Eyes and Ears PDF

Through the eyes, you see the world and all of its beautiful colours because receptors that are located in the retina convert images to be sent to and processed in the brain. The ears allow you to experience the different sounds around you. In this part, you will learn to identify the structures an...

Through the eyes, you see the world and all of its beautiful colours because receptors that are located in the retina convert images to be sent to and processed in the brain. The ears allow you to experience the different sounds around you. In this part, you will learn to identify the structures and functions of the eye and the ear. You will also define terms related to the eye, ear, and vision. **The Eye** The **eye (ocul/o; ophthalm/o)** is the receptor organ for the sense of sight. The eyes contain **photoreceptors **that respond to changes in light. The combining forms **optic/o** and **opt/o** mean vision or sight. The eyeball is sometimes called the \"**globe**\". There are three layers of the eyewall. Let us explore the different layers and structures of the eye in more detail. The sclera is the tough, white, fibrous tissue that covers the entire outside of the eye, except for the transparent anterior (front) portion. The sclera protects and maintains the shape of the eye. The **choroid**, which is the opaque, middle layer of the eyeball wall, provides blood supply for eye tissues. It is a part of the **uveal** layer of the eye. The retina is the innermost layer of the eye wall. It contains light-sensitive receptor cells called rods and cones. Rods are black and white receptors, and cones are colour receptors. Both types of receptors receive information and convert it to nerve impulses to be sent to the brain. Signals are triggered by a chemical reaction that occurs when light hits the appropriate receptor. **Macula (macula lutea) and Fovea Centralis** The macula is the yellowish area in the center of the retina that contains the highest concentration of receptor cells. This is the area of sharpest central vision because it contains both rods and cones. The fovea centralis, which is the pit in the middle of the macula and contains the highest density of cone receptors, is the area of sharpest colour vision. The cornea is the transparent anterior surface of the eye that extends like a dome over the iris and pupil. It focuses on the light rays that enter the eye. The conjunctiva is the transparent mucous membrane that lines the sclera and the underside of the eyelid. The conjunctiva does not cover the cornea. The iris is the pigmented, muscular layer that forms the pupil. The muscles of the iris contract to restrict the size of the opening (pupil). The colour of the iris is determined by the amount of melanin the iris contains. The pupil, which is the circular opening in the center of the iris, permits light to enter the eye. The lens is the clear, flexible, curved structure that focuses images on the retina. The ciliary body is a set of muscles and ligaments that adjust the shape and thickness of the lens to refine the focus. This body is located within the uvea/choroid layer. The optic disk is a small region where the optic nerve meets the retina. It contains no rods or cones and is considered a \"blind spot.\" The optic nerve is the second cranial nerve. It transmits nerve impulses from the retina to the brain. The aqueous chamber, which is the area between the cornea and ciliary body, is divided by the iris into anterior and posterior portions. It is filled with aqueous humor, which maintains the eye\'s shape and nourishes the intraocular structures. Aqueous humor is constantly drained to regulate the intraocular pressure. The vitreous chamber is filled with vitreous humor (or gel), a soft, clear, jelly-like substance that helps the eye maintain its shape. Unlike aqueous humor, vitreous humor is not naturally filtered or drained and replaced. The eye uses **refraction** to focus light and bend light rays. In the eye, this process is performed mainly by the cornea and lens. The muscles of the ciliary body produce a flattening of the lens to give you distant vision, and a thickening and rounding of the lens to give you close vision. It is the ability of the eye to adjust its focus on an object no matter the distance it lies from the eye. Light passes first through the cornea, then into the pupil which is regulated by the iris, the coloured part of the eye. Next, it hits the lens which focuses the light on the retina and then converts the image to be sent to the optic nerve where it finishes the process in the occipital lobe of the brain. That is the part of the brain that is responsible for vision only. The orbit is the bony cavity of the skull that contains and protects the eyeball and the adnexa. The orbit is sometimes called the eye socket. Three pairs of eye muscles are attached to each eye. The muscles of both eyes work together in coordinated movement. The simultaneous inward movement of the eyes as objects come nearer is called convergence. Convergence occurs to preserve binocular vision. The eyelids protect the eyes from foreign matter, excessive light, and impact. They also spread tears to cleanse the eyes. The upper and lower lids meet at the **canthus**. The inner canthus is closer to the nose, and the outer canthus is towards the side of the face. The eyelids, eyebrows and eyelashes all serve similar purposes in preventing foreign matter from reaching the eyes. Eyebrows keep sweat out of the eyes, and eyelashes protect the eyes from airborne particles. Because eyelashes are extremely sensitive to movement, they help to detect objects in close proximity, so the eyelid can close reflexively. **Lacrimal Glands** These structures secrete lacrimal fluid, or tears (lacrim/o; dacry/o). **Lacrimal Ducts** These ducts collect tears into the lacrimal sac. **Nasolacrimal Ducts** These ducts drain excess tears into the nasal cavity. The process of seeing begins when light waves enter the front of the eye. These light waves are interpreted for brightness and color by structures in the back of the eye and are then sent as signals to the brain, which interprets them as vision. On the outside of the eye is the **cornea**, which includes the **pupil**, a small opening through which light enters the eye. Once inside the eye, the light is bent, or refracted, by the **lens** and focused onto the **retina**, a layer of receptors that lines the inside of the eye. The retina contains two types of cells: - **Cones**, which interpret the colors of light. - **Rods**, which interpret the intensity (brightness) of light. These specialized photoreceptor cells convert the light information into nerve signals that travel through the **optic nerve** to the **occipital lobe** of the brain. In the brain, these signals are processed and interpreted as the images we see. **Career Spotlight** **Ophthalmologist:** A physician who specializes in diagnosing and treating disorders of the eye. **Optometrist:** Someone who specializes in measuring the accuracy of vision and prescribing corrective lenses. Optometrists are not medical doctors. They hold a Doctor of Optometry degree. - **Lens** - Bends light rays to focus them on the retina - **Retina** - Light-sensitive layer containing photoreceptor cells - **Rod** - Photoreceptor cells are essential for vision in dim light - **Cone** - Photoreceptor cells responsible for colour vision - **Sclera** - Tough, white outer coat of the eyeball - **Pupil** - Central opening of the eye - **Iris** - Allows more or less light into the eye - **Cornea** - Transparent anterior surface of the eye - **Conjunctiva** - Dedicated membrane under the surface of the eyelids **Combining Form** **Meaning** ---------------------- ----------------------------- optic/o; opt/o Vision phot/o Light ocul/o; ophthalm/o Eye scler/o Sclera corne/o; kerat/o Cornea conjunctiv/o Conjunctiva uve/o Uvea ir/o; irid/o Iris cor/o; pupill/o Pupil phac/o; phak/o Lens retin/o Retina aque/o Water vitre/o Glassy lacrim/o; dacry/o Tears blephar/o; palpebr/o Eyelid nas/o Nose -logist; -ist Specialist; one who studies -metry To measure -opia; opsia Vision condition - The ear is the receptor organ of hearing and balance, or equilibrium. The combining forms **acoust/o** and **audi/o** mean hearing. The ear is divided into three regions: **Outer ear** - Collects signals - **Middle ear** - Transmits and amplifies signals - **Inner ear** - Contains receptor cells for hearing and balance and transmits the signal information to the brain - **Pinna (auricle)** - The pinna collects sound waves and directs them into the auditory canal. - **External Auditory Canal** - The external auditory canal, which carries sound waves from the pinna to the middle ear, is lined with glands that produce cerumen, a material that lubricates and protects the ear. - **Tympanic Membrane (myring/o; tympan/o)** - The tympanic membrane, which is referred to as the eardrum, covers the inner end of the auditory canal and conducts sound to auditory ossicles by vibrating. - **Auditory Ossicles** - Auditory ossicles conduct and amplify sound waves. Three bones make up the auditory ossicles. They are the Malleus (hammer), the Incus (anvil), and the Stapes (stirrup). - **Eustachian Tubes** - The Eustachian tubes are narrow tubes that lead from the middle ear to the nasal cavity and throat. They open upon swallowing to equalize air pressure inside the middle ear with external air pressure. - The inner ear, or the labyrinth, holds the sensory receptors for hearing and balance. - **Oval Window** - The oval window is the membrane that separates the middle ear from the inner ear. The stapes rests on the oval window and vibrates when the tympanic membrane vibrates the ossicles. - **Cochlea (cochlea/o)** - The cochlea is a spiral, fluid-filled passage that leads from the oval window. It contains the Organ of Corti. The fluid in the cochlear duct vibrates when the membrane of the oval window vibrates. - **Semicircular Canals** - The semicircular canals contain receptors for movement and positioning that help the body maintain balance or equilibrium. When the liquid in the canals moves with the movement of the head, hairlike cells lining the canals also move and send impulses to the Central Nervous System (CNS) to help maintain balance or equilibrium. - **Career Spotlight** - **Otolaryngologist:** An otolaryngologist is an ENT specialist who focuses on head and neck surgery. These doctors specialize on the ear, nose, and throat regions. - **Audiologist:** An audiologist specializes in measuring hearing function and rehabilitating people with hearing impairments. +-----------------------------------+-----------------------------------+ | - **Combining Form** | **Meaning** | +===================================+===================================+ | acoust/o; audi/o | Hearing | +-----------------------------------+-----------------------------------+ | ot/o; aur/i; auricul/o | Ear | +-----------------------------------+-----------------------------------+ | myring/o; tympan/o | Eardrum | +-----------------------------------+-----------------------------------+ | staped/o | Stapes | +-----------------------------------+-----------------------------------+ | salping/o | Eustachian tube | +-----------------------------------+-----------------------------------+ | cochle/o | Cochlea | +-----------------------------------+-----------------------------------+ | laryng/o | Larynx | +-----------------------------------+-----------------------------------+ | **Suffix** | **Meaning** | +-----------------------------------+-----------------------------------+ | -cusis | Hearing condition | +-----------------------------------+-----------------------------------+