Ear (Stato-Acoustic Organ)
- It is the organ for hearing & balancing.
- It has 3 divisions: External ear, middle ear & inner ear.
External Ear
- Consists of pinna (ear lobe) & auditory meatus (ear canal).
- At the opening of ear canal, hairs are seen.
- Ear canal and skin of pinna contain ceruminous glands (modified sweat glands). They secrete wax (cerumen).
- Wax and hairs protect ears from foreign objects.
- Ear canal ends in tympanic membrane (Tympanum or ear drum). It is a semi-transparent membrane covered by a thin layer of skin on its outer surface and by mucous membrane on the inside.
Middle Ear
- Consists of tympanic cavity and ear ossicles.
- Tympanic cavity is an air-filled space that separates the external and inner ear portions.
- An auditory tube (Eustachian canal) connects middle ear to the pharynx. It maintains an equal pressure on either side of the eardrum.
- Ear ossicles include 3 small bones namely Malleus, Incus and Stapes. Malleus is attached to tympanum.
- Stapes is the smallest bone of the body. It is attached to membrane of oval window (fenestra ovalis) of inner ear.
Inner Ear
- It consists of bony labyrinth & membranous labyrinth.
- Bony labyrinth is a cavity filled with perilymph.
- The membranous labyrinth consists of cochlea and Vestibular apparatus.
Cochlea (organ of hearing)
- It is a coiled structure having 3 canals:
- Upper scala vestibula.
- Middle scala media.
- Lower scala tympani.
- Scala vestibula & scala media are separated by Reissner’s membrane.
- Scala media and scala tympani are separated by basilar membrane.
- S. vestibula & S. tympani are filled with perilymph and scala media is filled with endolymph.
- Resting on the basilar membrane and projecting into scala media is a complex receptor organ called Organ of Corti. It consists of a row of sensory hair cells. The hairs (stereo cilia) of these cells project upwards and lie in contact with tectorial membrane, which projects above them.
Vestibular Apparatus
- It consists of 3 semicircular canals and otolith organ.
- Two semicircular canals are vertical and one is horizontal. One end of each canal has a bulging called ampulla. Inside it is a lump called crista ampullaris. Long cilia of cells of crista are grouped together in a bundle (cupula).
- Otolith organ consists of utricle and saccule.
- Utricle & Saccule have a projecting ridge called macula.
- Crista and Macula are specific receptors in vestibular apparatus. They contain sensory hair cells. They are responsible for equilibrium & posture of body.
Mechanism of Hearing
Pinna collects sound waves → waves reach the tympanic membrane via ear canal → tympanic membrane vibrates → vibrations transmit to ear ossicles & oval window → perilymph in the vestibular canal vibrates → vibrations reach the scala tympani and force the basilar membrane to vibrate → hair endings of sensory hair cells press against tectorial membrane → sensory hair cells are excited → auditory nerve carries impulses to auditory centre of the brain → hearing.
Nose
- Organ of smell (olfaction).
- It contains mucus-coated receptors (olfactory receptors) made up of olfactory epithelium.
- They receive sense of smell. It contains 3 kinds of cells.
- The neurons of olfactory epithelium extend from the outside environment directly into a pair of broad bean-sized organs, called olfactory bulb. These are extensions of the brain’s limbic system.
Tongue
- Organ of taste (gustation).
- 4 primary tastes are sweet, salt, sour and bitter.
- Taste buds (Gustatoreceptors + supporting cells) are seen around the bases of taste papillae.
- Nose & tongue are chemoreceptors (detect dissolved chemicals).
- Senses of taste & smell are functionally similar and interrelated. The brain integrates different input from taste buds and a complex flavour is perceived.
Skin (Cutaneous Receptors)
- Largest sense organ.
- It contains receptors for heat, cold, touch, pain & pressure.
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