The Ear & Hearing

ear200.jpgYour hearing system is active 24 hours a day. Hearing is not a sense that you can ‘turn off’; it works on several levels:

  • With our hearing we perceive background sounds, such as traffic noise, or more relevant sounds, such as the ringing of an alarm clock. What is generally most important, however, is the ability to hear speech– to understand and communicate.
  • Hearing allows us to distinguish among a multitude of sounds – from the soft rustle of leaves to complex sentences. When our hearing ability is reduced, we are no longer able to hear sounds optimally.
The ear is a complex sensory organ that consists of three main parts:

Outer Ear
The auricle, which is also known as the pinna, and the external acoustic canal comprise the outer ear. The external acoustic meatus is created by the combination of the fleshy tube which lays inside the bony tube. Easily discernable with the naked eye, the pinna can be detected along the outside of the head, and is varied mildly in shape and size from individual to individual. A basic structural frame of cartilage is then covered with a stretchy connective tissue, and finally covered with skin. The ridge of the pinna is known as the helix. The fleshy lower section that flexes and hangs free is known as the earlobe, and is the only section of the out ear that is free of the cartilage framework. The auricle is held to the head with ligaments, as well as a combination of muscles which lack strength or high definition. These muscles are attached to the outer ear along the superior, anterior, and posterior segments of the ear.

The auricle receives the necessary blood supply via posterior auricular artery and the occipital artery. These are extensions which branch off from the external carotid artery and the superficial temporal artery. The pinna is designed structurally to help encourage sound waves to enter the ear, with its mild funneling style it acts somewhat like a sound wave scoop. The external acoustic canal measures approximately 1 inch in length and appears to hold the shape of a near S. The canal itself has a slight incline, extending upward toward the tympanic membrane. The canal is lined with sebaceous glands and fine, thin hairs that nearly circle the entrance, which helps to keep foreign objects from entering the ear canal.

The ceruminous glands are those which are buried deeper in the ear canal, secrete wax for ear protection and seeps out wax to keep the tympanic membrane soft, pliable, and waterproof. The secreted wax serves as an additional barrier to small foreign objects entering the ear canal. Cerumen, also known as ear wax, is thought to be an insect repellant.

The tympanic membrane measures only about 1 centimeter in diameter. It is quite thin despite its double layering, and is really a dividing element designed by epithelial cells. It segregates the external acoustic canal from the middle ear. The outer concave layer is designed out of stratified squamous epithelium while the inner convex layer is designed of low columnar epithelium. A firm connective tissue layer rests in between the epithelial layers.

Outer Ear

 

Middle Ear
The tympanic membrane serves as a segregating point between the middle ear and the external acoustic canal of the outer ear. This creates an air filled chamber known as the tympanic cavity, where the middle ear is is laterally compressed. It can be located in the petrous part of the temporal bone. The middle ear and the inner ear are segregated by a bony structure known as the vestibular window and the cochlear window. The vestibular window is ovular while the cochlear window is circular. The tympanic cavity hosts two openings.

Connecting the tympanic cavity with the mastoidal air cells, which reside in the mastoid process of the temporal bone, is the epitympanic recess. In order to equalize the air pressure on both sides of the tympanic cavity, the Eustachian tube, also known as the auditory tube, serves as the connecting piece between the tympanic cavity and the nasopharnyx. Reaching out across the tympanic cavity, there are three auditory ossicles which reach the vestibular window. These are the smallest bones within the human body. From smallest to largest these bones are named the malleus, the incus, and the stapes. Ligaments maintain the position these tiny bones hold within the structure of the ear.

The tympanic membrane echoes the sound required to vibrate these tiny bones, which transmit sound waves detected via vibration of these bones. The sound waves carried on the vibrations reach the vestibular window. From the vestibular window, these vibrations reach a fluid filled cavity deep within the inner ear, stimulates the receptors, which transmits sound waves into the brain which is recognized as sound. The ossicles serve as a systematic lever amplifier to deliver the necessary sound waves. The sound waves are once again subjected to amplification as they leave the larger tympanic cavity and enter the smaller vestibular window. This system allow for amplification of about 20 times the starting volume.

Middle Ear

 

Inner Ear
The inner ear is called a labyrinth. The labyrinth itself is comprised of a bone shell known as the bony labyrinth, which is designed to offer protection for the membranous labyrinth it encompasses. There is a space in between theses two labyrinths which is filled with fluid known as perilymph. Perilymph is a secreted fluid, constantly supplied by the cells which line the bony canal. A second fluid can be found within the structure of the tubular maze inside the membranous labyrinth. This fluid is known as endolymph. Both fluids are equipped to help discern vibrations to assist in the process of hearing as well as help the body maintain a sense of balance and equilibrium. There are three structural cavities within the bony labyrinth, the vestibule, the semicircular canals and the cochlea. The necessary organs for hearing and maintaining balance are embedded and supported within these three regions.

Inner Ear

 

 

Did you know?

  • Your ears aren’t just good for helping you hear. They help you keep your balance as well. Near the top of the cochlea are three loops called the semi-circular canals. These canals are full of fluid that moves when you move your head. It pushes up against the cilia and sends messages to your brain that tells it how your body is moving.
  • You know that feeling of dizziness after you have been spinning around? Well, the fluid in you ears spun as well. That makes the cilia move in all different directions and confused your brain.
  • Children have more sensitive ears than adults. They can hear a larger variety of sounds.
  • Dolphins have the best sense of hearing amongst all the animals. They are able to hear 14 times better than humans.
  • Too much fluid putting pressure on your eardrums causes earaches. They are often a result of infection, allergies, or a virus.