The Path of Light and Sound By: Kristin Jaskowiak The organ that gives us a sense of sight is our eyes. Of the five senses, our eyes allow us to learn the most about our surroundings. It is important to know the structures and anatomy that make up the eyes to gain a better understanding of how light and images are processed to become sight. ANATOMY The lacrimal gland, or tear duct, produces tears when stimulated by the facial nerve. The delicate, transparent mucous membrane that covers the inside of the eyelids and the anterior surface of the eye is the conjunctiva.

The sclera is the white, tough, fibrous connective tissue that makes up the whites of the eyes. The cornea is the window of the eye and helps to focus entering light rays. The iris is the colored portion of the eye. The muscles of the iris involuntarily increase or decrease the diameter of the pupil. The pupil is the dark, round central opening that allows light rays to enter the internal eye. The lens is the clear, flexible disk behind the pupil. The layer of tissue that lines the posterior cavity of the eye is called the retina.

Rods and cones are located in the retina. Rods are light sensitive cells that detect black and white and function in daytime and nighttime vision. Cones are light sensitive cells that detect color and respond to red, green, or blue light. The vitreous humor is the clear, gel-like substance that fills the posterior cavity of the eye. PROCESS OF VISION The process involves light waves that enter the eye through the cornea, then the crystalline lens, and then the vitreous humor till the reflected light waves can project onto the photoreceptors of the retina.

Due to the refraction of light on the cornea and lens, the image projected on the retina is inverted and reversed from left to right. The impulses from the retina converge to the optic nerve then to the brain where they are transferred into imagery. Continual adjustments of the lens and pupil regulate the entry and focusing of light. Moving on to sound… ANATOMY There are three components to the ear: the outer ear, the middle ear and the inner ear. All three are involved in hearing but only the inner ear is responsible for balance. The external ear consists of 3 parts: the auricle, the external auditory meatus, and the eardrum.

The auricle, or pinna, is the visible external ear. The external auditory meatus is the opening at the entrance to the external auditory canal. The eardrum, or tympanic membrane, covers the auditory canal and separates the external ear from the middle ear. The middle ear is an air filled space in the temporal bone that consists of the oval window, the auditory tube and the auditory ossicles. The auditory ossicles are made up of the malleus, incus, and stapes. They are 3 small bones bridge the eardrum and the inner ear, transmitting vibrations between these parts.

The auditory, or Eustachian tube, connects the middle ear to the throat. The internal ear is a complex system of communicating chambers and tubes called a labyrinth. The labyrinth consists of 3 semicircular canals, a cochlea, and a vestibule. The semicircular canals provide a sense of equilibrium, while the cochlea is the tightly coiled structure that is associated with hearing. The vestibule connects the two and contains two more balance and equilibrium related structures, the saccule and utricle. PROCESS OF HEARING The path that sound waves take upon reaching us is an interesting one.

First, sound waves enter the outer ear through the external auditory canal. The sound waves can also be described as vibrations. These vibrations are funneled into the canal until the eardrum is reached. When the sound waves strike the eardrum, it vibrates and transmits these waves to three tiny bones (ossicles) of the middle ear, which amplify the sound waves. The malleus (hammer), the incus (anvil), and the stapes (stirrup), are the names of the three bones. The stapes sends the waves through the oval window to the inner ear, where the cochlea is located.

The cochlea is filled with fluid. The cochlea contains the organ of Corti, which is made up of tiny hair cells. These cells are specialized sensory hearing cells, covered with fine hairs that project into the fluid of the inner ear. The fluid is moved by the vibrations. This stimulation of the hairs gives rise to internal changes in the sensory cells, which lead to the sound waves being converted into electrical nerve impulses. The impulses then travel to the brain through the vestibular and cochlear nerves, and the brain interprets the impulses as sound.