EAR
EAR
It consists of the auditory apparatus and the organs concerned with balance which record both rotary movements of the head and the direction of the gravitational field acting on it.
Ear is divided into three parts:
• External ear (outer ear)• Middle ear
• Internal ear (inner ear)
1. EXTERNAL EAR
It consists of the auricle and the external acoustic meatus.
The auricle collects the sound waves, and the external acoustic meatus transmits them medially to the tympanic membrane, which separates the external ear from the middle ear.
External acoustic meatus (external auditory meatus)
It runs medially with a slight anterior inclination.
It is almost exactly in line with the internal acoustic meatus, and their shadows are superimposed in a true lateral radiograph of the skull.
It is approximately 24 mm. long, of which the medial 2/3 is the bony part and the lateral 1/3 is the cartilaginous part.
The skin of the cartilaginous portion contains many ceruminous glands (which produce cerumen, or earwax) and hairs.
In the bony part of the canal the skin has no hairs and few if any glands.
Its diameter is not uniform; the narrowest point is called the isthmus which is about 5 mm. from the tympanic membrane.
Tympanic membrane(ear drum)
Is an elliptical disc which is stretched across the medial end of the external acoustic meatus, and forms the greater part of the lateral wall of the middle ear cavity.
2. MIDDLE EAR
It is a narrow chamber whose cavity (the tympanic cavity) lies between the tympanic membrane and the lateral wall of the internal ear, and communicates with the nasal part of the pharynx through the auditory tube (Eustachian tube).
Three small auditory ossicles (malleus, incus, and stapes) lie in the middle ear, and stretch across it from the tympanic membrane to the lateral wall of the internal ear to transmit the vibrations of the tympanic membrane to the internal ear.
3. INTERNAL EAR
It consists of a complex system of communicating cavities (the bony labyrinth) situated in the densest part of the petrous temporal bone.
It contains a similarly shaped (to the bony labyrinth), but narrower complex of membranous tubes and sacs which constitutes the membranous labyrinth, and on which the sensory nerve fibers of the vestibulocochlear nerve end.
