Ears


Ears


Sense Organs :

     There are two parts of auricle orpinna and external auditory meatus of external ear. Ears are very important organs of the body which do the work of hearing and make the body equilibrium. Sound is heard by the sense organ. 

     The formation of ear is very complicated, so it can be divided into following mentioned three parts in point of view of study.

  • External ear
  • Middle ear
  • Internal ear

External Ear:

     There are two parts of auricle orpinna and external auditory meatus of external ear. Auricle of pinna is the part of each which is attached to the head and it is made of flexible fibro cartilage and covered by skin. It is situated on both sides of the head. The size of auricle is zigzag and irregular.  Its outer region is called helix. The semicircular ridge inside the helix is called anthelix. Swallow layer situated in the upper portion is called triangular fossa and the deep part attached by the external auditory meatus is called concha. The small projection situated before the entry of external auditory meatus is called tragus. The hanging ear lobule is soft and is made of adipose connective tissue and there are many blood pipe in enough quantity in it. The auricle of ear defends the ear and help in sending sound inside the ear by collecting the rays of sound.

     The external auditory meatus is 2.5 centimeter long narrow piple and it goes to the tympanic membrane or ear drum from the tympanic membrane or ear in circular form of the English letter ‘S’ inside from the external auricle. It remains separate from the middle ear because of tympanic membrane.  Its one third-portion is made of cartilage and the rest of two third portions into temporal bone in the form of pipe which is made of bone. All the external meatus is coated by hairy skin which remain close by the skin that covers the auricle. There are sebaceous or ceruminous glands in the cartilagin portion of skin which secrete sebum (oily excretion) and ceriman (ear wax). External objects as insects and other foreign body are not able in reaching inside the ear because of ear wax, hair and zigzag external auditory meatus.    

Tympanic membrane:

     It is called ear drum too. It is a thin fibers seat in squarish cone shape which partition to the external ear and middle ear which remains in the continuation with the skin outwardly and mucus membrane that covers the middle ear inwardly. The tympanic membrane remains pressed towards the middle ear. This point is called umbo. At this same place, malleus situated at the middle portion remains attached inside the bone tympanic membrane. Most of margins of tympanic membrane are embedded in tympanic groove. This groove is situated on the surface of inner end of external ear. External and external auditory meatus send sound inwardly by collecting the waves of sound. When the waves knock to the tympanic membrane, vibration originates there which are sent intot he internal ear by the ossicles of middle ear.

Middle ear:

     There is a small chamber at the middle of middle ear, tympanic membrane and internal ear. In it there are tympanic cavity and auditory ossicles. 

     The tympanic cavity is a narrow air filled place of no shape situated in the petrous portion of the temporal bone. In it, there are frontal, last, middle and lateral four membranes and roof and floor which all are covered by mucus membrane. Thin plates of temporal bone make roof and floor which separate tympanic canal by middle cranial fosa on the up side and by vessels of neck on the below side. There is a hole in the posterior wall which opens in mastoid air cells and in it there is a small pyramid in the shape of cone of bone which is surrounded by stapedius muscle. The tendon of this muscle gets jointed to the stapes on the peak of pyramid by going through this hole. The lateral wall is made of tympanic membrane. The medial wall is a thin layer of temporal bone in which there is internal ear on the other side. There are two openings in it which are covered by membrane. The upper opening is in egg shaped which opens into the vestibule of internal ear and it is called fenestra ovalis. The second hole is circular which is called fenestra rotundum. This circular hole remains closed by the mucus membrane of middle ear and it is related to the caclia of internal ear. The anterior wall is made of temporal bone. In it, there is a hole close by the tympanic membrane which is the mouth of Eustachian tube. 

     Eustachian tube or auditory tube or pharyngotympanic tube make a relation between nasopharynx and canal of middle ear. The mucus membrane which covers to the middle ear covers this tube too in continuation. The middle ear has a relation with the mastoid air cells too. Often, throat infections reach into the middle of ear through this tube and can produce abscesses by reaching the cells of ear roots from here.

     The auditory ossicles are three small bones (malleus, incus or envil and stapes) which are situated from the tympanic membrane to the fenestra ovalis situated on the medieval membrane in a series. The end pf each bone (milieus) is jointed to the edge of other bone. The edge of first bone is attached to the tympanic membrane and the edge of last bone (stapes) is attached close by the fenestra ovails. The vibration produced by these waves of sound because of being a chain reaches to the internal ear by going through auditory ossicles and the external ear is attached to the internal ear.

Internal ear:

     It is a main organ of the ear which is situated in the petrous portion of temporal bone. In it, the organ of hearing and balancing are situated. The formation of internal ear is zigzag and complicated which appear like snail to some extant. In it, there are two major formation bony labyrinth and membranous labyrinth which remain inside of each other. Bony labyrinth situated at the petrous portion of temporal bone is a serioes of channels of tubes in zigzag and irregular size, which is filled with a liquid named Perilymph.

     The membranous labyrinth is situated in the bony labyrinth. It contains utricle, saccule, semicircular ducts and cochlear duct. There is a liquid named endolymph in all these formations and all the sensitive receptors of hearing and balancing are present. Semicircular ducts are present in the semicircular canals. There is a liquid named perilymph between the place of bony canals and ducts. Although, membranous labyrinth is situated inside the bony lebirinth and this is the reason that its size is same. This is the reason that the size of these is same. These have been described in bony labyrinth. 

There are three formations in bony labyrinth which has been mentioned below.

  • Vestibule
  • 3 Semicircular canals
  • Cochlea

Vestibule:

     It is the middle room of the labyrinth. There is Cochlea before it and semicircular canals behind it. There is a plate of stapes of middle ear in the egg shaped opening of external membrane of vestibule. The vestibule is directly attached to the middle ear through it. There is a veil of membrane attached in the same hole of this window between middle ear and vestibule. There are two sacs of membranous labyrinth filled with endolymph inside the vestibule. These are called utricle and saccule. Each sac has sensory patch which is called macule.

Semicircular canals:

     There are three bony labyrinth canals above, posterior and lateral of vestibule and they are attached to the vestibule. These three canals are perpendicular to one another. There are membranous ducts inside them and there is perilymph between the membranous semicircular ducts and bony semicircular canals and there is endolymph liquid inside the membranous ducts. The edge of each duct is some expanded which is called ampulla and there is sensory receptor crista ampullaris in it. There are hair cells named cupola in it. This is the place where the branch of eighth nerve of skull named vestibular nerve comes.

     The utricle, saccule and semicircular ducts are associated to sustain the equilibrium of body not to the activity of hearing. 

Cochlea:

     Cochlea is a serpentine duct which is covering on itself. It appears like the armor of snail. It situated at the middle of cone shaped bony axis pole which is called modiolus covers two or three times (2¾). The lower wheel is the biggest and the upper wheel is the smallest. The canal of cochlea is expanded to the fenestra rotunda a window with a hole by starting from the egg shaped opening window. In side the bony cochlea, there is a membranous cochlea duct of the same size. The liquid perilymph is filled in these two cannals.  

     Membranous cochlea gets divided into the three separate serpentine ducts in a perpendicular way by two membranes name vestibular and basilar.  

  • Scala vestibuli
  • Scala media or cochlear duct
  • Scala tympani

     Scala vestibuli is related to the vestibule and the end of scala tympani is on the window which has circular holes and the scala media is present between the other two ducts and endolymph is filled in it. There is perilymph in the scala vestibuli and scala tympani. These three ducts raise high by becoming in the form of serpentine by remaining arranged systematically parallel to the modiolus. There are spiral organ in the the Scala media which is called cochlear duct and these organs are called organ of corti.  

     Organ of Corti is an end organ of hearing which is made by the supporting cells and hair cells. The hair cells are arranged in the rows along with the total length of serpentine (kundali). External hair cells are arranged in three rows and internal hair cells are arranged in a single row along with the edges of basilar membrane. There are 3500 internal hair cells and about 20, 000 external hair cells in human being. There are sensory hairs or stereocilia like thin hair in these both internal and external hair cells. There are 80 to 100 sensory hairs in each external hair cells and there are 40 to 60 sensory hair in internal hair cells. Hairs are arranged in rows in each cell of hair which make the shape of English letter of ‘W’ and ‘U’. Tips of hairs make a perforated membrane by meeting altogether which is called tectorial membrane which is firmly bound over the spiral organ.

Nerves of the Ear:

     The vestibulocohlear nerve (eighth nerve) gets supplied into the ear. It is divided into two branches. One goes into vestibule and the second into cochlea. The branch which goes into the vestibule is called vestibular nerve. It is attached to the activity which keeps the body in equilibrium position. It transmits impulses into the cerebellum after getting stimulus.  

     The second nerve which goes into the cochlea is called cochlear nerve. It gets entry into the bony modiolus situated at the middle of cochlea and its many branches spread all around by reaching close to the organ of corti. The impulses brought by this nerve reach into the hearing center situated the temporal lobe of cerebrum where the sound is analyzed and we know the sensation of special sound.

Mechanism of hearing:

     Waves or vibrations take birth in the air because of sound and which transmit with the speed of 332 meter per second. Hearing of any word is possible only when the word has the capacity to produce waves in the air and then these waves may knock to the ears. The type of sound depends on the pitch, intensity, sweetness, harshness, frequency of the waves of air, size and form.

     The auricle of external ear gathers the waves of sound and transmits them into the ear by the external auditory meatus. It has been mentioned already. Thereafter, the waves of sound knock to the tympani membrane and produce the same vibration. The vibrations of tympanic membrane are transmitted by going through the middle ear because of the movement in the ossicles. The vibration of tympanic membrane reaches on the membrane attached on the fenestra ovalis in the egg shaped by going through the incus and stapes bones by the bones malleus attached close by it. After reaching vibration to this point, the waves take birth in the liquid perilymph situated in the vestibule. The perilymph situated in the scala vestibule and scala tympani is related to the perilymph of vestibule consequently it gets vibrated. The waves of scala vestibuli and scala tympani polarize endolymph of scala media or cochlear duct consequently the vibration takes birth in the basilar membrane and there is a stimulation in the auditory receptors in the hair cells of organ of corti. The nerve impulses reach into the hearing center situated in the temporal lobe of cerebrum by the cochlear branch of vestibulocochlear nerve. Here the sound is analyzed and special sound is felt.   

Equilibrium and balance of the body:

     The work to keep the body in equilibrium and balance is done by the vestibular apparatus of internal ear as utrical, saccule and semicircular canals. If there is any kind of change in the position of the head, the liquid of perilymph and endolymph gets shaken in which hair cells get turn and end organs of sensory nerve situated in utrical, saccule and amullae. The fibers of brain supply to the cerebellum of the brain. According to the order of brain, there is appropriate movement and actions in all the muscles because of which the condition of the body remains equilibrium.