External Acoustic (Auditory) Canal

It extends from the bottom of the concha to the tympanic
membrane and measures about 24 mm along its
posterior wall. It is not a straight tube; its outer part is
directed upwards, backwards and medially while its inner
part is directed downwards, forwards and medially.
Therefore, to see the tympanic membrane, the pinna has
to be pulled upwards, backwards and laterally so as to
bring the two parts in alignment.
The canal is divided into two parts: (a) cartilaginous
and (b) bony.
(a) Cartilaginous Part
It forms outer one-third (8 mm) of the canal. Cartilage is
a continuation of the cartilage which forms the framework
of the pinna. It has two deficiencies-the "fissures of
Santorini" in this part of the cartilage and through them
the parotid or supelficial mastoid infections can appear
in the canal, or vice versa. The skin, covering the cartilaginous
canal is thick and contains ceruminous and
pilosebaceous glands which secrete wax. Hair is only
confined to the outer canal and therefore furuncles
(staphylococcal infection of hair follicles) are seen only
in the outer one third of the canal.
(b) Bony Part
It forms inner two-thirds (16 mm). Skin lining the bony
canal is thin and continuous over the tympanic membrane.
It is devoid of hair and ceruminous glands. About
6 mm lateral to tympanic membrane, the bony meatus
presents a narrowing called the isthmus. Foreign bodies
lodged medial to the isthmus, get impacted, and are difficult
to remove. Antero-inferior part of the deep meatus,
beyond the isthmus, presents a recess called the anterior
recess which acts as a cesspool for discharge and debris
in cases of external and middle ear infections. Anteroinferior
part of the bony canal may present a deficiency
(foramen of Huschl<e) in children up to the age of four or
sometimes in adults, permitting infections to and from
the parotid.