ANATOMY OF THE INNER EAR

It is located within the petrous part of the temporal bone between the middle ear and the internal auditory canal. Its length is 20 mm and consists of a membranous labyrinth surrounded by a bony labyrinth:

• The bony labyrinth is a hollow bony capsule, which is a part of the petrous portion of the temporal bone.
• The membranous labyrinth is a group of delicate ducts and sacs.

The membranous labyrinth is filled with a fluid called endolymph and is surrounded by a fluid called perilymph, the membranous labyrinth contains the labyrinthine sensory end organs.

1. The Bony Labyrinth consists of:

1. Bony cochlea: A coiled canal that resembles a snail's shell having 2.5 turns.

2. Three bony semi-circular canals (lateral, superior and posterior), each canal forms 2/3 of a circle and they are perpendicular to each other.
3. Vestibule: Between the cochlea and the semi-circular canals.
   

2. The Membranous Labyrinth consists of:

1. Membranous cochlear duct: Within the bony cochlea contains the organ of corti à Cochlear division of the 8^th Cranial nerve.

2. Membranous semi-circular ducts: Within the bony semi-circular canalsà vestibular division of the 8^th Cranial nerve.
3. Utricle, and Saccule: Within the vestibule à vestibular division of the 8^th Cranial nerve.
4. Endolymphatic duct and sac.
   

The Cochlea:

1. The cochlea forms 2.5 turns around an axis called the modiolus.

2. Cochlear nerve fibers leave the cochlea through the modiolus.

3. The cavity of the bony cochlea is divided into three compartments:

• A middle compartment called scala media (cochlear duct), filled with endolymph. The organ of Corti (the sensory end-organ of the cochlea) lies in this compartment. It rests on a membrane called basilar membrane.
• An upper compartment called scala vestibuli, filled with perilymph.
• A lower compartment called scala tympani, filled with perilymph.

The Labyrinthine Sensory End Organs:

They are the sensory end organs of hearing and equilibrium, located within the membranous labyrinth

• The sensory end organ of the cochlea is called: the organ of Corti.
• The sensory end organ of the semi-circular duct is called: the crista.
• The sensory end organ of the utricle and saccule is called: the macula.

Category: Anatomy Notes