(b) Structure of the Eye \u2013<\/strong><\/p>\n![\"Biology](\"https:\/\/cdn1.byjus.com\/wp-content\/uploads\/2021\/02\/biology-chapter-21-image-2.png\" "\\"")
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Human eye is embedded in a bony socket of the skull and they are spherical. The walls of the eye ball consist of three layers, namely \u2013 The inner neurosensory coat, the middle vascular coat and the outer fibrous coat.<\/p>\n
Outer fibrous coat \u2013 it is a thick and tough covering protecting the eye ball and helping to maintain its form. It has two regions \u2013 cornea and sclera<\/p>\n
Sclera \u2013 consists of a dense white fibrous connective tissue, where only the white eye is visible, the remaining major part is orbiting. The white of the eye is composed of collagen fibers.<\/p>\n
Role \u2013 maintaining and protecting the shape of the eyeball.<\/p>\n
Cornea \u2013 It is the non-vascular transparent part of the outer fibrous coat that is visible and is covered by a thin, transparent vascular layer of stratified epithelium known as the conjunctiva. It is in the continuation with the lining of the eyelids.<\/p>\n
Role \u2013 The cornea refracts light which enters the eye and converges it into the lens.<\/p>\n
Middle vascular coat \u2013 it consists of three regions, namely \u2013 choroid, ciliary body, and iris.<\/p>\n
Choroid \u2013<\/p>\n
Highly vascular and made of loose fibrous connective tissue. It is in the continuation on the inner portion of the sclera. It finely layers over the posterior two-thirds of the eye ball and tends to turn thicker towards the front, imparting a bluish appearance. It consists of some pigmented cells.<\/p>\n
Role \u2013 nourishes the retina supplying it with oxygen. The pigmented cells absorb excessive light to avoid reflection in the eye ball.<\/p>\n
Ciliary body \u2013<\/p>\n
It is thick and forms the anterior part of the choroid. Comparatively, it is less pigmented and vascular and is composed of ciliary muscles and ciliary processes.<\/p>\n
\u2192Ciliary muscles \u2013 They are smooth muscles and are of two types \u2013 circular muscles and meridional muscles.<\/p>\n
\u2192Ciliary processes \u2013 the inner portion of the ciliary body has plenty of folds known as ciliary processes.<\/p>\n
Role \u2013 secrete aqueous humor.<\/p>\n
Iris \u2013 it is a fine, opaque and a pigmented structure, located at the junction of sclera and cornea. The color of the iris is imparted by the pigmented cells of the choroid. The color varies between black, dark-brown, blue or green. It consists of a pupil centrally, as an aperture. The iris consists of two types of smooth muscles \u2013 circular muscles and the radial muscles.<\/p>\n
Role \u2013 iris controls the eye size and hence the amount of light that enters. When radial muscles contract, pupil enlarges in dim light. When circular muscles contract, the pupil diminishes in bright light<\/p>\n
Inner neurosensory coat \u2013<\/p>\n
The retina forms the innermost, neurosensory fine layer of the eyeball. The outer surface of retina is in contact with the choroid and the inner surface is in contact with the vitreous humor.<\/p>\n
The external surface has four layers:<\/p>\n
Pigmented layer \u2013 it is made up of single layer of cells containing dark-brown pigment.<\/p>\n
The layer of photoreceptors \u2013 has two types of cells \u2013 rods and cones<\/p>\n
Rods \u2013 it is rod-shaped and elongated containing a purplish-red protein pigment known as rhodopsin or visual purple. It contains vitamin A derivatives. Rods do not respond to colours and are sensitive to dim light. They provide vision in the dark, hence known as twilight vision or scotopic vision<\/p>\n
Cones \u2013 these are sensitive to colors and bright lights, providing daylight or photopic vision. The pigment that is found in the cone cells is known as iodopsin. Three kinds of cone cells respond to green, red and blue light. Other colors are detected by the simultaneous trigger of cone cells of more than one kind. A sensation of white light is generated when all the three types of cells are simultaneously triggered.<\/p>\n
Cone cells are insensitive to the dim light hence in the dark, colour cannot be recognized.<\/p>\n
Layer of bipolar neurons, layer of ganglionic cells are the two layers. This layer contains the cell bodies of the ganglion cells which forms the optic nerve.<\/p>\n
Blind spot \u2013 The optic nerve exits the brain, retinal blood vessels enter the brain at a point where the photoreceptor cells are absent, this is the blind spot.<\/p>\n
Macula lutea \u2013 It is the yellowish pigmented spot that is present lateral to the blind spot. It is located exactly opposite the cornea\u2019s centre. It has a pit located centrally known as the fovea which lacks in rods and blood vessels. It has only cone cells and is the area of most distinct vision.<\/p>\n
Lens \u2013 It is elastic, transparent and biconvex in nature which is located just behind the iris. It is covered by a thin, elastic and transparent membrane known as the lens capsule. The lens is intact in its position due to the suspensory ligaments. These ligaments along with the lens segregates the eye ball into two chambers known as the aqueous chamber and the vitreous chamber.<\/p>\n
Aqueous chamber \u2013 it is the space between the cornea and the lens containing a thin watery fluid known as aqueous humor.<\/p>\n
Vitreous chamber \u2013 it is the space between the lens and the retina which is filled with a transparent gel known as vitreous humor.<\/p>\n
Ear<\/strong><\/p>\n![\"Biology](\"https:\/\/cdn1.byjus.com\/wp-content\/uploads\/2021\/02\/biology-chapter-21-image-3.png\" "\\"")
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The human ear has two sensory functions \u2013 enables hearing and maintains the balance of the body. It can be divided into three main sections \u2013 inner ear, outer ear, middle ear.<\/p>\n
\u2192Inner ear \u2013 also known as the labyrinth, it is split into membranous labyrinth and the bony labyrinth. Membranous labyrinth is filled with endolymph while bony labyrinth is filled with perilymph.<\/p>\n
The Membranous labyrinth is segregated into two portions \u2013 vestibular apparatus and the cochlea. The vestibular apparatus consists of three semi-circular canals and otolith. Each semi-circular canal lies in a different plane at right angles to each other. The membranous canals are suspended in the bony canals(perilymph). The base of the canals is swollen and is known as ampulla containing crista ampullas \u2013 a projecting ridge which has hair cells.<\/p>\n
The utricle and the saccule have a projecting ridge known as macula. The macula and the crista are the particular receptors of the vestibular apparatus that has a role to play in maintaining the posture and body balance.<\/p>\n
Saccules has a coiled and long outgrowth \u2013 cochlea which is the chief hearing structure consisting of three membranes. A hearing organ, the organ of corti, is situated on the basilar membrane possessing hair cells.<\/p>\n
\u2192Outer ear \u2013 it has the pinna and the external auditory canal (meatus). The pinna gathers the vibrations in the air that generate sound. The external auditory canal extends up to the ear drum (tympanic membrane). It has very fine hair, wax-secreting glands in the skin of the meatus and the pinna. The tympanic membrane consists of connective tissues covered with mucous membrane inside and with skin on the outside.<\/p>\n
\u2192Middle ear \u2013 it consists of three ossicles known as the malleus, stapes and incus that are linked to one another in a chain pattern. The malleus is linked to the tympanic membrane and the stapes is linked to the oval window of the cochlea. The ear ossicles increase the efficiency of transmission of sound waves to the inner ear. The middle ear cavity is connected to the pharynx through the Eustachian tube which aids in equalizing the pressure on both sides of the ear drum.<\/p>\n
<\/span>2. Compare the following:<\/strong><\/span><\/h3>\n<\/span>(a) Central neural system (CNS) and Peripheral neural system (PNS)<\/strong><\/span><\/h3>\n<\/span>(b) Resting potential and action potential<\/strong><\/span><\/h3>\n<\/span>(c) Choroid and retina<\/strong><\/span><\/h3>\nThe comparison is as shown below:<\/p>\n
(a) Central neural system (CNS) and Peripheral neural system (PNS)<\/p>\n
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\n\n\n| \n Central neural system (CNS)<\/strong><\/p>\n<\/td>\n| \n Peripheral neural system (PNS)<\/strong><\/p>\n<\/td>\n<\/tr>\n\n| \n Consists of the spinal cord and the brain<\/p>\n<\/td>\n | \n It consists of the spinal nerves and the cranial nerves<\/p>\n<\/td>\n<\/tr>\n | \n| \n Spinal column is protected by the vertebral column whereas the brain is protected by the skull<\/p>\n<\/td>\n | \n No protective structures<\/p>\n<\/td>\n<\/tr>\n | \n| \n No subdivisions<\/p>\n<\/td>\n | \n It is divided into autonomic nervous system and the somatic nervous system<\/p>\n<\/td>\n<\/tr>\n | \n| \n Processes information and regulates the responses to impulses.<\/p>\n<\/td>\n | \n Nerves of PNS passes impulses to the CNS and responses from the CNS to various structures of the body<\/p>\n<\/td>\n<\/tr>\n | \n| \n Group of neurons is known as nuclei<\/p>\n<\/td>\n | \n Group of neurons is known as ganglia<\/p>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n (b) Resting potential and action potential<\/strong><\/p>\n | | |
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