stapes). The tympanic membrane is a thin layer of skin on its outer surface, and on the inner surface, it is covered with mucous membrane. Sound is transmitted from the auditory canal through the auditory meatus. The sound is conducted by the change in pressure on the eardrum and then the three ossicles vibrate.
The auditory or eustachian tube connects the middle ear to the throat or nasopharynx and mouth. The tube helps maintain equal air pressure on both sides of the eardrum, which is essential for normal hearing.
The inner ear, embedded in the temporal bone, consists of three semicircular canals and a cochlea. The canals provide a sense of equilibrium, and the cochlea contains the organ of Corti, the hearing receptors. The space between the bony and membranous labyrinths is filled with perilymph, whereas the membranous labyrinth contains endolymph. These fluids are essential in the hearing function of the inner ear. In the inner ear, sound is conducted via the organ of Corti receptor cells and nerves.
When studying the material in this chapter, it will be helpful to refer to Figures, which indicate the major parts of the eye and ear.