Good day everyone, today we are going to discuss fully on the ear:
Learning outcomes
After studying this section, you should be able to:
- describe the structure of the outer, middle and inner parts of the ear
- explain the physiology of hearing.
The ear is the organ of hearing and is also involved in balance. It is supplied by the 8th cranial nerve, i.e. the cochlear part of the vestibulocochlear nerve, which is stimulated by vibrations caused by sound waves.
With the exception of the auricle (pinna), the structures that form the ear are encased within the petrous portion of the temporal bone.
Structure
The ear is divided into three distinct parts : the outer ear, middle ear (tympanic cavity) and inner ear.
The outer ear collects the sound waves and directs them to the middle ear, which in turn transfers them to the inner ear, where they are converted into nerve impulses and transmitted to the hearing area in the cerebral cortex.
Outer ear
The outer ear consists of the auricle (pinna) and the external acoustic meatus (auditory canal).
The auricle (pinna)
The auricle is the visible part of the ear that projects from the side of the head. It is composed of fibroelastic cartilage covered with skin. It is deeply grooved and ridged; the most prominent outer ridge is the helix.
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The lobule (earlobe) is the soft pliable part at the lower extremity, composed of fibrous and adipose tissue richly supplied with blood.
External acoustic meatus (auditory canal)
This is a slightly ‘S’-shaped tube about 2.5 cm long extending from the auricle to the tympanic membrane (eardrum). The lateral third is embedded in cartilage and the remainder lies within the temporal bone. The meatus is lined with skin continuous with that of the auricle. There are numerous ceruminous glands and hair follicles, with associated sebaceous glands, in the skin of the lateral third. Ceruminous glands are modified sweat glands that secrete cerumen (earwax), a sticky material containing protective substances including the bacteriocidal enzyme lysozyme and immunoglobulins. Foreign materials, e.g dust, insects and microbes, are prevented from reaching the tympanic membrane by wax, hairs and the curvature of the meatus. Movements of the temporomandibular joint during chewing and speaking ‘massage’ the cartiaginous meatus, moving the wax towards the exterior. The tympanic membrane (eardrum) completely separates the external acoustic meatus from the middle ear. It is oval-shaped with the slightly broader edge upwards and is formed by three types of tissue: the outer covering of hairless skin, the middle layer of fibrous tissue and the inner lining of mucous membrane continuous with that of the middle ear.
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Middle ear (tympanic cavity)
This is an irregular-shaped air-filled cavity within the petrous portion of the temporal bone. The cavity, its contents and the air sacs which open out of it are lined with either simple squamous or cuboidal epithelium.
The lateral wall of the middle ear is formed by the tympanic membrane.
The roof and floor are formed by the temporal bone.
The posterior wall is formed by the temporal bone with openings leading to the mastoid antrum through which air passes to the air cells within the mastoid process.
The medial wall is a thin layer of temporal bone in which there are two openings:
- oval window
- round window.
The oval window is occluded by part of a small bone called the stapes and the round window, by a fine shee of fibrous tissue.
Air reaches the cavity through the pharyngotympanic (auditory or Eustachian) tube, which links the nasopharynx
and middle ear. It is about 4 cm long and lined with ciliated columnar epithelium. The presence of air at atmospheric pressure on both sides of the tympanic membrane is maintained by the pharyngotympanic tube and enables the membrane to vibrate when sound waves strike it. The pharyngotympanic tube is normally closed but when there is unequal pressure across the tympanic membrane, e.g. at high altitude, it is opened by swallowing or yawning and the ears ‘pop’, equalising the pressure again.
Auditory ossicles
These are three very small bones only a few millimetres in size that extends across the middle ear from the tympanic membrane to the oval window. They form a series of movable joints with each other and with the medial wall of the cavity at the oval window. The ossicles are held in place by fine ligaments and are named according to their shapes.
The malleus. This is the lateral hammer-shaped bone. The handle is in contact with the tympanic membrane and the head forms a movable joint with the incus.
The incus. This is the middle anvil-shaped bone. Its body articulates with the malleus, the long process with the stapes, and it is stabilised by the short process, fixed by fibrous tissue to the posterior wall of the tympanic cavity.
The stapes. This is the medial stirrup-shaped bone. Its head articulates with the incus and its footplate fits into the oval window.
