THE EAR

The ear is made up three main parts called the external ear, the middle ear, and the internal ear.

THE EXTERNAL EAR

The auricle.The auricle consists of a thin plate of elastic cortilage covered on both sides by true skin. The skin is closely adherent to the cartilage. Hair follicles, sebaceous glands, and sweat glands are present in the skin.

The external acoustic meatus.The wall of the external acoustic meatus is made up partly of elastic cartilage (in its outer part) and partly of bone (in its inner part). The meatus is lined by skin. The skin lining the bony part is thin and is firmly adherent to the underlying bone. In the cartilaginous part the skin is thick and contains hair, sebaceous glands, and ceruminous glands. The ceruminous glands secrete the wax of the ear. They are modified sweat glands lined by a columnar, cuboidal or squamous epithelium.

THE MIDDLE EAR

The tympanic membrane.The tympanic membrane has three layers. The middle layer is made up of fibrous tissue, which is lined on the outside by skin (continuous with that of the external acoustic meatus), and on the inside by mucous membrane of the tympanic cavity.

The fibrous layer contains collagen fibres and some elastic fibres. The fibres are arranged in two layers. In the outer layer they are placed radially, while in the inner layer they run circularly.

The mucous membrane is lined by an epithelium which may be cuboidal or squamous.

The tympanic cavity.The walls of the tympanic cavity are formed by bone which is lined by mucous membrane. The mucous membrane also covers the ossicles (malleus, incus, stapes). The lining epithelium varies from region to region. Typically it is cuboidal or squamous. At places it may be ciliated columnar. The ossicles of the middle ear consist of compact bone, but do not have marrow cavities.

The auditory tube.The wall of the auditory tube is partly bony (lateral part) and partly cartilaginous (medial part, nearer the nasopharynx). The bone or cartilage is covered by mucous membrane which is lined by ciliated columnar epithelium. Near the pharyngeal end of the tube the epithelium becomes pseudostratified columnar. Goblet cells and tubulo-alveolar mucous glands are also present.

INTERNAL EAR

The internal ear is in the form of a complex system of cavities lying within the petrous temporal bone. It has a central part called the vestibule. Continuous with the front of the vestibule there is a spiral shaped cavity called the cochlea. Posteriorly, the vestibule is continuous with three semicircular canals.

Because of the complex shape of these intercommunicating cavities the internal ear is also called the labyrinth. The space bounded by bone is bony labyrinth. Its wall is made up of bone that is more dense than the surrounding bone. Its inner surface is lined by periosteum. Lying within the bony labyrinth there is a system of ducts which constitute the membranous labyrinth. The space within the membranous labyrinth is filled by a fluid called the endolymph. The space between the membranous labyrinth and the bony labyrinth is filled by another fluid called the perilymp.

The bony labyrinth consists of a central part called the vestibule. The vestibule is continuous anteriorly with the cochlea; and posteriorly with three semicircular canals. .

The cochlear part of the bony labyrinth is divisible into two parts. One part, the scala vestibuli opens into the vestibule; while the second part called the scala tympani opens into the middle ear through an aperture called the fenestra cochlea.

Within each semicircular canal the membranous labyrinth is represented by a semicircular duct. The part of the membranous labyrinth (bounded by vestibular membrane and basilar membrane) present in the cochlea is called the duct of the cochlea. The part of membranous labyrinth that lies within the vestibule is in the form of two distinct membranous sacs called the saccule and the utricle.

The bony cochlea.The cochlea has a striking resemblance to a snail shell. It is basically a tube that is coiled on itself for two and three-fourth turns. The “turns” rest on a solid core of bone called the modiolus.

Ampullary crest.One ampullary crest is present in the ampulated end of each of the three semicircular duct. Each crest is an elongated ridge projecting into the ampulla, and reaching almost up to the opposite wall of the ampulla. A long axis of the crest lies at right angles to that of the semicircular duct. The crest is lined by a columnar epithelium in which two kinds of cells are present. These are hair cells which are specialized mechanoreceptors, and supporting (or sustentacular) cells.

Spiral organ of Corti.The spiral organ of Corti is so called because (like other structures in the cochlea) it extends in a spiral manner through the turns of the cochlea. In sections it is seen to be placed on the basilar membrane and to be made up of epithelial cells that are arranged in a complicated manner. The cells are divisible into the true receptor cells or hair cells, and supporting elements which are given different names depending on their location. The cells of the spiral organ are covered from above by a gelatinous mass called the membrana tectoria.

Questions:

1. Three main parts of the ear.

2. The main parts of the internal ear.

3. The main parts of the bony labyrinth

4. Structural features of the bony cochlea.

5. Structural features of the ampullary crest.

6. Structural features of the spiral organ of Corti.

Cytotrophoblast

Syncytiotrophoblast

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