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External Ear Anatomy

“Otology is almost unique even ill the later part of the 20th century in not being able to explain at least a few of its diseases ill biochemical terms”
– Ruben 1975.

Ear is divided into three parts:
1. External ear
2. Middle ear
3. Inner ear
It consists of pinna and external auditory canal. Pinna develops from six tubercles around the 1s1 branchial cleft whereas external auditory canal develops from the 1st branchial cleft.
External auditory canal is the only cul-de­ sac in the body lined by skin.

Pinna or Auricle
Pinna (or auricle) is the prominent part of the external ear composed of a single sheet of yellow elastic cartilage covered by fat, subcutaneous tissue and skin. It has two surfaces-medial (cranial) and lateral. The lateral surface is concave with folds and hollows. The medial surface is convex. The most prominent outer fold is called the helix and the fold in front is the anti­ helix. In front of the anti-helix is a hollow called cavum conchae.

Cavum concha leads inwards to the external auditory canal. Anterior to the cavum conchae there is a small cartilaginous projection known as tragus. In the upper part of the cavum conchae, in front of the anti-helix, there is a triangular space known as fossa triangularis. There is also a boat shaped space in between the upper part of helix and anti-helix known as scaphoid fossa.

The whole of the pinna is composed of a single sheet of cartilage except in the lobule and in the space between the cress of the helix and tragus. This space is called incisura terminalis. Since this area is devoid of cartilage, otologists can safely give an incision here for procedures in the ear to avoid postoperative perichondritis. The skin lining on the lateral or outer surface of the pinna is firmly adherent to the perichondrium of the cartilage with minimal or no subcutaneous tissue. Hence, the outer surface of the pinna is more prone to frost bite. In the cranial surface there is more subcutaneous tissue and the skin is loosely adherent to the underlying cartilage. Cysts like sebaceous cyst are commonly seen on this surface. The cartilage of the pinna extends medially to form the cartilaginous part of the external auditory canal.

External Auditory Meatus
External auditory meatus is S-shaped and approximately 2.5 cm in length. It has two parts – the outer one-third is cartilaginous and inner two third is bony. The cartilaginous part is a continuation of the auricular cartilage. It is firmly attached to the bony part by fibrous tissue. In infants, the cartilaginous meatus may remain collapsed because of the non development of the bony part. Hence, to examine the deeper part of the meatus in an infant, one has to either pull the pinna upward or downward.
It has fissure known as fissure of Santorini from where infection from mastoid and parotid can pass into each other.

The external auditory canal is directed first inward, backward and upward and then goes forward, downward and medially. Isthmus is the narrowest part of the canal lying medial to the junction of bony and cartilaginous parts, nearly 5 mm lateral to the tympanic membrane. To examine the deeper part of the EAC in adult one has to pull the pinna upward, outward and laterally. The roof and posterior wall of the external auditory canal are shorter than the floor and anterior wall. Thus, the tympanic membrane fits obliquely in the deeper end of the canal.
The anterior wall goes sharply forward to the tympanic membrane to form a blind pouch known as the anterior recess. Examination of this area is likely to be missed on routine otoscopic examination unless one takes care. This may be a common site for foreign body impaction lodgement.

Bony part in anteroinferior area has a deficiency known as foramen of Huschke. It persists till age of four.
Skin lining of the external auditory canal has its own peculiarity. The cartilaginous part has both epidermis and dermis whereas the deeper bony part has only the epidermal layer. Furunculosis, therefore, occurs more commonly in the outer part of the canal.

Relation
It is closely related to the temporomandibular joint and parotid gland anteriorly. Mastoid antrum and mastoid air cells are the posterior relations.

Nerve Supply
Nerve supply of pinna
Greater auricular nerve is common to lower one­ third of both the surfaces. Upper two-third of the medial surface is supplied by the lesser occipital nerve and the upper two-third of the lateral surface is supplied by the auriculotemporal nerve.
Nerve supply of external auditory canal
The anterior wall, floor and contiguous portion of the tympanic membrane are supplied by the auriculotemporal nerve. Rest of the canal and posterior part of the tympanic membrane are attached to the handle of malleus. When light is reflected over the tympanic membrane, the anteroinferior part is the most illuminated part in the pars tensa.
2. Pars flaccida (Shrapnell’s membrane; attic) is a triangular area above the malleolar folds which is thin and devoid of fibrous tissue and annulus. It fits into the notch of Rivinus.
The ear drum measures approximately 10 mm in vertical diameter and 5 mm in horizontal diameter. It is oval in shape and placed obliquely at an angle of 55 degrees with the floor of the meatus. The inner surface is convex. The point of its greatest curvature is called umbo which corresponds to the tip of the handle of malleus on the inner surface.

Blood Supply
The outer surface of the tympanic membrane is supplied by mandibular artery whose origin is not known and also by the deep auricular branch of the maxillary artery.
The inner surface of the tympanic membrane is supplied by the following arteries
• Anterior tympanic branch of the maxillary artery.
• Posterior tympanic branch of stylomastoid artery.
• Inferior tympanic artery, a branch from the ascending pharyngeal artery.
• Arteria nutricia incudomallea, a twig from the middle meningeal artery.

Venous Drainage
The external jugular vein provides drainage for the outer surface. The inner surface is drained b; the transverse sinus and the venous plexus located around the Eustachian tube.

Nerve Supply
The outer surface is supplied by the auriculotem­ poral nerve in the anterior half and tympani branch of vagus in the posterior half. Tympani plexus supplies the inner surface.

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