We observe things by our eyes. The vision is that sensation on which a person depends very much. Vision is a complicated process in which the direct knowledge of sensationalism towards light rays, structure, distance, color etc is present. Eyes are very complicated senses which are situated on both left and right side enclosed in an orbital cavity. These are almost circular and their diameter is about 2.5 centimeter. These are called eyeballs. The orbital cavity is in cone shaped. There is a foramen in the apex of the eyes of which the tract of secondary optic nerve makes. The roof of this orbital cavity is made of frontal bone and the floor of it is made of maxilla whereas lateral layers are made of cheekbones and sphenoid and the medieval layer are made of lachrymal, maxilla, ethmoid. In the same cavity, the eye ball is internally situated and remains safe in the fatty tissues.
Structure of the eye:
The wall of eyeball is made of three layers of tissues.
1. Outer fibrous layer
2. Middle vascular layer
3. Inner nervous layer
Outer fibrous layer:
It is the supporting layer of outside in the eyeballs which is made of thick membrane of strong fibrous connective tissue. In it, there are sclera and cornea.
The sclera is a white layer of opaque and strong fibers tissues of posterior segment of eyeballs. Only 5/6 posterior segment of the eyeball is covered. Its external surface is white and makes the white portion and on it all the muscles are attached which do the work to move the eye. It defends soft internal layers of eyeballs and makes it shape constant.
The cornea is an anterior transparent portion of fibrous layer which is situated on the frontal bulgy portion of eyeball. The cornea makes 1/6 portion of front side. In it, there is a complete lack of blood vessels. The rays of light reach on the retina by going through the cornea.
The sclera and cornea remain in continuity. External fibrous layer covers the eyeball completely except the exterior portion where there is a small hole in the sclera and through which fibers of optic nerve reach into the brain from the eyeball.
Middle vascular layer:
It is a middle layer of eyeball and there are many blood vessels in it. Choroids, ciliary body and iris are included in it.
The color of this layer is dark because of pigments which is helpful in making the internal portion of the eye in which rays of light reflect on the retina completely light proof. The 2/3 posterior portion of vascular layer is made of thin membrane which is called choroid. There is a thin layer of connective tissue and blood vessels between the sclera and retina. This choroid is of dark brown color and supplies blood of the retina.
By becoming thick on the anterior portion, the vascular layer makes ciliary body in which there are muscular and glandular tissues. The ciliary muscles control the size of the lens and help in centralizing the rays of light of distance or nearby. These are called the muscles of accommodation. Cilliary glands excrete a liquid like water which is called aqueous humor. It is filled in the middle portion of the lens and cornea of eye and goes into the veins through the small pores situated at the middle angle of the cornea and iris.
The anterior extension of choroid is in the form of thin muscle layer which is called iris. It is the colorful portion of the eyeball by which can be seen through the cornea. It is situated between the cornea and lens and divides this place into anterior and posterior chambers. The middle portion between cornea and iris is anterior chamber and the middle portion between iris and lens is called posterior chamber. In the iris, there are muscular tissues in the form of circular and radiating fibers. The circular fibers contract the eyeball and radiating fibers expand them.
There is a round hole in the iris which is called pupil. You see it as a small doll in your reflected image. The size of the eyeball keeps on increasing and decreasing according to the intensity of light. It gets contracted in too much light to check the entry of too much light into the eyes and it gets spread in dim light so that excessive light may reach on the retina.
Inner nervous layer:
The innermost layer of the eyeball is called retina which is made of neurons and several layers of neural fibers and it is situated at the posterior chamber of eyes. There is choroid of its one side and remains close by it and there is liquid named vitrous humor filled on the other side. It comes to an end just behind the ciliary body. It is an egg shaped. There is one thin and one thick layer on the retina. The thick layer is of nervous tissue which is called neuroretina and it joints to the optic nerve. Behind it, there is a thin layer of pigmented epithelium which is attached to the choroids and checks the reflection behind the retina. There are cell rods like long poles and cones in the shape of cells which are very sensitive towards the light because of the acceptable pigments situated in it. Beside these, there are several neurons in the neuroretina. There are 125 million rods and seven million cones in each eyes of which work are different. Most of cones are situated at the middle of retina just behind the region of lens which is called yellow spot or macula lutea. There is a small pit at the center of macula lutea and it is called fovea or fovea centralis. The rest portion of the retina which is called peripheral ratina and there are rods and some cones in it. Macula lutea is that portion of retina where the clearest image appears. Cones make us feel about the color whereas rods make us feel about the shades. With the help of cones, we are able in observing things in dim light.
Optic nerves come out from the eyeball on the distance of 3 millimeter at the tip of nose of macula lutea. This little place is called optic disc and because it is insensitive to the light. This is the reason that it is called blind spot. The cause behind not making the image on the blind spot is that there is a complete lack of rods and cones on this spot.
Rods and cones are the real receptors of vision and the light which reach on that produce impulses of which transmission is done by the optic nerve in the vision center of brain where visual impressions take birth.
Cavities of the eyeball:
The eyeball has been divided into three cavities. The middle region between cornea and iris is called anterior chamber and the region between iris and lens is called posterior chamber. There is a transparent, thin, watery liquid in these both chambers which is called aqueous humor. The intraocular pressure remains same because of aqueous humor. It supplies oxygen, glucose and amino-acids to the lens and cornea and there is no blood vessel for the nutrition. The production of aqueous humor is done by capillaries of ciliary body. It reaches into the anterior chamber by going through the posterior chamber and it spread scelar venus cinus situated on the base of cornea which is called Canal Of Schlemn.
The third and the biggest cavity of eyeball is called vitreous chamber which is in about 80 percent portion of eyeball. It fills the complete space behind the lens. There is a transparent colorless substance like jelly in this chamber which is called vitreous humor which is the transformation of connective tissues. Its work is to check the eyeball from coming out due to external pressure. In it, beside cologne and hayloronic acid, there are other components are like aqueous humor. The production of aqueous humor is done by the ciliary body. It supplies nutrition for the lens and retina.
The lens is situated just behind the iris in the eyeball of lens. It is transparent, firm but flexible, circular biconves formation in which there are blood vessels. It remains closed in a transparent and flexible capsule and is attached to the ciliary body. These suspensory lingaments keep the lens in good position and the ciliary muscles give pressure on the lens by the help of these suspensory lingaments. The lens raises high on the both sides because of the relaxation of suspensory lingaments viz the convexity of the lens gets increased and the convexity of the lens gets decreased because of the contraction and it becomes flat. Such activity gets completed automatically according to the distances of different objects which is called the accommodation of eyes.
Accessory Structures of the Eye:
Eyes are very sensitive organs which remain safe because of some supporting formation. These structures have been given below.
- Orbit or orbital cavity
- Lacrimal apparatus
- Muscles of the eye
Eyebrows- Small hairs with crookedness grown on the orbital cilia of the frontal bone are called eyebrows. Safety is the main work of eyebrows. They check the sweat of the forehead coming downwards and avoid eyes from fierce sunlight.
Eyelids: These are two movable folds of areolar tissue covered with thin skin up and down before each eye. The upper eyelid is much enlarged and much moveable between these two eyelids. The meeting point of upper and lower eyelids is called canthi. The internal canthus towards the nose is called medial canthus and the external canthus towards the ear is called lateral canthus. The eyelids can be divided into four layers: skin layer in where there are eyelashes 2. Muscular layer in which there is an orobicularis oculi which brings the eyelid on the eye 3 Layers of fibrous connective tissue in which there are many sebaceous glands which are called mebomian glands and the oily substance secreted from these glands save the eyes from sticking to each other 4 the innermost layer is covered with thin pink membrane which is called conjunctiva.
The eyelid keeps the dust particle and foreign body away from entering into the eyes. Beside it, the eyelids check the sudden entry into the eyes. The secretion of glands gets spread on the eyeballs by blinking the eyelids after a few seconds again and again. The cornea remains humid by it. The closed eyelids check the vaporization of secretions during sleep. When anything comes near the eye, the eye gets closed itself reflexively.
Small and thick hairs are coming out from the margins of eyelids which are called eyelashes. They check the entry of foreign body into the eyes. There are 200 eyelashes in the eyelid of each eye. Each eyelash falls within 3 to 5 months and new eyelash grows in place of that. If the hair follicles get infected, styes appear in the eyes.
Conjunctiva is a transparent thin mucus membrane which covers the internal portion of the eyelids and after reverting it covers the surface by coming over the eyeball. It comes to an end by coming on the transparent cornea which is covered. The portion which covers the eyelid is called palpebral conjunctiva and the portion which covers the whiteness of the eye is called bulbar conjunctiva. There are two conjunctival sacs between these two portions. There is movement in the eyeballs because of these sacks. The eye drop which is dropped into the inferior conjuctival sac by pulling the eyelid backwardly.
Each eye has a lacrimal apparatus. This apparatus comes into existence by the lacrimal gland, lacrimal sac, lacrimal ducts and nasolacrimal duct.
Lacrimal glands are situated on the lateral canthus under the upper eyelid of each eye. The eyeball remains wet because of secretion of these glands. Each lacrimal gland is like an almond in size and is situated in the cavity of eye of the frontal bone. It is made of the cells of oozing epithelium. Several vessels come from these glands which open in the upper portion of conjunctiva. Tears are produced in these glands which come down by the lacrimal ducts and after spreading on the frontal portion of the eyeball, they go towards the medial canthus of each eye where they get entered into two lacrimal canaliculi. Both these lacrimal canaliculi go ona small red colored bulge which is called cauncle one from up side and other from below side and then enter into the lacrimal sac. Lacrimal sac opens in the inferior meatus of nose by crossing the nasal cavity of bony wall at the below side through which tears reach into the nose.
Each eye keeps on blinking after each 2 to 10 seconds of which the lacrimal gland secretes a sterile liquid by getting excited. This secretion is called tears. There is water, salt, mucin and an antivirus enzyme named lysozyme.
- Tears keep eyes lubricate and wet. Eyelids move easily because of this wetness.
- Tears clean eyes by washing foreign bodies and dust particle.
- The enzyme named lysozyme has the power to kill virus.
- The tears supply water and nutrition to the cornea.
- Tears provide clean, wet and smooth surface to the eyeball.
Muscles of the eye ball:
The movement of the eyeball in its socket is done by the set of six muscles. Among these muscles, four muscles are rectus (straight) and two are oblique.
1. Medial rectus
2. Lateral rectus
3. Superior rectus
4. Inferior rectus
1. Superior oblique muscle
2. Inferior oblique muscle
All these muscles are called extrinsic muscles because they remain outside of the eyeball. One end of each muscle is connected with skull bone and the second end is connected with the sclera. With the help of all these muscles, the eyeball is moved in every direction easily. Eyes are moved because of the co-ordination in the movement of these muscles in this way eyes are centered on a thing. Both eyes work with each other’s help and this is the reason that we see only one thing even when we see by both eyes. In the condition of muscles’ weakness or in the condition of illness, eyesight of both eyes does not fall on one center and double images of a thing are seen.
Movements of the muscles:
When the movements of the above mentioned muscles are just opposite to each other viz when one set of muscles gets contracted, the set of just opposite muscles does the activity of relaxation. As there is a relaxation in the inferior rectus along with the contraction in the superior rectus and just opposite of it when the inferior rectus gets contracted, there is relaxation in the superior rectus of which the eyes move downwardly. In this way, medial and lateral muscles of eyes move eyes up and down. Eyes move up and down and outwardly with the help of oblique muscles. In addition to, they help eyes in moving in all the directions.
There are following mentioned three smooth muscles inside the eyes which are called intrinsic muscles. Ciliary muscles reduce the tension of suspensory ligament and let the lens change in its shape for adjusting the eye properly. Circular muscle or sphincter pupilla- this muscle surrounds the eyeball from all sides in a circular way and contracts the eyeball. Radial muscle or Dilator pupilla-it starts from the external circle of iris and is situated on the edges of eyeball in the radial direction.
Muscles of the eye lids:
The muscle named orbicularis ocule flings the eyelids for closing the eyes and Levator palpebrae superioris muscle raises the eyes for opening the eyes. Superior tarsal muscle is a smooth muscle of which nerve supply is done by the sympathetic nervous system. It is helpful in raising eyelids upwardly. The upper eyelid hangs downwards if it gets paralyzed.
Optic nerve and visual pathway:
Optic nerves are nerves of the light sensation which do the work of seeing. The origin of these nerve fibers is in the retina of eyes which make optic nerve by converging about 0.5 centimeters towards the nose from the macula lutia and by connecting to each other. This nerve emerges from the posterior portion of the eyeball and fuses with other optic nerve by coming close to the pituitary gland. This crossing point is called optic chiasma. By coming on this point, the half fiber of optic nerves of both eyes of (asal side) march forwardly by crossing to each other. The nerve fibers of each retina of the rest of optic nerves (of the lateral side) reach into the mid brain by marching both sides without crossing to each other. The nerve fibers are called optic tracts after going through the optic chiasma. Each optic nerve has the nasal fibers of other side of eye and the lateral fibers of the same side in the optic nerve.
Each optic nerve goes backwardly by going through the cerebrum and synapses with the neurons of nucleolus in the thalamus which are called lateral geniculate body. From there, the axons of the neurons of the geniculate body are launched in the primary visual cortex in the occipital lobe of cerebral cortex. Here the impulses of eyesight are described and analyzed.
Mechanism of vision/sight:
Light is considered as the form of radiant energy. The light runs with the speed of 3, 00, 000 kilometer per second in the rays through the medium of air.
Generally, the rays of light run in the parallel line but when they go through from one density to another density, they bend. This bend is called refraction. The rays of light which enter into the eyes with the external air get refracted and center on focus point by conversing on one point viz by meeting on a point.
The rays of light go through the refracting media of eye-cornea, aqueous humur, lens and vitreous humur. Among these, lens is such formation which has the capacity to bend or refrect to the rays of light whereby they make image on the retina at the back side of the lens by conversing.
Eyeballs are compared with camera in the point of view of formation in which the eyelids do the work of shutter for the eyes like camera and the cornea is like a window for the entry of light. The iris controls to the quantity of light; the crystalline lens focuses the rays of light; the middle vascular layer does the work light detent box of camera and the retina does the work of photosensitive plate like the camera.
The opposite image appears of a thing which remains present before the camera. Similarly, the opposite images of the things seen by the eyes appear on the retina. The rays of light come out from the thing and fall on the retina by crossing aqueous, humur, lens and vitreous humur. Lens and cornea center parallel rays of light on the retina. The distance from lens to focus point in the camera is definite. So in the eye, the clear image of a thing is got on the retina by changing the distance of focus point by the contracting action of ciliary muscles.
The enlarged information of an image reaches into the occipital lobe of cerebral cortex by the optic nerve. Here it gets developed into consciousness. In this way, sensations reach into the visual area by the optic nerve. By it, the brain has sensation to see. Showing right image of a opposite reflection is the deed of brain.
There is a need to bend or refract to all the parallel rays of light which enter into the eyes for centralizing on the retina. Much refraction is not essential to the rays of light which come from the things to see the things which are kept 6 meter away but as the thing come much close, the rays of light which come from the thing need to refract very much.
Parallel rays of light emerged from the distant things fall on the right angle of a lens’ vertical axis. Eyes are adjusted in this way that these rays are bent by the lens wherewithal those rays focus on the retina to make clear image. The rays of near by object (less than six meter) are divergent and fall on the lens in an oblique way generally these rays focus behind the retina. The lens should be thicker and rounds up so that these rays may focus on the retina properly. This activity is completed by the ciliary muscles. In addition to, the number of rays which enter into the eyes gets reduced for the clear image of the object because of the contraction in the iris. In this way, the process in which the pupil becomes small because of the change in the size of lens and contraction of iris is called accommodation. This process is must when near by objects are observed. The power of getting change in the thickness (convexcity) of lens according to different distances is called accommodation. These auto adjusting activities of the focus distance of the lens of eye is called accommodation capacity. Objects which have been kept 25 centimeter far can be seen properly by the healthy eyes. This distance is called lowest distance of clear eyesight. A point where some things are present and the eyes can see them properly. This point can be said far point of eyes.
When anything is near by to the eyes, both eyes move inwardly for getting clear focus on the retina of both eyes. This process of eyes is called accommodation.