Ear ossicles : malleus, incus ,stapes : short and easy description

Ear osssicles :

The middle ear cavity contains a chain of three small bones the malleus , the incus and the stapes .

Conduction of sound through the middle ear to internal ear is transmitted by these three ossicles. Each ear ossicles is ossified from one ossification centre in the fourth month of intra-uterine life and they attain adult size at birth .

Malleus :

It is the largest bone of middle ear.

Shape: like a round-headed club

Parts:

Head: it lies in the epi- tympanic part of middle ear cavity. Head articulate with the incus forming a siddle shaped synovial joint

Neck:  it situated in between head and handle and lies just medial to pars flaccida of tympanic membrane. Its medial side is related with chorda tympani nerve

Handle: it is attached with medial side of tympanic membrane and its upper part receives insertion of tendon of tensor tympani muscles.

Anterior process: situated just below the neck: it receives attachment of anterior ligament which extends up to spine of sphenoid. This ligament derived from perichondrium of 1^st pharyngeal arch cartilage.

Lateral process: mallear folds are attached to the apex of the lateral process.

Incus

Shape : like premolar tooth

Development : 1^st pharyngeal arch

Parts :

Body : articulate with head of malleus

Short process : directed posteriorly

Long process : it is situated parallel to the upper part of handle of malleus and tip of incus forms a knob known as lentiform nodule which articulate with head of stepes . 

Stepes:

Shape : it looks like stirrup

Development :2^nd pharyngeal arch

Parts :

Head : articulate with lentiform nodule of incus

Neck : receive insertion of stepedius muscle

Anterior and posterior limbs

Base or foot- plate : it is attached with fenestra vestibule by annular ligament

What are the structure seen through auroscope

Lateral process of malleus

Long process of incus

Two small muscles are associate with ear ossicles

1.      Stapedius :

Attachement : origin : interior of hollow pyramid

Insertion : neck of stepes

Nerve supply :  facial nerve

Functions: contraction of this muscle draw the stapes laterally thus tilting its foot plate in the oval window.

 it prevent excessive movement of stapes so its paralysis leads  to uninhibited movement of stapes and excessive acuteness of hearing (hyperacusis) 

SN: paralysis of the stapedius causes an abnormally increased power of

2.      Tensor tympani:

Attachment : origin : bony part of auditory tube

Insertion : handle of malleus

Nerve supply : mandibular division of trigeminal nerve

Functions: contraction of this muscle draw the handle of malleus medially (inward) which makes tympanic membrane more concave laterally  and tense. It prevent wide excursions of the ear ossicles and potential damage to the inner ear when exposed to loud sounds

          Functions of both muscles: damp high frequency vibrations