د.حسين جاسم                                                                                            The ear

L.1 Anatomy
    The ear is an organ responsible for hearing sensation & balance composed of 3- main parts:

1: the external ear: - consist of three parts: a. the auricle  b. external                      ear canal or meatus  c. tympanic membrane(ear drum).
2: the middle ear: - it contain the auditory ossicles & this part communicates with:
     a. Eustachian tube     b. mastoid air cells.
3:  the inner ear :- consist of 2- parts:
a.	bonny labyrinth: localized in the temporal bone & consist of the semicircular canal & the cochlea.
b.	Membranous labyrinth: consist of :
1. Sac of the endolymph         2. Sac of the perilymph.

The external ear
 
A.	The auricle:
    It’s also called the (pinna) & is attached to the lateral surface of the head, consisting of a yellow elastic cartilage covered by perichondrium & skin .It has a medial & a lateral surfaces, the lateral surface is bridged & the skin is closely adherent to this surface, while the medial surface is smooth & the skin is loosely attached to it.
    The lobular part of the auricle is free of cartilage & consists of skin & fibrous tissue.

B.	The external auditory canal (EAC):
        It extend from the auricle laterally to the tympanic membrane  
     medially,its length is( 24 mm ) . The direction of the EAC is not like an artificial tube, it’s a convoluted tube & this serves a physiological function, so in its lateral (1/3)(8 mm) its directed upwards &backwards, deep in the head in its medial (2/3)(16 mm) its directed downwards &forwards, this is important in ear examination especially if we want to see the ear drum.
           The EAC is composed of 2- parts:
1.	Cartilaginous part: which represent the lateral 1/3 of the EAC 
& is covered by skin which contains hair  & sebaceous glands
& wax secreting glands , the skin here is thick & undergoes exfoliation towards the surface & obliquely outwards,the wax is a bactericidal material containing various amino acids,fatty acids ,lysozymes & immunoglobulines.
2.	Bonny part: its located deep in the petrous bone,representing the medial 2/3 of the EAC & reaches the tympanic membrane it’s also covered by skin which is thin,loose & easily damaged,there are no hairs & no wax secreting glands.

The Middle ear

          Its nearly a box shaped cavity (biconcave medially & laterally) its also called the tympanic cavity or the middle ear cleft which include the mastoid air cells & the Eustachian tube, its situated in the temporal bone between the EAC & the inner ear structures,the middle ear cavity is larger than the tympanic membrane.
          It has a superior, inferior, lateral, medial, anterior & posterior walls with the following dimentions:length(15 mm),height(15 mm) & width(3.6 mm).
          It’s divided into 3- parts:
a.	Epitympanum: it lies above the tympanic membrane.
b.	Mesotympanum: it lies opposite to the tympanic membrane.
c.	Hypotympanum: it lies below the tympanic membrane.
         The contents of the middle ear cleft are:
1.	Chorda tympani nerve: which is a branch of the facial nerve that supplies the parotid salivary gland & runs in the fallopian canal.
2.	2- small muscles:
a. Tensor tympani: attached to neck of the malleus.
c.	stapedius: attached to the neck of the stapes.
3.	The auditory ossicles: which are three bones connected to each other:
a.	Malleus: the handle of the malleus is firmly attached to middle layer of the tympanic membrane (the fibrous layer) & its head articulates with the body of the incus. 
b.	Incus: has 2-processes:
1-	Short process: attached to the tip of the aditus.
2-	Long process: attached to the head of the stapes.
c.	Stapes: resembles stirrup & its footplate occupies the oval window.

Walls of the middle ear:

1.	The lateral wall: it’s formed mainly by the tympanic membrane that separates the middle ear from the EAC.
2.	The medial wall: it separate the middle ear (ME) from the inner ear & is formed mainly by the bony cochlea & 2-openings the oval & the round windows, these windows are not opened but is covered by membrane & are important in conducting sound waves to the inner ear. The oval window is above the promontory (the bony cochlea) while the round window lies below the promontory. The facial nerve run horizontally above the promontory & is covered by a thin layer of bone.
3.	The superior wall: it separates the (ME) from the middle cranial fossa or cavity.
4.	 The inferior wall: it separates the ME from the bulb of the internal jugular vein.
5.	The anterior wall: it separate the ME from the internal carotid artery,in the lower part of the wall there is an opening for the Eustachian tube (which connect the ME to the nasopharynx).tensor tympani muscle enter this wall through a canal lies above the Eustachian tube opening.
6.	The posterior wall: it has the opening of the aditus, which connect the ME to the mastoid antrum.

  The tympanic membrane: 

     It separate the EAC from the ME, its oval in shape, semitransparent, pearly gray in color like a finger nail, its longitudinal diameter is larger than the transverse diameter , its concave laterally towards the EAC .
     Structurally its divided into unequal parts, the larger part is called the pars tensa which consist of 3-layers:
1.	Single epithelial layer (laterally).
2.	Fibrous layer (middle).
3.	Mucous layer (thin & located medially). 

While the other part of the eardrum is called: pars flaccida, which is smaller than the pars tensa & has a similar layers but lacking the fibrous layer in the middle.
 The tympanic membrane also has a bright surface which reflect light located in the anteroinferior part of the tympanic membrane & end up in a depression called: umbo this surface is triangular in shape & called the cone of light.

Communications of the ME:
  The ME communicates with the nasopharynx through the Eustachian tube & with the mastoid antrum through the aditus.
 
Eustachian tube:
    It’s a tube connecting the ME with the nasopharynx, it’s 36-mm in length & is directed downward, medially & forwards. In children the tube is shorter, wider & more horizontal than in adults.
    Its composed of 2-parts ; the bony part(lateral 1/3) & the cartilaginous part(medial 2/3).
    The whole tube is lined by mucous membrane that contains goblet cells, physiologically the tube is closed at rest, but is opened during swallowing &yawing. The tensor palati muscle is attached to its wall & when it contract it causes E.tube opening. 

Mastoid air cells:
   Its located in the mastoid process (that appears as a projection behind the auricle) it gives origin to the sternomastoid muscle & considered as part of the temporal bone.
   The mastoid air cells is a communicating cells (not isolated) the number & the size of the cells vary between individuals, sometimes its completely absent & here the mastoid process is called scleroting bone, the mastoid process is normally absent at birth but it appears as a process at the age of 2- years.
    The function of the air cells is as a reservoir of air in normal atmospheric pressure , but during pressure changes it may be filled by blood , fluid , pus.

The inner ear

It’s called: the labyrinth & is an important part of the ear & most of the symptoms & diseases are related to it, its located in the petrous part of the temporal bone, between the ME & the EAC.
It measures about 20-mm length & consists of 2-parts; the membranous labyrinth located within the 2nd part the bony labyrinth, which is a bony cavity, the membranous part consist of a delicate sacs & canals filled with fluid called: endolymph, while the space between the bony & membranous labyrinth is filled with fluid called: perilymph.
The membranous labyrinth consist of 2-parts:
1.	The cochlea: which is a snail like structure (2 & ½ turns) this part contain the organ of Corti responsible for hearing sensation by its neuroepithelia located on the basilar membrane, when the sound waves reach the ear it causes vibration of the ossicles, which transmit the vibration to the oval window & causes vibration of the fluid in the cochlea leading to vibration of the tectorial membrane & stimulation of the auditory cells (neuroepithelium) which passes to the higher centers through the cochlear part of the 8th cranial nerve.

2.	The vestibule: it consists of 3-semicircular canals (horizontal, anterior vertical & posterior vertical) & 2-sacs (utricle & saccule) , the neuroepithelia in this part of the inner ear is responding to any change in position or movement & hence is responsible for balance & equilibrium .
            The stimulation of this system causes vibration of the fluid in the membranous vestibule & cause movement of the cupola (gelatinous material) causes stimulation of the hair cells which transmit the impulses to the vestibular part of the 8th cranial nerve.the balance & equilibrium is a function of 3-systems the ear, the joints & the cerebellum (it need 2-systems healthy to gain a normal balance. Loss of balance or equilibrium causes vertigo while affection of the cochlea causes deafness.