Page 401 - Clinical Anatomy

ECA6 7/18/06 6:54 PM Page 386

386 The central nervous system

presents the fenestra cochleae (round window), closed by the secondary tym-
panic membrane; the fenestra vestibuli (oval window), occupied by the base
of the stapes; the promontory, formed by the first turn of the cochlea; and the
prominence caused by the underlying canal for the facial nerve (Fig. 263).
The floor is a thin plate of bone separating the cavity from the bulb of the
jugular vein.
The roof is formed by the thin sheet of bone known as the tegmen
tympani, which separates it from the middle cranial fossa and the temporal
lobe of the brain.
Anteriorly, the cavity communicates with the pharynx by way of the
pharyngotympanic or Eustachian tube.
Posteriorly, it communicates with the mastoid or tympanic antrum and
the mastoid air cells.
The mastoid antrum is a small cavity in the posterior part of the petrous
temporal bone connected to the epitympanic recess of the middle ear by
way of the narrow aditus. Its importance is twofold: it is in communication
with the mastoid air cells (hence the portal through which infection may
spread to these spaces from the middle ear) and it is intimately related pos-
teriorly to the sigmoid sinus and the cerebellum, both of which may be
involved from a middle ear infection.
The mastoid air cells arise postnatally as diverticula from the tympanic
antrum, becoming obvious first at two years. They may invade not only the
mastoid process but also the squamous part of the temporal bone. They are
lined by a mucoperiosteum continuous anteriorly with that of the tympanic
cavity.
The pharyngotympanic (Eustachian) tube reaches downwards, forwards
and medially from the anterior part of the tympanic cavity to the lateral
walls of the nasopharynx. In all it is about 1.5in (37mm) long, the first 0.5in
(12mm) being bony while the rest is cartilaginous. It is lined by a ciliated
columnar epithelium. The mucous membrane is thin in its bony part but
the cartilaginous segment contains numerous mucous glands and, near its
pharyngeal orifice, a considerable collection of lymphoid tissue termed the
tubal tonsil. This may become swollen in infection, producing blockage of
the tube. The tube is widest at its pharyngeal end and narrowest at the junc-
tion of the bony and cartilaginous portions.
Conduction of sound through the middle ear is by way of the malleus,
incus and stapes. The malleus is the largest of the three and is described as
having a handle, attached to the tympanic membrane, a rounded head,
which articulates with the incus, and a lateral process, which can be seen
through the tympanic membrane and from which the malleolar folds
radiate. The incus comprises a body, which articulates with the malleus, and
two processes, a short process attached to the posterior wall of the middle
ear and a long process for articulation with the stapes. The shadow of the
long process can often be seen through an auroscope running downwards
behind the handle of the malleus. The stapes has a head for articulation with
the incus, a neck and a base, which is firmly fixed in the fenestra vestibuli
(the oval window).
Two small muscles are associated with these ossicles: the stapedius,