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Home , Human eye

THE HUMAN

By Jyoti Joshi nursing tutor Sight and the eye

• The eye is the of sight, • It is situated in the orbital cavity and supplied by the optic (2nd cranial nerve), • It is almost spherical in shape, • 2.5 cm in diameter, • The space between the eye and the orbital cavity is occupied by adipose tissue.

Internal Structures: The Eyeball

• It is a hollow sphere • Its wall is composed of three layers • Its interior is filled with fluids called humors that help to maintain its shape. • These three layers are- 1. Fibrous or avascular layer- sclerotic protection 2. Pigmented or vascular layer 3. Photosensitive layer- Fibrous/Avascular layer

• Outermost stiff layer made-up of connective fibers called sclerotic. • Divided into two parts i.e. 1. Larger and posterior – • White, outer layer of eyeball • Helps to maintain the spherical shape of eyeball 2. Frontal and smaller –

• Transparent layer

• Front part of eyeball through which waves pass

• No blood vessels but lots of nerve ending • Opening present at the junction of sclera and cornea called canals of schlem.

• Excessive amount of aqueous humor is drained into the canal of schlem, comes in the vascular layer and diffused into the blood.

• - thin, transparent mucous membrane that covers the eyeball. Pigmented/vascular layer

• Includes 1. - • Present below sclera • Largest part of pigmented layer • Act as diaphragm of camera to prevent reflection of light • It is very rich in blood vessels 2. - • Dark coloured rounded structure present beneath the cornea. • Distribution of melanin in iris determine the eye 3. - rounded opening of iris through which light passes • surrounded with circular and radial muscles • Radial muscles are supplied by sympathetic nerve • Circular muscles are supplied by parasympathetic nerve. • Parasympathetic stimulation constricts the pupil and sympathetic stimulation dilates it. 4. Ciliary bodies- • The is the anterior continuation of the choroid • Associated with coat, through suspensory ligament to hold the lens. • Capillaries present in the ciliary process produce humour which provide the intraoccular pressure. 5. Lens- • highly elastic circular biconvex body • The lens bends (refracts) light rays reflected by objects in front of the eye. • The lens bends (refracts) light rays reflected by objects in front of the eye. • which is achieved by changing its thickness. Photosensory layer

Retina – • The retina is the innermost layer of the wall of the eye. • Its surface contains photoreceptors (cells responsible for converting light into nerve impulses- rods and cone) 1. Rod cells- • Cylindrical in shape • 110-125 millions • Has visual pigment . • Helps in vision during dim light. • Does not differentiate colours. • Provide peripheral vision • Found on the edges of the retina 2. Cone cells- • Cone shaped • 6-7 millions • Has visual pigment iodopsin • Helps in vision during intense light. • Helps in differentiation of primary coloure- , green & red. • Found near the center of retina. Yellow spot- • Present in retina on the axis of lens • Bears only cone cells - • Part of retina without rod and cone cells • are come out from the retina Ora- serrata- Differentiate the photosensory and non photosensory area. Neurons- bipolar neuron and amacrine cells are found in retina. Accessory Organs & Eye Protection • Orbital cavities (bony sockets)- house & protect the ... • Adipose tissue- cushions the eye • Lacrimal glands- produce tears that lubricate and have a germicidal effect. • - protect against foreign articles, perspiration & direct rays of light.

• - fold of skin that cover the surface of eye; close by action when an object approaches.

• - secrete oils that prevent lids from sticking together. Muscles of eye

1. Extrinsic muscles-

• Muscle locates outside the eye

• Control certain eye movements, such as moving eyeball from side to side or rolling the 2. Intrinsic muscles

• Muscles located inside the eye

• Hold the eye lens in place and modify its shape Cavities within the eyeball

1. Anterior cavity • Filled with • Divided into anterior & posterior chamber through iris • Provide intraoccular pressure 2. Posterior cavity • Filled with viterous humour • Semi solid • Provide a part of IOP to maintain shape of eyeball • IOP- 10-20mmHg BLOOD SUPPLY: • The eye is supplied with arterial blood by the ciliary and the central . • Venous drainage is by a number of , including the central retinal • The and vein are encased in the optic nerve, which enters the eye at the . • Optic (second cranial nerves): • The fibres of the optic nerve originate in the retina • they converge to form the optic nerve about 0.5 cm to the nasal side of the (yellow spot)macula lutea. • Optic chiasma – nerve from both side cross each other. Physiology of vision

1. of eye.

2. Image formation

3. Biochemistry of vision Few Properties of light

• Light waves travel at speed of 300000km per second. • Light is reflected into the eyes by object within the field of vision. • White light is the combination of all the colours of the visual spectrum (rainbow), i.e. red, orange, yellow, green, blue, indigo and violet. • Red light has the longest wavelength and violet the shortest.

Refraction of the light rays • Light travels from one medium to another medium then its bends or its path will change. • When light travel from rarer medium to denser medium then it bends towards normal. • Similarly in our eye: the biconvex lens bends and focuses light ray . Accommodation of eye

• Accommodation is the ability of eyes to see the object at different distances. • To see the nearer object/Close vision: • to focus on near objects, i.e. within about 6 meter's, the eye must make the following adjustments. • constriction of the • convergence • changing the power of the lens. • Diameter of the pupil decreases, • Convexity of the lens increases, • Focal length of decreases. To see the distant object/distant vision

• Objects more than 6 metres away from the eyes are focused on the retina without adjustment of the lens or convergence of the eyes. Image formation

• Light coming from the object enters the retina • which passes through the aqueous humour, lens, viterous humour with several it finally fall on the retina. • Where images are formed. • Real, smaller than object and inverted image formed on the retina. Biochemistry of vision

• The retina is the light-sensitive (photosensitive) part of the eye. • The light-sensitive nerve cells are the rods and cones • Light rays cause chemical changes in light- sensitive pigments in these cells • they generate nerve impulses which are conducted to the occipital lobes of the cerebrum via the optic nerves • The rods are much more light sensitive than the cones. • so they are used when light levels are low. • Stimulation of rods leads to monochromic (black and white) vision. • The cones are sensitive to bright light and colour. • Light split rhodopsin (Visual purple) into pigment retinene (retinol) and a scotopsin. • The process of splitting is called bleaching. • In dark, rhodopsin is resynthesizes from retinene and scotopsin making rods functional. Binocular vision • Binocular or stereoscopic vision enables three- dimensional views. • although each eye ‘sees’ a scene from a slightly different angle. • The images from the two eyes are fused in the cerebrum so that only one image is perceived. • Binocular vision provides a much more accurate assessment of one object relative to another.