The organ of hearing

The external ear

-The external ear consists of the auricle (commonly called the ear) and the external auditory meatus.

-The greatest part of the auricle (auricula) is made up of a singe crumpled plate of cartilage which is lined on both sides by skin. The free curved margin called the helix, the antihelix, located parallel to it, the anterior prominence, the tragus, and the antitragus situated behind it.

-The lowest part of the auricle is soft and consists only of the connective tissue. This part is called the lobule.

-The largest depression of the auricle called the concha: it leads into the external acoustic meatus.

-The external auditory meatus (meatus acusticus externus) conducts sound waves from the concha to the tympanic membrane. The canal is S-shaped. It consists of two parts: cartilaginous and bony.

-The skin that covers the auricle continues into the external auditory meatus. In the cartilaginous part of the meatus the skin is rich in sebaceuous glands and the ceruminous glands, which produce cerumen (ear wax).

The tympanic membrane is a thin, translucent partition between the external acoustic meatus and the middle ear.

-It is oval in shape, placed obliquely, faces downwards, forwards and laterally.

-The membrane has outer and inner surfaces.

The outer surface of the membrane is lined by thin skin. It is concave.

The inner surface provides attachment to the handle of the malleus which entends up to its centre. The inner surface is convex. The point of the maximum convexity lies at the tip of the handle of the malleus and is called umbo (umbo membranae tympany.

-The membrane is fixed to the tympanic sulcus of the temporal bone. The greater part of the membrane is tightly stretched and is called the tense pars (pars tense). The upper part is soft and less tightly stretched and is called the flaccid part (pars flaccida).

The middle ear

The middle ear consists of the tympanic cavity and the auditory tube through which it communicates with the nasopharynx.

-The tympanic cavity (cavitas tympany) is situated in the base of the pyramid of the temporal bone. Six walls are distinguished in the tympanic cavity.

1. The lateral or membranous wall (parties membranaceus) is formed by the the tympanic membrane and the bony plate of the external auditory meatus. The upper part of the tympanic cavity, the epitympanic recess (recessus epitympanicus), contains 2 auditory ossicle: the head of the maleus and the anvil.

2. The medial wall belongs to the labyrinth and is called labyrinthine wall (parties labyrinthicus). It has 2 opening:

-the fenestra cochleae opening into the cochlea and closed with the secondary tympanic membrane (membrana tympani secundaria);

-the fenestra vestibule opening into the vestibulum labyrintii.

3. The anterior or carotid wall (parties caroticus) is called so because it is closely adjoined by the internal carotid artery separated from the cavity of the middle ear only by a thin bony plate. In the upper part of this wall is the tympanic opening of the pharyngotympanic tube (ostium tympanicum tubae auditivae), this explains the frequent penetration of infection from the nasopharynx into the cavity of the middle ear.

4. The posterior or mastoid wall (parties mastoideus) presents the following features:

-The pyramid of the tympanum (eminentia pyramidalis)? Containing the stapedius muscle.

-There is an opening or aditus through the epitympanic recess communicates with the mastoid (tympanic) atrium.

-The fossa incudis is a depression which lodges the short process of the incus.

-There is the posterior canaliculus for the chorda tympany through which the nerve enters the middle ear cavity.

5. The roof or tegmental wall (parties tegmentalis) is separates the middle ear from the middle cranial fossa.

6. The floor or jugular wall (parties jugularis) is formed by a thin plate of bone which separates the middle ear from the superior bulb of the temporal bone.