Page 249 - Biology_F5

Biology for Advanced Level Secondary Schools

prevent damaging the ear drum. There is Unlike the outer ear and middle ear which
a eustachian tube (auditory tube) which is filled with air, the inner ear comprises a
connects the middle ear to the pharynx. complex system of fluidfilled tubes. The
Via this tube, air enters and leaves the cochlea, a coiled structure, is an organ for
middle ear during swallowing to equalise hearing while the semicircular canal and
pressure (Figure 4.31). the vestibule are organs for body balance.
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Pinna
Semicircular canals
Vestibular nerve
Facial nerve
Auditory nerve

Cochlea

Eustachean tube

Stapes Ear
Incus
Malleus ossicles
External auditory meatus (ear canal) Tympanic membrane
Outer ear Middle ear Inner ear
Figure 4.31: Structure of a mammalian ear

The structure of the membranous as the vestibular apparatus. The cochlear
labyrinth of the mammalian inner ear duct is located within the bony scaffolding
There are two labyrinths in the inner the cochlea. It is held in place by the spiral
ear, namely the bony labyrinth and lamina. The saccule and utricle are two
membranous labyrinth; one is inside membranous sacs located in the vestibule.
the other. The membraneous labyrinth
is comprised of the cochlea, vestibule The utricle is larger than saccule, and it
and three semicircular canals. All these receives the three semicircular ducts.
structures contain a fluid called perilymph. The saccule is globular in shape and
Membranous labyrinth lies within the receives the cochlear duct. Endolymph
bony labyrinth. drains from the saccule and utricle into
the endolymphatic duct. The duct extends
The membranous labyrinth is a continuous through the vestibular aqueduct to the
system of ducts filled with endolymph. It posterior aspect of the petrous part of the
is comprised of the cochlear duct, three temporal bone. Here, the duct expands to a
semicircular ducts, saccule and the utricle. sac where endolymph can be secreted and
The cochlear duct is situated within the absorbed. Semicircular ducts are located
cochlea and is the organ of hearing. The within the semicircular canals, and share
semicircular ducts, saccule, and utricle are their orientation (Figure 4.32).
the organs for balance, and are also known

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