EAR

Author Dr.R.Menaka

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EAR

- This is an organ associated with hearingg and equillibrium.
- Ear may be considered as vestibulo-cochllear organ.
  Sound waves provide mechanic stimuli; which are received and transformed into   electrical signals
  by the cochlea, while neuro receptors within the vestibule organ provide the   animal with a perception
  of position and movement with respect to gravity.
- The receptors of both organs are part off the inner ear, which is located in   the petrous temporal bone.
- The two organs are linked anatomically aand functionally by the vestibulo-cochlear   nerve.
- The ear comprises of 3 parts,
1. External ear- i. Auricle / or pinna and External acoustic meatus.
2. Middle ear - i. Tympanic cavity ; ii. Eustachian tube and iii. Auditory   ossicles
3. Internal ear- i. Vestibule; ii. Cochlea, iii. Semicircular canals and iv. Organs of corti.

External Ear:

- External ear can be divided into 2 partss,
i. Auricle or pinna
ii. External acoustic meatus

i. Auricle or pinna:

- The external ear of the domestic mammalss varies greatly in size and shape between species and breeds.
- In most animals the external ear is highhly mobile and is important for the communication between individuals.
- Auricle can be moved in desired positionn by muscles.
- Pinna is a funnel shaped structure and aact as sound receiver.
- Pinna have two surfaces, two borders, a base, and an apex.
- Anterior surface is concave, posterior ssurface is convex and covered by hairy skin.
- There are 3 cartilages in the external eear
i. Conchal cartilages- which forms the actual shape of the ear.
ii. Scutiform cartilage
iii. Annualr cartilage- which forms a small imcomplete ring.

ii. External Acoustic Meatus:

- The external acoustic meatus is a canal,, which leads to middle ear.
- In carnivores and pigs the cartilagenouss part is relatively long and curved.
- The external acoustic meatus contains seebacous and tubular ceruminous glands, which secrete earwax (cerumen).
In the ruminants and horse, these glands are located within the distal cartilagenous part of the external acoustic meatus.
In carnivores, the glands are located along the whole of the external acoustic meatus.
- Tympanic membrane or ear drum separates the middle ear from the external acoustic meatus.
- It transmits sound waves into the auditoory ossicles of the middle ear.
- The lateral surface is concave and the mmedial surface is convex. The handle of malleus is attached to this medial surface.

Middle Ear or Typanic Cavity

- The middle ear comprises:

a. Typanic cavity
b. Auditory ossicles
c. Auditory tube

a. Typanic cavity:

- The tympanic cavity is housed within thee petrous temporal bone and situated between the tympanic membrane and the internal ear.
- The tympanic cavity contains ear ossiclees, part of chorda tympani, muscles and air.
- The outer wall is formed by the tympanicc membrane and the inner wall is formed by the lateral wall of internal ear.
- The inner wall contains two windows.
> i. Oval vestibular window - fenestra vestibuli
ii. Round cochlear window (foramen cochlea)

b. Auditory ossicles

- The transmission of viberations from thee tympanic membrane across the tympanic cavity to the inner
ear is mediated by the three auditory ossicles.
i. Malleus
ii. Incus
iii. Stapes
Note: MIS- These are the smallest variety of bones in the body.
- They are small lamellar bones that are jjoined each other by syndesmoses to form a chain which extends
from the tympanic membrane to the vestibular window.

i. Malleus

- Malleus is a hammer shaped bone.
- It consists of head, neck, a short proceess and a long process or the handle.
- The handle is attached to the tympanic mmembrane
- The head of malleus articulates with inccus.

ii. Incus

- It is an anvil shaped structure.
- Incus presents a head and a long processs.
- Long process of incus articulates with tthe head of stapes.
- The head of incus articulates with that of malleus.

iii. Stapes

- The stapes consists of a head, a neck, ttwo limbs, a bsae and a muscular process.
- The base articulates at the oval vestibuular window at the lateral wall of internal ear.

Muscles of tympanic cavity are
i. Tensor tympani and stapedius
- Stapedius originates from the posterior wall of the tympanic cavity and is inserted to the neck of the stapes.
- Tensor tympani muscle originates in the rostro-medial part of the tympanic cavity and inserts to the manubrium of the malleus i.e. handle or long process of malleus.
- contraction of this muscle tensions the chain of auditory ossciles and the tympanic membrane, thus resulting in a higher sensitivity of the transmission system.
- Auditory ossicles donot only transmit thhe vibrations of the tympanic membrane, but also magnify them by atleast 20 times.
- This is essential to initiate waves in tthe endolymph of the internal ear.
An important role in the enchancement mechanism is played by two antagonistic muscles associated with the ossicles.
- Two antagonistic muscle sare, tensor tymmpani and stapedial muslce.
- Chorda tympani is a branch of facial nerrve.

c. Auditory tube or Eustachian tube:

- The auditory tube is a narrow, slit likee tube which connects the tympanic cavity with the nasopharynx.
- The auditory tubes serves to equalise attmospheric pressure on the two sides of the tympanic membranes.
- They temporarily open during yawning or swallowing. Sometimes, the pressure becomes unbalanced eg. during a ride in an elevator and its sudden
restoration causes the ears to "pop"
- In the horse (equines), the auditory tubbe forms a large diverticulum known as guttural pouch.
- Guttural pouch has a capacity of about 5500ml and occupies the space between the base of the skull and the atlas dorsally and the pharynx and the begining of the esophagus.
- Guttural pouch disorders are relatively common in the horse.

Internal Ear:

- The internal ear is a composite organ, tthat consists of an inter connected series of membranous fluid- filled ducts and chambers.
- It consists of a osseous labyrinth, whicch accomudates membraneous structure the membraneous labyrinth.
- Osseous labyrinth and membranous labyrinnth both are complex structure within the petrous temporal bone.
- The space between the two labyrinth are filled with a fluid, known as perilymph which resembles cerebrospinal fluid.
- The osseous labyrinth comprises of
i. Vestibule
ii. Cochlea
iii. Semicircular canals.

i. Vestibule:

- The vestibule is the central chamber of the osseous labyrinth.
- It communicates with the cochlea rostrallly and with the semicircular canals caudally.
- The lateral wall of the vestibule has twwo windows.
1. Oval vestibular or window- stapes attached.
2. Cochlear window

- The medial wall is perforated for the paassage of the filaments of vestibula cochlear nerve and opening of aqueduct of vestibule.

ii. Cochlea:

- It is a bony spiral canal and is very siimilar in shape to a snail shell.
- Cochlea forms a spiral around a central hollow core of bone, the modiolus which contains the cochlear nerve.
The spiral consists of 3 turns- in carnivores
The spiral consists of 2.5 turns- in horse
The spiral consists of 4 turns - in pig
The spiral consists of 3.5 turns - in ruminants
- Projecting into spiral canal from the moodiolus is an osseus shelf, the spiral lamina which imcompletely bisects the
lumen into two partition within the canal, which is completed by the basilar membrane.
- The basilar membrane extends from the eddge of the lamina to the outer wall of the spiral canal and thus divides the canal into,
a. Scala vestibuli
b. Scala tympani

- These two scala are continous at the apeex of the cochlea termed helicotrema.

iii. Semicircular canal:

- Semicircular canals are three in number,,
Anterior
Posterior
Lateral
- The terminal parts of the tubes at theirr junctions with the vestibule are dilated to form ampullae.

Membranous Labyrinth:

- Membranous labyrinth includes
1. Cochlear duct
2. Saccule and utricle
3. 3 ducts of semicircular canals.

- The cavity of the membranous labyrinth iis filled with endolymph.
- This spiral tube contains the organs of corti, which is the sense organ of hearing.

Organ of Corti:

- This organ is situated on the basilar meembrane.
- This consists of
Hair cells
Tunnel of corti
Tectorial membrane
Spiral ganglion
- Hair cells are placed on the basilar memmbrane
- Hair cells are arranged in upper and lowwer rows and there by encloses the tunnel of corti.
- Tectorial membrane is attched to the outter border of spiral lamina.
- Spiral ganglions are situated on the bonny spiral lamina.
- The axons of these spiral ganglionic nerrve cells constitute the cochlear division of the auditory cells.

Nose- Olfactory organ - olfactory area.

Integument- Skin- Sweat glands, sebaceous gland. Subcutis- contarction of these muscles causes twitching of the skin.
Hoof, horn, nail etc - modified
Ergots - vestiges of second and fourth digits (Horse)- Small horny area placed at the postero-medial surfac i.e. fetlock joint in all 4 limbs.
Chestnut: vestiges of first digits (Horse)- Placed at the medial surface of carpus and tarsus of all the 4 limbs.

Fowl- Skin appendages- comb, wattles, ear lobe, feathers, scales and claws.

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