a
c
Malleus
Incus
Stapes
Styloid
Stylohyoid
ligament
Hyoid bone
b
Figure. 1.1.1 (a) The ear is divided into three compartments: external, middle, and inner. The pinna is
composed of cartilage covered by skin. The shape of the cartilage is very important, since it gives the shape
of the auricle. Any necrosis of the cartilage may lead to cosmetic deformity. The external auditory canal
measures approximately 2.5 cm. The outer one-third is cartilaginous and the inner two-thirds is bony. There
is a narrowing at the bone–cartilage junction which causes foreign bodies to get stuck in this area. The skin
of the bony part is very thin lying on the periosteum and does not contain glands, hair follicles, and any
adnexal structures. There are two or three fissures in the cartilaginous external auditory canal which are
called “Santorini fissures.” These fissures provide a potential route for the spread of infection from the
external ear to the parotid area or infratemporal fossa and also of tumors from the parotid area to the external ear. The eustachian tube connects the middle ear to the nasopharynx. The posterior one-third of the
adult eustachian tube is bony and lies within the petrous portion of the temporal bone. The anterior twothirds is cartilaginous. In adults the tube lies at an angle of 45° in relation to the horizontal plane, whereas
this inclination is only 10° in infants. The tube is longer in the adult than in the infant and young child. (b)
Illustration showing the organs of hearing and the cerebellum. Sound waves are channeled by the pinna
(visible part of the ear) into the auditory canal (pink) toward the eardrum. The eardrum transmits the vibrations to three tiny bones – the malleus, incus, and stapes – in the middle ear. The stapes passes the vibrations to the inner ear structures (purple), the semicircular canals and the cochlea (spiral). Auditory
sensations are picked up by the cochlear nerve (yellow) and transmitted to the medulla (brainstem), the
thalamus, and ultimately the cerebral cortex (visual photos). (c) The external and middle ear develop from
the branchial apparatus. The middle ear cavity is derived from the endodermal first branchial cleft. The
inner ear develops from the otic placode. The first arch, or Meckel’s cartilage, contributes to the malleus
and the incus. The tensor tympani muscle derives from the first branchial arch and is innervated by the
nerve of the first branchial arch, which is mandibular branch of the trigeminal nerve. The second branchial
arch, or Reichert’s cartilage, contributes to the suprastructure of the stapes. The stapes muscle is innervated
by a facial nerve, which is the nerve of the second branchial arch. The chorda tympani nerve, a branch of
the facial nerve (second arch nerve), joins the first arch nerve to the mandibular lingual nerve. The footplate
of the stapes is derived from the otic capsule. Thus, a congenital abnormality can occur in one part while
the other parts may develop normally