Briefly describe the structure of the following :
(a) Brain
(b) Eye
(c) Ear

Options:

None

Solution:
Structure of brain :
The brain is the central information processing organ of the body,
- The human brain is well protected by the skull.
- Inside the skull, cranial meninges cover the brain. These are tough tissue layers.
- Meninges consist of 3 layers which are as follows :
(i) The outermost - layer is dura mater
(ii) The middle layer is arachnoid
(iii) The inner layer is piamater.
The brain can be divided into three major parts which are given below :
(i) forebrain
(ii) midbrain
(iii) hindbrain
(i) Forebrain:
1. Cerebrum consists two cerebral hemisphere on the dorsal surface. It is connected by a tract of nerve fibres called corpus collasum. Cerebral hemispheres are covered by the layer of cells called cerebral cortex and are thrown into prominent folds referred as grey matter. Inner part of the cerebral hemisphere is white matter.
2. Diencephlon is the posterior part of fore-brain. It consists of thalamus and hypothalamus.
- Thalamus is a major co-ordinating centre for sensory and motor signaling. It forms $80 \%$ of diencephlon.
- Hypothalamus contains a number of centres which control many functions Like-hunger, thirst, sleep, sweating, body temperature and emotions
(ii) Midbrain:
It is located between the thalamus/hypothalamus of the forebrain and pons of the hindbrain.
It forms the brain stem with the hindbrain. Anterior part of mid-brain contains two cerebral peduncles, which controls the muscle of limbs and Posterior part of mid-brain in four optic lobs called corpora quadri gemiana i.e. two upper and two lower.
(iii) Hindbrain:
Hindbrain consists of pons. Cerebellum and medulla of longata.
- Pons is present below the midbrain and upper side of medulla oblangata. It posseses pneumotaxic area of respiratory centre.
- Cerebellum is the 2 nd largest part of brain, which lies behind cerebrum and provides the additional space for many neuron and maintains equillibrium or posture of the body.
- Medulla oblongata lies below cerebellum and continues into spinal-cord. It contains respiratory centre for regulating breatheing, Cardiac centre for regulating heart beat and blood pressure and also has reflex centre for swallowing, coughing, sneezing, etc.
(b) Structure of eye :
- Our paired eyes are located in sockets of the skull called orbits.
- The adult human eye ball is nearly a spherical structure.
- The wall of eye ball is composed of three layers which are given below:
- The external layer is composed of a dense connective tissue and is called the sclera. The anterior portion of this layer is called the cornea.
- The middle layer is called choroid contains many blood vessels and looks bluish.
- The choroid layer is thin over the posterior twothirds of the eye ball, but it becomes thick in the anterior part to form the ciliary body.
- The ciliary body itself continues forward to form a pigmented and opaque structure called the iris which is the visible coloured portion of the eye.
- The eye ball contains a transparent crystalline lens which is held in place by ligaments attached to the ciliary body.
- In front of the lens, the aperture surrounded by the iris is called the pupil. The diameter of the pupil is regulated by the muscle fibres of iris.
- The inner layer is the retina and contains three layers of cells- ganglion cells, bipolar cells and photoreceptor cells.
In the human eye, there are three types of cones which possess their own characteristic photopigments that respond to red, green and blue lights.
The sensations of different colours are produced by various combinations of these cones and their photopigments. When these cones are stimulated equally, a sensation of white light is produced.
The optic nerves leave the eye and the retinal blood vessels enter it at a point medial to and slightly above the posterior pole of the eye ball.
Photoreceptor cells are not present in that region and hence it is called the blind sport. At the posterior pole of the eye lateral to the blind spot, there is a yellowish pigmented spot called macula lutea with a central pit called the fovea.
The fovea is a thinned-out portion of the retina where only the cones are densely packed. It is the point where the visual acuity (resolution) is the greatest.
(c) Structure of ear :
Anatomically, the ear can be divided into three major sections called the outer ear, the middle ear and the inner ear.
Outer ear. The outer ear consists of the pinna and external auditory meatus (canal). The pinna collects the vibrations in the air which produce sound.
The external auditory meatus leads inwards and extends up to the tympanic membrane (the ear drum).
There are very fine hairs and wax-secreting sebaceous glands in the skin of the pinna and the meatus.
The tympanic membrane is composed of connective tissues covered with skin outside and with mucus membrane inside.
Middle ear : The middle ear contains three ossicles called malleus, incus and stapes which are attached to one another in a chain-like fashion. These ossicles transmit sound waves further inside the ear.
An Eustachian tube connects the middle ear cavity with the pharynx. The Eustachian tube helps in equalizing the pressures on either sides of the ear drum.
Inner ear the fluid-filled inner ear called labyrinth consists of two parts, the bony and the membranous labyrinths. The bony labyrinth is a series of channels. Inside these channels lies the membranous labyrinth, which is surrounded by a fluid called perilymph. The membranous labyrinth is filled with a fluid called endolymph. The coiled portion of the labyrinth is called cochlea.
The membranes constituting cochlea, the reissner's membrane and basilar membrane, divide the surrounding perilymph filled bony labyrinth into an upper scala vestibuli and a lower scala tympani.
The organ of corti is a structure located on the basilar membrane which contains hair cells that act as auditory receptors. The hair cells are present in rows on the internal side of the organ of corti.
The inner ear also contains a complex system called
vestibular apparatus, located above the cochlea. The vestibular apparatus is composed of three semi-circular canals and the otolith organ consisting of the saccule and utricle.
The base of canals is swollen and is called ampulla, which contains a projecting ridge called crista ampullaris which has hair cells. The saccule and utricle contain a projecting ridge called macula. The crista and macula are the specific receptors of the vestibular apparatus responsible for maintenance of balance of the body and posture.

Topic: Biology, Neural Control and Coordination