Core Curriculum Syllabus: Review of Anatomy - Temporal Bone and Ear

Temporal Bone
The temporal bone contains the sensory organs of hearing and balance, and structurally contributes to the cranial vault. The temporal bone consists of five parts: the squamous, the mastoid, the tympanic, zygomatic and petrous segment. It contains portions of the carotid artery and jugular venous drainage system, and is intimately related to the dura of the middle and posterior fossa. Anteriorly, it articulates with the condyle of the mandible. Posteriorly, and superiorly, the mastoid air cell system communicates with the middle ear. The facial nerve passes through the temporal bone en route to the muscles of facial expression.

Ear
Both functionally and anatomically, it can be divided into three parts.

A. External Ear - that portion external to the tympanic membrane. It serves chiefly to protect the tympanic membrane, but also collects and directs sound waves and plays a role in sound localization. The skin of the external ear normally migrates laterally from the umbo of the malleus in the tympanic membrane to the external auditory meatus (at a rate of 2-3 mm per day). This is a unique and essential mechanism for maintaining patency of the canal.

• The Auricle - elastic cartilage covered with closely adherent skin. The configuration is intricate, and extremely difficult to duplicate.

  

• External Auditory Canal
  • Lateral Portion - cartilaginous with thick, loosely applied skin containing ceruminous and sebaceous glands.
  • Medial Portion- very thin skin directly over bone, no skin appendages. Curves anteriorly and medially in adults, which may obscure the anterior tympanic membrane. It comprises two-thirds of the total canal in adults, less in infants and children.

B. The Middle Ear - This is an air-containing space which communicates with the nasopharynx via the eustachian tube. It is normally sealed laterally by the tympanic membrane. Its function is to transmit and amplify sound waves from tympanic membrane to the stapes footplate converting energy from air medium to a fluid medium of the membranous labyrinth. The relationship of the three ossicles is depicted below.

• Ossicles - three small bones which are involved in sound conduction. From lateral to medial, these are the malleus, the incus, and the stapes. The handle and lateral process of the malleus is attached to the tympanic membrane and can be easily seen on physical exam. The long process of the incus can often be seen through the posterior superior quadrant of the membrane. The stapes is attached to a foot plate which is in direct contact with the fluid of the inner ear. (See diagrams 1 and 3).
• Spaces - the middle ear cleft is wider than the tympanic membrane, and is conventionally divided into spaces in reference to the annulus.
  • Epitympanum - superior to the tympanic membrane. Contains the body of the incus and the head of the malleus. Communicates with the mastoid via the aditus.
  • Mesotympanum - on a level with the ear drum. The oval and round windows, located posterosuperiorly on the medial wall, communicate with the inner ear. The long process of the incus projects into the posterior quadrant to articulate with the stapes which sits in the oval window. The facial nerve, usually covered by a bony canal, crosses the posterior superior quadrant superior to the stapes, then courses inferiorly between the middle ear and mastoid air cells.
  • Protympanum - in this anterior recess of the middle ear, the eustachian tube exits to communicate with the nasopharynx. This tube runs in close proximity to the carotid artery.
  • Hypotympanum - the jugular bulb curves through the hypotympanum. It is usually covered by bone, but may be dehiscent and extend into the middle ear space.

C. Inner Ear - consists of a fluid-filled labyrinth which functions to convert mechanical energy into neural impulses. The bony labyrinth is subdivided into smaller compartments by the membranous labyrinth. Fluid surrounding the membranous labyrinth is called perilymph; fluid within is called endolymph. There are three main divisions of the bony labyrinth.

• Vestibule - just medial to the oval window, and contains the utricle and the saccule, two organs of balance. The vestibule is an antechamber, leading to both the cochlear and the semicircular canal.
• The Cochlea - a snail-shaped chamber anterior to the vestibule. It bulges into the middle ear and its bony covering is the promontory. The cochlea also communicates with the middle ear via the round window. In this organ, sound waves are converted into neural impulses with elaborate coding.
• The Semicircular Canals - three in number; project posteriorly from the vestibule. These organs detect angular acceleration. They consist of a superior, posterior and lateral, or horizontal canals.

The nerve fibers from the labyrinth make up the auditory nerve which consists of a cochlear nerve and a superior and inferior vestibular with both afferent and efferent fibers from the respective sensory end organs. This nerve enters the cranial cavity via the internal auditory canal.

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Bobby R. Alford Department of Otolaryngology-Head and Neck Surgery
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Last modified: Jan. 23, 2006