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

(I) (a) Structure of brain. The brain is divided into three regions – Forebrain, Midbrain and Hindbrain. Procencephalon. It is made up of olfactory lobes and the cerebrum. Olafactory lobes are concerned with the sense of smell. Cerebrum is made up of two cerebral hemispheres, which are joined, by band of nerve fibres called corpus callosum. Each hemisphere is divided into lobes viz, Frontal, Parietal, Temporal and Occipital. Diencephalon. The roof of diencephalon is called hypothalamus. The anterior part of hypothalamus is vascular and folded to form the anterior choroid plexus. The hypothalamus gives off a body called the pineal body suspended by the pineal stalk. The sidewalls of the diencephalon are called the thalamus while its floor is called the hypothalamus. The optic nerves coming out of the eyes cross at a point called optic chiasma in front of the hypothalamus. The pituitary gland (hypophysis) is directly attached to the hypothalamus by a stalk called infundibulum. (II) Midbrain. It exists in the form of two pair of round protrusions called coproa quadrigemina. The anterior pair is called superior colliculi and the posterior pair is called inferior colliculi. Two bundles of fibres lie on the lower surface of midbrain and are called cerebral peduncles. (III) Hindbrain.  1. Cerebellum. It consists of two cerebellar hemispheres joined by a central worm- shaped parte, the vermis. It controls muscular activities concerned with maintaining body equilibrium. 2. Pons Varoli. It is situated in front of the cerebellum, below the midbrain and above the medulla oblongata. It consists of nerve fibres connecting the two-cerebellar hemispheres. 3. Medulla Oblongata. It extends from the pons varoli to the spinal cord. It also possesses a non-vascular folded structure called posterior choroid plexus. (b) Structur of eye: It is a hollow spherical structure measuring about 2.5 cm in diameter. Its wall is composed of three coats - Outer fibrous coat made of sclera and cornea. Sclera: It contains many collagen fibres. It functions to protect and maintain the shape of the eyeball. Cornea: It is transparent in nature that helps to admit and focus light. It is a vascular. Middle vascular coat made of chorod, ciliary body and iris. Choroid: It lies adjacent to scleral and is abundant in blood vessels that provide nutrients and oxygen. Ciliary Body: It extends towards the inside of the eye from Choroid. It consists of ciliary muscles and ciliary processes. The ciliary processes secrete aqueous humour. Suspensory ligaments are attached from the ciliary body to the lens and hold the lens in place. Iris: It is the pigmented region that gives eye its colour. It separates the aqueous humour region into anterior and posterior region. It has an opening in the center called the pupil. The iris controls the amount of light entering the eye with the help of muscles. Retina (Inner nervous coat): The retina is the neural and sensory layer of the eye. A small oval yellowish area of the retina lying opposite the centre of the cornea is called Macula lutea or yellow spot, which contains the fovea centralis. The fovea centralis has cone cells only and no rod cells. (c) Structure of ear. Each ear consist of three portions – External Ear. It comprises of a pinna and external auditory meatus. Pinna. The pinna is a projecting elastic cartilage covered with skin. It performs the function of collecting the sound waves. External Auditory Meatus. It is a tubular passage lined by hairy skin and ceruminous glands. The functions of the meatus is to pass on the sound waves and that of ceruminous glands is to secrete cerumen (ear wax) to prevent the entry of foreign bodies into the ear. Middle Ear. It contains the following parts – Tymphanic Membrane. It separates the tymphanic cavity from the external ear. It is thin and semi transparent. Tymphanic cavity. It is the cavity of the middle ear that is connected with the mesopharynx via the eustachian tube that maintains pressure. The tymphanic cavity contains a small flexible chain of three small bones called ear ossicles – the malleus (hammer shaped), the incus (anvil shaped) and the stapes (stirrup shaped). The middle ear is connected to the inner ear through two small openings – (i) Fenestra ovalis (oval window) (ii) Fenestra rotunda (round window) Internal Ear. It consists of the membranous labyrinth. The membranous labyrinth of the middle ear consists of the following parts: Semi Circular Canals: There are three semi circular canals – the anterior, posterior and the lateral semi circular canals which are perpendicular to each other. Utricle, saccus endolymphaticus and saccule. The three semicircular canals are connected to the utricle. The sacculus is joined to the utriculus via an utriculo- saccular duct. From this duct, a long tube called ductus endolymphaticus arises which ends blindly as the saccus endolymphaticus. Cochlea. It is the main hearing organ, which consists of three fluid filled chambers, the upper scala vestibuli, middle scala media and the lower scala tympani. The cochlea contains the organ of corti, which has hair cells that help in hearing.