Brieflydescribe the structure of the following:

1. Brain (b) Eye (c) Ear

(A) Brain: Brain is the main coordinating centre of thebody. It is a part of nervous system that controls and monitors everyorgan of the body. It is well protected by cranial meninges that aremade up of an outer layer called dura mater, a thin middle layercalled arachnoid, and an inner layer called pia mater.

It is divided into three regions − forebrain, midbrain, andhindbrain.

Forebrain: It is the main thinking part of the brain. Itconsists of cerebrum, thalamus, and hypothalamus.

(a) Cerebrum:

Cerebrum is the largest part of the brain and constitutes aboutfour-fifth of its weight. Cerebrum is divided into two cerebralhemispheres by a deep longitudinal cerebral fissure. Thesehemispheres are joined by a tract of nerve fibre known as corpuscallosum. The cerebral hemispheres are covered by a layer of cellsknown as cerebral cortex or grey matter. Cerebrum has sensory regionsknown as association areas that receive sensory impulses from variousreceptors as well as from motor regions that control the movement ofvarious muscles. The innermost part of cerebrum gives an opaque whiteappearance to the layer and is known as the white matter.

(b) Thalamus:

Thalamus is the main centre of coordination for sensory and motorsignalling. It is wrapped by cerebrum.

(c) Hypothalamus:

It lies at the base of thalamus and contains a number of centres thatregulate body temperature and the urge for eating and drinking. Someregions of cerebrum, along with hypothalamus, are involved in theregulation of sexual behaviour and expression of emotional reactionssuch as excitement, pleasure, fear, etc.

Midbrain:

It is located between the thalamus region of the forebrain and ponsregion of hindbrain. The dorsal surface of midbrain consists ofsuperior and inferior corpora bigemina and four round lobes calledcorpora quadrigemina. A canal known as cerebral aqueduct passesthrough the midbrain. Midbrain is concerned with the sense of sightand hearing.

Hindbrain:

It consists of three regions − pons, cerebellum, and medullaoblongata.

(a) Pons is a band of nerve fibre that lies between medullaoblongata and midbrain. It connects the lateral parts of cerebellarhemisphere together.

(b) Cerebellum is a large and well developed part ofhindbrain. It is located below the posterior sides of cerebralhemispheres and above medulla oblongata. It is responsible formaintaining posture and equilibrium of the body.

(c) Medulla oblongata is the posterior and simplest part ofthe brain. It is located beneath the cerebellum. Its lower endextends in the form of spinal cord and leaves the skull throughforamen magnum.

(B) Eye: Eyes are spherical structures that consist of threelayers.

(a) The outer layer is composed of sclera and cornea.

(i) Sclera is an opaque tissue that is usually known as whiteof the eye. It is composed of a dense connective tissue.

(ii) Cornea is a transparent anterior portion of eye thatlacks blood vessels and is nourished by lymph from the nearby area.It is slightly bulged forward and helps in focusing light rays withthe help of lens.

(b) The middle layer of eye is vascular in nature and containschoroid, ciliary body, and iris.

(i) Choroid lies next to the sclera and contains numerousblood vessels that provide nutrients and oxygen to the retina andother tissues.

(ii) Ciliary body: The choroid layer is thin over posteriorregion and gets thickened in the anterior portion to form ciliarybody. It contains blood vessels, ciliary muscles, and ciliaryprocesses.

(iii) Iris: At the junction of sclera and cornea, the ciliarybody 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 justbehind the iris. It is known as lens. The lens is held in position bysuspensory ligaments attached to the ciliary body. The lens dividesthe eye ball into two chambers – an anterior aqueous andposterior vitreous chamber.

(c) The innermost nervous coat of eye contains retina. Retinais the innermost layer. It contains three layers of cells –inner ganglion cells, middle bipolar cells, and outermostphotoreceptor cells. The receptor cells present in the retina are oftwo types – rod cells and cone cells.

(a) Rod cells –The rods contain the rhodopsinpigment (visual purple) that is highly sensitive to dim light. It isresponsible for twilight vision.

(b) Cone cells –The cones contain the iodopsinpigment (visual violet) and are highly sensitive to high intensitylight. They are responsible for daylight and colour visions.

The innermost ganglionic cells give rise to optic nerve fibre thatforms optic nerve in each eye and is connected with the brain.

(C) Ear: Ear is the sense organ for hearing and equilibrium.It consists of three portions – external ear, middle ear, andinternal ear.

1. External ear:

It consists of pinna, external auditory meatus, and a tympanicmembrane.

(a) Pinna is a sensitive structure that collects and directsthe vibrations into the ear to produce sound.

(b) External auditory meatus is a tubular passage supportedby cartilage in external ear.

(c) Tympanic membrane is a thin membrane that lies close tothe auditory canal. It separates the middle ear from external ear.

2. Middle ear:

It is an air-filled tympanic cavity that is connected with pharynxthrough eustachian tube. Eustachian tube helps to equalize airpressure in both sides of tympanic membrane. The middle ear containsa flexible chain of three middle bones called ear ossicles. The threeear ossicles are malleus, incus, and stapes that are attached to eachother.

3. Internal ear:

It is also known as labyrinth. Labyrinth is divided into bonylabyrinth and a membranous labyrinth. Bony labyrinth is filled withperilymph 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 intoutriculus and sacculus. A special group of sensory cells calledmacula are present in sacculus and utriculus.

Vestibular apparatus also contains three semi-circular canals. Thelower end of each semi-circular canal contains a projecting ridgecalled crista ampularis. Each ampulla has a group of sensory cellscalled crista. Crista and macula are responsible for maintaining thebalance of body and posture.

(b) Cochlea:

Cochlea is a long and coiled outgrowth of sacculus. It is the mainhearing organ. Cochlea consists of three membranes. The organ ofcorti, a hearing organ, is located on the basilar membrane that hashair cells.