Lens is colourless, transparent and fibrous crystaline structure made up of protein and enclosed in lens membrane. It is ectodermal in origin. Lens is lodged in eye ball by suspansory ligament of ciliary body. Suspansory ligaments are known as "Zonula of Zinn". In man lens is biconvex while in frog it is elliptical.
Lens divide the eye ball in 2 chamber outer aqueous chamber (partially divided into a large anterior and a smaller posterior chamber) filled with aqueous humor (watery) formed by ciliary body and inner vitreous chamber filled with vitreous jelly (or Wharton's jelly) containing 99% water, some salt a little mucoprotein (vitrein) and hyaluronic acid. It is the part between lens and retina. At its periphery, the jelly like fluid is condensed to form a hyaloid membrane. A narrow hyaloid canal runs through the vitreous humor obliquely from the region of blind spot upto the middle of posterior face of the lens. An intrinsic network of fine collgen fibres and some rounded cells hyalocytes are suspended in the vitreous humour.

: It is innermost, thin and transparent, purplish red due to the present of the eye pigment rhodopsin (in rods) or visual purple which was extracted by Kuhne (1876) and named 'Schpurpur' (Visual purple). Made up of 4 distinct layer –
 Cuboidal pigmented epithelium (towards choroid).
 Layer of rods and cones.
 Layer of bipolar neurons.
 Layer of ganglia (Towards vitreous chamber innermost).
Inverted retina : Light rays can enter into a vertebrate eye, not through sclerotic and choroid, but only through the cornea and lens. Obviously, the light rays penetrate the retina from its inner, ganglionic layer. Then, the impulses of photoreception, set up by rod and cone cells, pass back in reverse direction through bipolar ganglionic cells and finally into the fibres of optic nerve. Due to such an arrangement this is an "inverted retina" and the eye is said to be inverted eye.
 Area centralis of retina : A little part of retina that lies upon the optical axis is called area centralis. Here, the retina is very thin and contains only cone cells filled with a yellow pigment. Hence, this part is called yellow spot or maculla lutea. In man (Rabbits) and other mammals, but not in frogs, this area has a small shallow dispression called fovea centralis. The latter is the most sensitive part of an eye, i.e. the area of most acute vision. It is also claimed that the cone cells in foven centralis are placed somewhat obliquely. So that these can form magnified images of object.
 Blind spot (Optic disc) : At this point, the optic nerve turns towards the outer side, pierces through the whole thickness of the wall of eyeball, forming an optic foramen and runs to the brain. Obviously, the region of optic foramen has no retina. It therefore, does not take part in image formation and is called blind spot.

(a) Mechanism of light perception : The human eye has two functional parts – Dioptric or Focussing part and Receptor part.
 Focussing part : It consists of conjunctiva, cornea, aqueous humour lens and vitreous humour. These part are transparent and act as lenses. They refract the light rays passing through the eye to bring them to a focus on the retina. Maximum refraction is caused by the cornea, which places the image approximately on the retina. The lens effects fine adjustment and brings the image into a sharp focus.
 Receptor part : It comprises the retina. The image formed on the retina is inverted and smaller. It converts the energy of specific wave lengths of light into action potential in nerve fibre.
(b) Pathway of sensory impulses from eye to brain : The nerve impulses generated in the retina of the eye in response to light follow a definitive path and terminates in visual cortex in each optic lobe which act as primary visual center.

# Biochemistry of eye : The receptor cells of eye are called photoreceptor or visual cells. They are of two types – Rod cells and Cone cells named after their shapes. Both have light sensitive pigments. Specific wavelengths of light breakdown the light sensitive pigments and this stimulates the receptor cells to set up nerve impulses.

# (1) Rod cells : The rod cells contain a purplish pigment called visual purple or rhodopsin. They function in dim light and at night. They produce poorly defined images. Bright light splits rhodopsin into a lipoprotein scotopsin and a carotenoid pigment retinal (retinine) a process called bleaching. The spiliting of rhodopsin depolarizes the rod cell and it releases a neurotransmitter, passing the nerve impulse via bipolar neuron and ganglion cells to the optic nerve. In the dark, rhodopsin is resynthesized from scotopsin and retinal. This process is called 'dark adaptation'. It makes the rods functional. It takes sometime for rhodopsin to be reformed. This is why on entering a dark room at daytime or on coming out of a well lighted room at night we feel blind for a while, when we go from darkness into bright light, we feel difficulty in seeing properly for a moment till rhodopsin is bleached and cones become functional

# (2) Cone cells : Cones contain iodopsin which is visual violet and made up of photopsin + retinal. The 3 types of cones are erythrolobe (775 nm sensitive tored), cyanolabe (430 nm sensitive to blue) and chlorolabe (sensitive to green 535 nm). However, if all the cone, types are simultaneously stimulated by equal amounts of coloured light than sensation for white light is perceived.
Diurnal animals are adapted to see during day light (Photopic vision) and can perceive colour. In dark, colours are not perceived. Such animals have more cones in their eye than rods.

# Accommodation and types of vision
(1) Accommodation : Light passes through many refractive surfaces before it is focussed on the retina forming an inverted and true image. The main sites of refraction are cornea  aqueous humor – iris – lens (position can be altered by ciliary body : accommodation) – posterior chamber (= vitreous humor)  retina ( in fovea). The refractive index of the eye varies from 59 diopter (when the lens is at rest) to about 71 diopter (when lens is bulging in maximum accomodation). The accommodation reflex occurs when the eye changes its focus from a far away object to nearer one. The change in strength of the lens provides the physiological basis of accommodation. Radial and circular muscle fibres of ciliary muscles play an important role in this as they contract reflexively (parasympathetic control) and increase lens strength. The pupil constricts. This facilitates increase in sharpness of image. Ageing causes loss of accomodation.

# Types of vision
 Binocular vision : Man has binocular vision in which both the eyes are focussed on the same object but from slightly different angles. The visual fields of both eyes overlap and the foveae of both are focused on the same object. This provides depth to the images, i.e., gives stereoscopic or 3D effect and enables man to judge distances correctly.
 Vision in other animals : Primates and predatory animals, such as owl and cat, have binocular vision. In some animals, such as rabbit, birds, each eye is focussed on a separate object. This is termed monocular vision.
 Colour vision : It is the ability of some animals to detect colours in an object. Humans, apes, monkeys, and most fishes, amphibians, reptiles and birds have strong colour vision. The insects and crayfish also have colour vision. In vertebrates, colour vision results from the activity of cone cells. Most domestic and nocturnal mammals and sharks lack colour vision. They probably see objects in shades of grey (monochrome vision).
 Nocturnal and Diurnal vision : Man has both day vision and night vision as he has both rods and cones in considerable numbers in the retina. Most birds have only day vision as their retina contains mainly cones. Owls have much better night vision than day vision for they possess a large number of rods and few cones in their retina.
 Range of vision : The visible range of spectrum varies in animals. Bees, ants, spiders and goldfish can see ultraviolet light, which is invisible to man.

Eye movement
 In eye orbit eyeball remain attached with 6 extrinsic muscles.
 Out of six, four are rectus and two are oblique muscles.
 Four rectus muscles are
(1) Anterior rectus or Internal ractus
(2) Posterior rectus or External ractus
(3) Inferior rectus
(4) Superior rectus
 Oblique muscles are
(1) Inferior oblique muscle (2) Superior oblique muscle

(1) Myopia
 Also known as near sightness.
 Short sightness.
 Near object is clear. Far object is not clear.
 Eyeball become longer.
 Image is formed before retina. Can be removed by concave lens.
(2) Hypermetropia
 Also known as hypermetropia or long sightness.
 Far sightness.
 Far object is clear, near object is not clear.
 Eye ball become short.
 Image is formed behind the retina.
 Can be removed by convex lens or lens convient.
(3) Astigmatism
 Curvature of cornea become irregular and image is not clearly form.
 Can be removed by cylindrical lens.
(4) Cataract
 It is due to defective protein metabolism.
 During this lens or cornea sometime both become opaque.
 Operation is needed.
(5) Gloucoma
 It is due to increase in intraocular pressure in aqueous chamber.
 Operation is needed at early stage due to blockage of shlemm’s canal.
(6) Trachoma
 It is increased in redness of eye and more secretion of watery fluid.
 It is due to infection of bacteria, chlamidia trachamastis.
 Due to this follicles may form in conjunctiva.
(7) Xerothalmia
 It is due to deficiency of vitamin A. (A2)
 During this conjunctiva or cornea becomes keratinized.
 It may lead to blindness.
(8) Strabimus
 In this type eyeball remain in some what in bended position.
 It is due to long extra ocular muscles during development of eye.
 Operation is needed at early stage.
 Also associated with squint.
(9) Presbiopia
 During this power of accommodation of lens decreases due to age factor and defected metabolism.
 Also known as age sightness.
 Can be removed by bifocal lens.
(10) Photofobia
 No clear image in bright light.

Also known as stato-acuostic organ. It is the receptor for balancing and hearing which is sensitive for gravity and sound waves. It is also sensitive in orientation of body. It is also known as mechano receptor because of it change mechanical energy of sound waves in to action potential.
(i) Structure of Ear : Ear of mammal is divided in to 3 parts –
(a) External ear (b) Middle ear (c) Internal ear
(a) External ear : It is made up of pinna and auditory meatus. Pinna is found in only mammals. Its upper rounded part is helix and lower is ear lobe. It is made up of adipose connective tissue and elastic cartilage and has ear muscles which are vestigeal in case of human beings. Pinna collect the sound waves and drive towards auditory meatus.
Auditory meatus is 25 mm. long canal and made up of fibro elastic cartilage. It possesses ceruminus gland which secrete cerumin (ear wax). Cerumin trap the dust particles and microbes.
Tympanic membrane : It is also called ear drum and present at the junction of auditory meatus and tympanic cavity. Tympanum (Ear drum) is made up of fibrous connective tissue covered by stratified epithelium on external side and mucous membrane on the inner side. Its central region is known as umbo.
(b) Middle ear : The cavity of middle ear is known as tympanic cavity which is enclosed by tympanic bulla bone of skull and filled with air.
Ear ossicle : A chain of three small, movable bones, the auditory or ear ossicles crosses the tympanic cavity. The outer ossicle is attached to the inner surface of the tympanic membrane.

Muscles :
Tencer tympani : It connect malleus to tympanic cavity.
Stepadius : It connect stapes to incus. It is the smallest muscles of the body.
Eustachian tube : It is made up of elastic cartilage and it connect middle ear to nosopharynx. It maintain equilibrium in and out side of the tympanic membrane. Blocking of eustachian tube impairs hearing due to imperfect vibrations of drum. Lining of eustachian tube is pseudostratified epithelium (P.S.E.). P.S.E. is also present in trachea, bronchi and Larynx.
Fenestrae : Between middle ear and internal ear a thin bony membrane is present which possess two apertures (Windows).
Fenestra ovalis : It is upper window, connect middle ear to internal ear and guarded by membrane. End of stapes is fit on the upper window. It is towards vestibule so it is also known as F. vestibuli.
Fenestra rotundus : It is ventral window, connect middle ear to internal ear and guarded by membrane. It is towards scala tympani so it is also known as F. Tympani (also known as F. cochleae).
(c) Internal ear (Membranous labyrinth) : Internal ear is also known as membranous labyrinth and enclosed by bony labyrinth. Bony labyrinth is formed by periotic bone or petrous. A cavity is present between membranous labyrinth and bony labyrinth known as perilymphatic space. It is filled with perilymph and endolymph is found in membranous labyrinth. The membranus labyrinth consists of 2 parts – Vestibule and Cochlea.
 Vestibule : The vestibule is a central sac like part. It further consists of 2 chambers large – Utriculus (Upper) and smaller – sacculus (lower).

# Semicircular canal : From utriculus 3 semicircular canals arise these are
- Anterior semicircular canal (Superior)
- Posterior semicircular canal (Inferior)
- Horizontal semicircular canal (External)
They are perpendicular each other.
Crus commune : A common part of anterior and posterior semicircular canal arise from dorsal region of utriculus is known as crus commune.
Ampulla : Terminal part of the each semicircular canal is enlarged to form an ampulla.
Crista : Each ampulla has a sensory spot called crista ampullaris or simply crista, for equilibrium.
Sacculus : It is a lower chamber of vestibule. From the lower part of the sacculus arises a short tube, the ductus reuniens, that joins the cochlear duct.
Ductus endolymphaticus : It is filled with endolymph and arises from the junction of utriculus and sacculus.
Macula : are present in utriculus and sacculus. it is a group of sensory cells. In man (Rabbit) 2 maculas are present. (A crista resembles a macula in structure except that lies on an elevation, the acoustic ridge, its sensory cells have longer "hair", and its gelatinous mass is dome shaped, lacks otoliths and is called cupula.)

(a) Hearing : The ear not only detects sound but also notes its direction, judges its loudness and determines its pitch (frequency) sound waves are collect by the pinna and directed inward through the external auditory meatus (frequency 430 cycle per second). Here they strike the tympanic membrane. The latter begins to vibrate at the same frequency as that of the sound waves. From the tympanic membrane, the vibration are transmitted across the tympanic cavity by the ear ossicles to the membrane of the fenestra ovalis. The force of vibrations is considerably increased in the middle ear by leverage of the ossicles and also by much smaller surface area of the membrane of fenestra ovalis than that of the tympanic membrane. (The frequency is 2400 cycle/sec). Increase in frequency is important because the sound wave are transmitted from air to a fluid medium. The membrane of fenestra ovalis transmits the vibrations to the perilymph of the scala vestibuli and hence via Reissner's membrane to the endolymph in the scala media. From here the vibrations are transferred to the basilar membrane and the perilymph in the scala tympani.
Vibration of the endo lymph of the scala media cause the basilar membrane of this chamber vibrate. Vibrations of the basilar membrane make the "sensory hair" of receptor cells in the organ of corti move in the overlying gelatinous membrane (Tectorial membrane) and get distorted. This stimulation causes depolarisation of the receptor cells and initiation of nerve impulse in the fibres of the auditory nerve. The nerve impulse travels via relay centers.
# Human ear can hear a frequency of 2000 cps. However, it can hear the complete range of frequencies from 20 cps – 20,000 cps only with intense sound. Sound energy is measured in terms of units called decibels (dB). Sounds in our city homes average 40 – 50 dB, but street noise averages 70 – 80 dB. Sounds up to 80 dB are considered bearable by man, but higher sound intensity are hazardous, causing nervous stress, irritability, increased blood pressure etc. Non stop noise of 90 or more dB produces temporary deafness. 160 dB sound can cause total deafness by rupturing our ear drum.

# (1) Static equilibrium and linear acceleration : Maculae detect changes in the head (or body) with respect to gravity (static equilibrium) and in the movement in one direction (linear acceleration). With a change in the position of the body, the otoliths, being heavier than the endolymph, press upon the sensory hairs of the maculae. This stimulates the sensory cells which initiates nerve impulse in the fibres of the auditory nerve. The macula of utricle responds to vertical movements of the head, and the macula of saccule responds to lateral (sideways) movement of the head.
On rapid forward movement, the otoliths, because of having greater inertia than the surrounding endolymph, lag behind and press back the sensory hair, stimulating the sensory cells to generate nerve impulses.

# (2) Dynamic equilibrium : Cristae detect turning or rotational movements of the head (angular acceleration). When the head is turned, the endolymph in the semicircular ducts, due to its inertia, does not move as fast as the head and the sensory cells of the crista, but continues to move after the head stops moving. Because of this difference in the rate of movement, the sensory hair of the cristae are swept through the endolymph and become bent over. This disturbance stimulates the sensory cells and sets up action potential in the fibres of the auditory nerve, which transmits it to the brain. Since the three semicircular ducts are arranged in three different planes, a movement of the head in any direction will stimulate the sensory cells of at least one crista.

(iii) Defects of ear
(a) Labyrinthine disease : Malfunction of inner ear.
(b) Meniere's disease : Loss of hearing due to defect in cochlea.
(c) Otitis media : Acute infection of middle ear.
(d) Eustachitis : Inflammation of eustachian tube.
(e) Myringitis (Tymanitis) : Inflammation of eardrum.
(f) Otalgia : Earache (pain in ear)