Tuesday, March 29, 2011
External Ear
In chapter 10, Shubin discusses ears. There are three parts to the ear: the external, the middle, and the inner. Shubin says that "the part of the ear that we can see... is a relatively new evolutionary addition to bodies" (159). Why did this part of the ear come to be? What is the purpose of the external ear? Also, mammals are the only animals that have pinna (a flap of the external ear). Earlier animals, like bony fish, amphibians, and reptiles don't have this pinna. Fish don't even have an external part to the ear. Why are mammals the only animals with pinna, and what is the purpose of this? Danielle Webb (dwebb456@gmail.com)
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Mammals are the only animals with pinna; this may be because this structure of the "funnel-like shape" and the "complex folds of skin and cartilage" helps in the external ear's purpose of guiding noise to the eardrum. It also allows for sound to be amplified, to be heard more clearly by the animal. The outer, visible part of the ear is also lined with tiny hairs and cerumen, a yellow, "wax-like" substance. This helps in preventing dust and other foreign substances from entering and reaching the eardrum. The reasons for the evolution of this external ear in mammals may be due to the fact that hearing in air and water is different. In water, the perceived sound is wholly dependent on the "tonality rather than the volume" of the sound. Hearing can often be impeded underwater at extreme tones, especially deep under the surface, which can often be a problem for divers. Thus, since the majority of mammals live on land, during this transition to a terrestrial environment, it became beneficial for mammals to have this external ear, which facilitates the traveling of the sound to the eardrum, as well as allows for the sound to be amplified. Furthermore, the overall structure of the ear also became more complex in mammals, with a three-boned ear rather than one, like in reptiles and amphibians. As Neil Shubin explains, this allows for mammals to hear sounds of "higher frequency" (Shubin 162). Mammals may be more dependent on sound for survival (detecting predators, food sources, etc.) since most live on land, rather than in the water, and thus the ear developed over time to have a more intricate strucute, one which included structures such as the pinna.
ReplyDeleteKathy Li, kathy2132@gmail.com
Sources:
http://www.globalbioweather.com/function_of_the_pinna.html
http://library.thinkquest.org/28170/36.html
The pinna is also known as the external/outer ear and this is essential for collecting sound waves and directing them into the ear through the middle ear and the inner ear. Sound travels in the form of mechanical waves that are generated by the differences of pressure applied to another object (“Sound”). Not all frequencies can be heard by humans, only the ones between 20 Hz (hertz) and 20,000 Hz (“Sound”). While humans can perceive these frequencies, other animals such as dogs can perceive frequencies higher than 20,000 Hz, and this depends on the animal and its necessities for the kinds of sounds it should be able to hear for its protection and survival.
ReplyDeleteThe auricle, the very outer part of the outer ear, also known as the area with the grooves, is responsible for collecting sounds from external sources. The outer ear then directs these sound waves through the middle ear which is about 1 inch long. When people clean their ears, this is the area that they are cleaning. The cilia trap all of the dust and other particles that are trying to seek their way in.
Sound then travels through them middle ear. The middle ear is also connected to the rear end of the nasal cavity through a 32 mm long, and 2mm wide tube called the Eustachian tube (Pasquesi). This tube is responsible for sending some of the air we breathe into the middle ear to keep it healthy. However, the tube remains closed most of the time to prevent infections from traveling through the throat and sinuses. Also, in addition to the trapped particles, there is also a fluid that tries to get rid of the “dirt” in the ear. This fluid is generated in the inner ear in the semicircular canals and the cochlea. The Pharyngeal Orifice is located about the hard palate of the nasal cavity and it is responsible for draining the fluid in the middle ear down the throat to the stomach. This also helps in equalizing pressure after you have yawned or swallowed (Pasquesi). Sound from the outer ear travels to the middle ear through the ear drum. The vibrations of the ear drum are sent through the three tiny bones called the Mallus, Incus, Stapes, or Hammer, Anvil, Stirrup, or Ossicles (Hain).
After the vibrations reach the stapes, the vibrations finally enter the inner ear. Their first stop is the semicircular canals which are responsible for balance and they achieve this through the amount of fluid in the canals. Then, the cochlea, also known as the snail’s shell, is responsible for transduction where the sound pressure impulses from the outer ear are converted to electrical impulses which can be decoded by the brain (Nave). These electrical impulses are sent to the brain through the auditory canal. The auditory canal takes these impulses to the temporal lobes located by the ears and the lobes truly decode the sounds and this is when we understand what we are hearing.
Mammals are the only ones with a pinna because they do not have the capacity to move and direct their ears to the source of the sound. All they can do is move their face or body but not their ears alone. Other animals such as a dog, can move its ears in the direction they choose, so the auricle and the outer ear is not mandatory in order to capture sound waves. Not only this, but some aquatic mammals can use their pinna to generate heat. This serves as an adaptation for attracting other mates or for their own protection.
Chandrika D.
(shiningstar0393@gmail.com)
Hain, T. “Ototoxicity From Ear Drops”. September 18, 2010. Retrieved on March 30,
2011, from
http://www.dizziness-and-balance.com/disorders/bilat/ototoxic_drops.htm
Nave, R. “The Inner Ear”. Retrieved on March 30, 2011, from
http://hyperphysics.phy-astr.gsu.edu/hbase/sound/eari.html
Pasquesi, A. “How Are Your Ears Connected to Your Throat?”. 2011. Retrieved on
March 30, 2011, from
http://www.ehow.com/how-does_5415609_ears-connected-throat.html
“Sound”. 2011. Wikipedia. Retrieved on March 30, 2011, from
http://en.wikipedia.org/wiki/Sound