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

(N/A) Brain
The brain is the main coordinating centre of the body. It is a part of the nervous system that controls and monitors every organ of the body. It is well protected by cranial meninges,which consist of an outer layer called dura mater,a thin middle layer called arachnoid,and an inner layer called pia mater. It is divided into three regions: forebrain,midbrain,and hindbrain.
Forebrain: It is the main thinking part of the brain. It consists of the cerebrum,thalamus,and hypothalamus.
Cerebrum: It is the largest part of the brain and constitutes about four-fifths of its weight. The two cerebral hemispheres are joined by a tract of nerve fibres known as the corpus callosum. The hemispheres are covered by a layer of cells known as the cerebral cortex or grey matter. The cerebrum has sensory regions known as association areas that receive sensory impulses from various receptors,as well as motor regions that control the movement of various muscles. The innermost part of the cerebrum is known as the white matter.
Thalamus: It is the main centre of coordination for sensory and motor signalling and is wrapped by the cerebrum.
Hypothalamus: It lies at the base of the thalamus and contains centres that regulate body temperature and the urge for eating and drinking. Some regions of the cerebrum,along with the hypothalamus,are involved in the regulation of sexual behaviour and emotional reactions such as excitement,pleasure,and fear.
Midbrain: It is located between the thalamus region of the forebrain and the pons region of the hindbrain. The dorsal surface of the midbrain consists of four round lobes called corpora quadrigemina. $A$ canal known as the cerebral aqueduct passes through the midbrain. The midbrain is concerned with the sense of sight and hearing.
Hindbrain: It consists of three regions: pons,cerebellum,and medulla oblongata.
Pons: It is a band of nerve fibres that lies between the medulla oblongata and the midbrain. It connects the lateral parts of the cerebellar hemispheres.
Cerebellum: It is a large and well-developed part of the hindbrain located below the posterior sides of the cerebral hemispheres and above the medulla oblongata. It is responsible for maintaining the posture and equilibrium of the body.
Medulla Oblongata: It is the posterior and simplest part of the brain,located beneath the cerebellum. Its lower end extends as the spinal cord and leaves the skull through the foramen magnum.
$(b)$ Eye
The eye is a spherical structure consisting of three layers:
$(i)$ The outer layer is composed of the sclera and cornea. The sclera is an opaque tissue known as the white of the eye,composed of dense connective tissue. The cornea is the transparent anterior portion that lacks blood vessels and is nourished by lymph.
$(ii)$ The middle layer is vascular and contains the choroid,ciliary body,and iris. The choroid lies next to the sclera and contains numerous blood vessels. The ciliary body is the thickened anterior portion of the choroid containing ciliary muscles. The iris is the thin,coloured,visible partition at the junction of the sclera and cornea.
$(c)$ Ear
The ear is the sense organ for hearing and equilibrium. It consists of $3$ portions: external ear,middle ear,and internal ear.
External Ear: It consists of the pinna,external auditory meatus,and tympanic membrane. The pinna collects sound vibrations. The external auditory meatus is a tubular passage. The tympanic membrane is a thin membrane separating the middle ear from the external ear.
Middle Ear: It is an air-filled tympanic cavity connected to the pharynx through the eustachian tube,which helps equalize air pressure. It contains three ear ossicles: malleus,incus,and stapes.
Internal Ear: It is also known as the labyrinth,divided into bony and membranous labyrinths. The bony labyrinth is filled with perilymph,while the membranous labyrinth is filled with endolymph. It includes the vestibular apparatus (utriculus,sacculus,and semi-circular canals) for balance and the cochlea for hearing,which contains the organ of Corti.