In man and other higher animals there are five types of special senses to sense the environment . These are sense of sight , hearing , touch , taste and smell . Correspondingly , there are five types of sense organs :
- Sense organs of sight or Photoreceptors.
- Sense organs of hearing-cum-equilibrium or Phonoreceptors cum statoreceptors.
- Sense organs of touch or Mechanoreceptors.
- Sense organs of taste or Gustatoreceptors.
- Sense organs of smell or Olfactory receptors.
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THE EYES-Sense organs of sight (Photoreceptors)
The sense organs of sight or photoreceptors are a pair of eyes. Each eye is a spherical ball- like structure measuring about 2.5 cm in diameter and is called eyeball.
The eyeballs are attached in the eye orbits of skull and can rotate with the help of six muscles. Two of these muscles are oblique and four are rectus muscles.
Protection of eyes The eyes are a very delicate structure. These are protected by various accessory structures such as :
(i) Adipose tissue : Below the eyeball is present a cushion of adipose tissue. It protects the eyeball from any kind of mechanical injury and shock.
(ii) Eyelids : The upper and lower eyelids are movable. They protect the eye from any kind of injury. They also clean the surface of eyeball by regular blinking.
Moreover, the eyelids spread the lubricating secretions over the eyeball for proper functioning. They also close the eyes and act as shutters during night and bright light.
(iii) Eyelashes : These are very small but thick hairs present on the free margins of eyelids in a row. They act as strainers. They do not allow the dust and other foreign particles to enter in the eye. Moreover, in the hair follicles of these eyelashes open special oil glands that secrete lubricating liquid. Infection of these glands is called sty.
(iv) Eyebrows : These eminences of skin above the eyes and bear hair. They protect the eyes from dust, sweat and rain-water. are arched
(v) Lacrimal glands : These are present on the lateral aspects of eyes. A lacrimal gland is of the size and shape of an almond present below the eyelids. Their secretions are called tears that wash away any dust and grit from the eyeballs. The tears contain lysozymes which are bactericidals and kill bacteria. The tears spread over the surface of eyeball and lubricate them.
(vi) Nose : Present between the two eyes, it acts like a shield. It prevents the eyes from being hit by any flat object.
FUNCTIONS OF TEARS
Tears secreted by the lacrymal glands perform the following functions:
- They wash away the dust particles and clean the surface of eyes.
- They lubricate the surface of eyeballs.
- They contain lysozymes which kill bacteria.
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Structure of eyeball
The adult eyeball is a hollow spherical structure. Out of its total surface area, only the front one sixth remains exposed. The rest of the eyeball is embedded in a bony socket. Six eye muscles attach the eyeball to the bones. The muscles hold the eyeball in position and their contractions also permit certain amount of movement. Anatomically the wall of eyeball is divided into three concentric layers outer sclerotic or sclera (fibrous), middle choroid (vascular) and inner retina.
1. Sclerotic layer or Sclera :
It is made- up of tough non-elastic fibrous tissue and is white in colour. It surrounds the entire eyeball making white portion on the front of the eye. The sclera bulges out and becomes transparent in the front region (anterior portion of the eye) to make cornea (the bulging part of the eye). The cornea derives oxygen by diffusion therefore excessive use of contact lens is not recommended as it could lead to the drying of the cornea.
This cornea sometimes becomes opaque (white) and non-functional (defective) which can be then replaced by a healthy cornea from a donated eye. Cornea remains alive up to nearly 40 hours after the death of a person. If donated, the eye can be removed within 4 hours after death and can be grafted in place of the defective cornea and the vision can be restored.
Covering the entire anterior front part of the eye is a thin membrane called conjunctiva. It is continuous with the inner lining of the eyelids. Over the cornea it is reduced to a single layer of transparent epithelium, which when gets infected by a virus, turns this thin membrane red and develops a common eye disease called conjunctivitis.
Inner to sclera is a deeply pigmented layer of tissue called choroid. It is richly supplied with blood vessels for providing nourishment to the eye. It is dark black in colour and darkens the inner side of eyeball. In the front of the eye, choroid expands to form a ciliary body which lies at the junction of choroid and iris. It consists of ciliary processes and ciliary muscles.
It is connected to the lens by suspensory ligament which alters the shape of the lens. Continuous with the choroid is a coloured disc of tissue called iris, partially covering the lens and leaving a circular opening in the centre called pupil. The blue, brown or black colour of eye refers to the colour of the iris. The iris contains radial muscles to widen and circular muscles to constrict the pupil which regulates the amount of light entering the eye.
3. Retina :
It is the innermost, light sensitive layer of the eyeball covering the choroid and ending at the edge of the ciliary body. The retina contains two types of photosensitive cells called Rods and Cones.
(a) Rods : The rods contain the pigment rhodopsin and are concentrated more towards the periphery of retina. They respond to light of low intensity (dim light) and do not respond to colour. They help to discriminate between shades of dark and light and see shapes and movement.
(b) Cones : The cones contain pigment iodopsin and are most densly concentrated in the fovea centralis or yellow spot or macula lutea of the eye. The yellow spot or macula lutea or fovea centralis is a particular spot in the retina, lying almost at the centre of the horizontal axis of the eyeball. Cones are specialized for colour vision and sharpness of vision in bright light.
As a result the yellow spot is called as the area of the best or brightest vision of a normal eye. This is the reason why you move your eye from word to word as you read a line through a printed page. Lying below the yellow spot is a small area on the retina called blind spot which contains opening through which the nerve fibres from all the sensitive cells of the retina converge and bundle together to leave the eyeball in the form of optic nerve.
There are no sensory cells in the blind spot and therefore this is the point of no vision and image striking it cannot be perceived.
Lens of the eyes :
Lens of eye lies behind the iris as a transparent, biconvex crystalline body. It is held by fibres called suspensory ligament which attaches it to the ciliary body. It is enclosed in a transparent and elastic membrane called lens capsule. It is an elastic structure, the thickness of which can be increased or decreased depending upon the distance of the object.
The function of lens is to refract the light and converge it on the retina. It also helps to give a clear vision of near and distant objects by changing its focal length by combined effort of the suspensory ligament and ciliary muscles.
Functions of various parts of eye :
1. Sclera :
(i) It provides and maintains the shape of the eyeball.
(ii) It protects the inner parts.
(iii) It provides surface for the insertion of muscles.
2. Cornea :
Being curved it refracts light entering the eye. It helps in focussing.
3. Conjunctiva :
Protects the surface of the eye.
4. Choroid :
Its dark surface helps in the absorption of light. It prevents total internal reflection that may cause the blurring of image Its blood vessels nourish the inner layer of retina.
5. Ciliary body :
The ciliary processes have folds which secrete aqueous humour. The ciliary muscle helps in accommodation.
6. Iris :
It regulates the amount of light entering the eye.
7. Pupil :
Light enters the eyes through pupil.
8. Retina :
Provides surface (screen) for image formation. The rod cells assure night vision. Whereas the cone cells assure day vision and colour perception.
9. Optic nerves :
Carry impulses from the sensory cells (rods and cones) to the brain for interpretation.
The optic system of the eye
It is a complex system of lenses. It forms an inverted, smaller and real image on the retina. The conjunctiva, cornea, aqueous humour, lens and vitreous humour, all being transparent act as lenses or optic system of the eye. They focus the light rays coming from the object on the yellow spot.
Cornea focusses the light rays on the retina and fine focussing is done with the help of lens. All other parts i.e. aqueous and vitreous humour help in the refraction of light. The image formed at the yellow spot Is an inverted, real and smaller one.
Working of the eye and image formation
Light rays from the object enter our eyes through conjunctiva, cornea, aqueous humour, lens and vitreous humour. The curvature of the cornea and the lens bend and converge the light rays to form an image on retina. The vitreous humour serves as a refractive medium. The image formed on retina is inverted, smaller and real. The light energy of the image stimulates the rod and cone cells.
These convert light energy into chemical energy. In this way these changes generate nerve sensory cells impulses. The nerve impulses so generated are conducted to the brain through the optic nerve. It is the visual area of cerebrum of brain which analyses size, distance and colour of the object. It is here that the inverted image is perceived as an erect image.
Power of accommodation
Ability of the eye to see the objects equally clear from various distances is called power of accommodation. All the parts of the eye that help process form the accommodation apparatus. It is brought about mainly by changing the curvature of the elastic lens. It makes the lens thinner or in this thicker, thereby changing its focal length.
The accommodation apparatus that changes the curvature of lens includes ciliary muscles, suspensory ligaments and the elasticity of lens itself. When the eye is to be adjusted to see the distant objects (beyond 20 ft.), it is said to be at rest. The ciliary muscles are fully relaxed. The suspensory ligaments are fully contracted and under maximum tension (peripheral pull).
The thickness of lens is reduced thereby decreasing the power of refraction. Whereas during adjustment eye to see the near objects (less than 20 ft), the eye is said to be at stress or maximum tension. In this condition ciliary muscles are fully contracted. It pulls the choroid forward making the suspensory ligament loose.
It shortens the radius of suspensory ligaments and thereby making it loose. As a result of this loosening, the lens becomes thick due to its elasticity. It is more so on the anterior side than the posterior side due to thick vitreous humour. It will increase the power of refraction and helps in focussing the object on yellow spot. In this way eye can be adjusted according to the distance of the object.
Light and dark adaptations
When you enter in a dark room (say a cinema hall) from a bright lighted area, momentarily you are not able to see anything.
Gradually, your vision is improved and you can see the objects in the dark room. It is called dark adaptation. This adaptation is due to :
(i) The dilation of pupil allowing more light to enter the eyes.
(ii) The regeneration of rhodopsin (visual purple) pigment of the rod cells which was earlier broken down in bright light.
Common abnormalities or defects of eye
(i) Near or short-sightedness (Myopia)
Myopia is a condition in which the near vision is clear while distant vision is blurred. This occurs because light rays entering the eyes converge too soon and are brought to focus before reaching the retina (in front of retina). It is because of two reasons :
(a) The lens become too convex or curved.
(b) Eyeball of too great depth (eyeball lengthened from front to back)
Correction of the defect : It is corrected by wearing concave lenses which causes parallel rays of light to diverge before they converge and focus on th retina. (Power of glasses used is mentione in minus “-“)
(ii) Far or long-sightedness (Hyper- opia)
Hyperopia is a condition in which the distant vision is clear while near vision 1s blurred. This occurs because the light rays entering the eyes converge and are brought to focus behind the retina.
It is because of two reasons :
(a) The lens becomes less convex or too flattened.
(b) Eyeball of too short depth (eyeball shortened from front to back)
Astigmatism is condition in which the curvature of the corno a more complicated or the lens is irregular. It causes focussing of horizontal and vertical rays at two different points on the retina. It is corrected by using cylindrical lenses.
This condition is due to the loss of flexibility of the lens, generally after the age of 40. It creates difficulty in focussing on near objects. It is corrected by using convex lenses.
It is a condition in which the lens becomes opaque either due to ageing or some disease. It leads to blindness.
The only cure of cataract is the removal of defective lens: by surgery and using convex lens in spectacles. It compensates the removed lens. Nowadays, a small plastic lens is implanted behind or in front of iris.
Over-production of vitreous humour increases pressure on the delicate retina. It crushes the delicate cells of retina causing blindness.
It may also be due to the increased pressure of aqueous humour in the anterior chamber because of the blocking of its drainage.
This condition of increased intraocular pressure is called glaucoma. An operation needs to be performed to drain excess fluid and restore normal pressure.
(vii) Night blindness
Difficulty to see in dim or diffused light is called night blindness. It is because of the failure of formation of visual purple (rhodopsin) pigment by the rod cells. Formation of rhodopsin requires vitamin-A. Deficiency of vitamin A causes this disease.
(viii) Colour blindness
Inability to distinguish various colours is called colour blindness, such as, failure to distinguish red and green colours. It is a genetic disorder and is more common in males as compared to females.
It occurs when the two eyes are not properly attached in the eye orbits but converge more leading to what is called “cross eyes”. In the opposite condition, the eyes diverge outwards leading to what is called “wide eye”. Both conditions may be corrected by surgical operation and suitable exercises.
(x) Detachment of retina
In this defect retina detaches from the choroid and fluid accumulates between these layers. It leads to distorted vision and blindness in the corresponding field of vision.
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