Briefly describe the structure of the following:
Brain (b) Eye (c) Ear
(A) Brain: Brain is the main coordinating centre of
the body. It is a part of nervous system that controls and
monitors every organ of the body. It is well protected by cranial
meninges that are made up of an outer layer called dura mater, a
thin middle layer called arachnoid, and an inner layer called pia
It is divided into three regions
midbrain, and hindbrain.
Forebrain: It is the main thinking part of the brain. It
consists of cerebrum, thalamus, and hypothalamus.
Cerebrum is the largest part of the brain and constitutes about
four-fifth of its weight. Cerebrum is divided into two cerebral
hemispheres by a deep longitudinal cerebral fissure. These
hemispheres are joined by a tract of nerve fibre known as corpus
callosum. The cerebral hemispheres are covered by a layer of
cells known as cerebral cortex or grey matter. Cerebrum has
sensory regions known as association areas that receive sensory
impulses from various receptors as well as from motor regions
that control the movement of various muscles. The innermost part
of cerebrum gives an opaque white appearance to the layer and is
known as the white matter.
Thalamus is the main centre of coordination for sensory and motor
signalling. It is wrapped by cerebrum.
It lies at the base of thalamus and contains a number of centres
that regulate body temperature and the urge for eating and
drinking. Some regions of cerebrum, along with hypothalamus, are
involved in the regulation of sexual behaviour and expression of
emotional reactions such as excitement, pleasure, fear, etc.
It is located between the thalamus region of the forebrain and
pons region of hindbrain. The dorsal surface of midbrain consists
of superior and inferior corpora bigemina and four round lobes
called corpora quadrigemina. A canal known as cerebral aqueduct
passes through the midbrain. Midbrain is concerned with the sense
of sight and hearing.
It consists of three regions
cerebellum, and medulla oblongata.
(a) Pons is a band of nerve fibre that lies between
medulla oblongata and midbrain. It connects the lateral parts of
cerebellar hemisphere together.
(b) Cerebellum is a large and well developed part of
hindbrain. It is located below the posterior sides of cerebral
hemispheres and above medulla oblongata. It is responsible for
maintaining posture and equilibrium of the body.
(c) Medulla oblongata is the posterior and simplest part
of the brain. It is located beneath the cerebellum. Its lower end
extends in the form of spinal cord and leaves the skull through
(B) Eye: Eyes are spherical structures that consist of
(a) The outer layer is composed of sclera and cornea.
(i) Sclera is an opaque tissue that is usually known as
white of the eye. It is composed of a dense connective tissue.
(ii) Cornea is a transparent anterior portion of eye that
lacks blood vessels and is nourished by lymph from the nearby
area. It is slightly bulged forward and helps in focusing light
rays with the help of lens.
(b) The middle layer of eye is vascular in nature and
contains choroid, ciliary body, and iris.
(i) Choroid lies next to the sclera and contains numerous
blood vessels that provide nutrients and oxygen to the retina and
(ii) Ciliary body: The choroid layer is thin over
posterior region and gets thickened in the anterior portion to
form ciliary body. It contains blood vessels, ciliary muscles,
and ciliary processes.
(iii) Iris: At the junction of sclera and cornea, the
ciliary body continues forward to form thin coloured partition
called iris. It is the visible coloured portion of eye.
The eye contains a transparent, biconvex, and elastic structure
just behind the iris. It is known as lens. The lens is held in
position by suspensory ligaments attached to the ciliary body.
The lens divides the eye ball into two chambers
- an anterior aqueous and posterior vitreous
(c) The innermost nervous coat of eye contains retina.
Retina is the innermost layer. It contains three layers of cells
- inner ganglion cells, middle bipolar cells,
and outermost photoreceptor cells. The receptor cells present in
the retina are of two types - rod cells and
(a) Rod cells -The rods contain the
rhodopsin pigment (visual purple) that is highly sensitive to dim
light. It is responsible for twilight vision.
(b) Cone cells -The cones contain the
iodopsin pigment (visual violet) and are highly sensitive to high
intensity light. They are responsible for daylight and colour
The innermost ganglionic cells give rise to optic nerve fibre
that forms optic nerve in each eye and is connected with the
(C) Ear: Ear is the sense organ for hearing and
equilibrium. It consists of three portions -
external ear, middle ear, and internal ear.
1. External ear:
It consists of pinna, external auditory meatus, and a tympanic
(a) Pinna is a sensitive structure that collects and
directs the vibrations into the ear to produce sound.
(b) External auditory meatus is a tubular passage
supported by cartilage in external ear.
(c) Tympanic membrane is a thin membrane that lies close
to the auditory canal. It separates the middle ear from external
2. Middle ear:
It is an air-filled tympanic cavity that is connected with
pharynx through eustachian tube. Eustachian tube helps to
equalize air pressure in both sides of tympanic membrane. The
middle ear contains a flexible chain of three middle bones called
ear ossicles. The three ear ossicles are malleus, incus, and
stapes that are attached to each other.
3. Internal ear:
It is also known as labyrinth. Labyrinth is divided into bony
labyrinth and a membranous labyrinth. Bony labyrinth is filled
with perilymph while membranous labyrinth is filled with
endolymph. Membranous labyrinth is divided into 2 parts.
(a) Vestibular apparatus
Vestibular apparatus is a central sac-like part that is divided
into utriculus and sacculus. A special group of sensory cells
called macula are present in sacculus and utriculus.
Vestibular apparatus also contains three semi-circular canals.
The lower end of each semi-circular canal contains a projecting
ridge called crista ampularis. Each ampulla has a group of
sensory cells called crista. Crista and macula are responsible
for maintaining the balance of body and posture.
Cochlea is a long and coiled outgrowth of sacculus. It is the
main hearing organ. Cochlea consists of three membranes. The
organ of corti, a hearing organ, is located on the basilar
membrane that has hair cells.
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