The eye is optically equivalent to the usual photographic camera, for it has a lens system, a variable aperture system, and a retina that corresponds to the film. The lens system of the eye is composed of (i) the interface between air and the anterior surface of the cornea, (ii) the interface between the posterior surface of the cornea and the aqueous humor, (iii) the interface between aqueous humor and the anterior surface of the lens, and (iv) the interface between the posterior surface of the lens and the vitreous humor.
The difference between the refractive indices on the two sides of each surface is one of the factors that determine the focusing strength of each surface. In exactly the same manner that a glass lens can focus an image on a sheet of paper, the lens system of the eye can also focus an image on the retina. The image is inverted, and reversed with respect to the object. However, the mind perceives the object in the upright position despite the upside-down orientation of the retina because the brain is trained to consider an inverted image as the normal.
The formation of an image on the retina requires four basic processes, all concerned with focusing light rays: (i) refraction of light rays, (ii) accommodation of the lens, (iii) constriction of the pupil, and (iv) convergence of the eyes. Accommodation and pupil size are functions of the smooth muscle cells of the ciliary muscle and the dilator and constrictor muscles of the iris. They are termed intrinsic eye muscles, since they are inside the eyeball. Convergence is a function of the voluntary muscles attached to the outside of the eyeball called the extrinsic eye muscles.
The ciliary muscle is controlled almost entirely by the parasympathetic nerves. Stimulation of the parasympathetic fibres to the eye contracts the ciliary muscle, which in turn relaxes the ligaments of the lens and increases its refractive power. With an increased refractive power, the eye is more capable of focusing on objects that are nearer to it than the eye with less refractive power. Consequently, as a distant object moves toward the eye, the number of parasympathetic impulses impinging on the ciliary muscle must be progressively increased for the eye to keep the object in focus.
Presbyopia
As a person grows older, his lens loses its elastic nature and becomes a relatively solid mass, probably because of progressive denaturation of the proteins. Therefore the ability of the lens to assume a spherical shape progressively decreases, and the power of accommodation decreases, from approximately 14 diopters shortly after birth to approximately 2 diopters at the age of 45 to 50. Thereafter, the lens of the eye may be considered to be almost totally non-accommodating, which condition is known as presbyopia.
Hypermetropia
A normal eye is considered to be ’emmetropic’, when the ciliary muscle is completely relaxed, parallel light rays from distant objects are in sharp focus on the retina. Hypermetropia, also known as ‘far-sightedness’, is due either to an eyeball that is too short or to a lens system that is too weak when the ciliary muscle is completely relaxed. In this condition, parallel light rays are not bent sufficiently by the lens system to come to a focus by the time they reach the retina. In order to overcome this abnormality, the ciliary muscle must contract to increase the strength of the lens. Therefore in old age, when the lens becomes presbyopic, the far-sighted person often is not able to accommodate his lens sufficiently to focus even distant objects, much less to focus near objects.
Myopia
In myopia, or ‘near-sightedness’, even when the ciliary muscle is completely relaxed, the strength of the lens is still enough so that light rays coming from distant objects are focused in front of the retina. This is usually due to too long an eyeball, but it can occasionally result from too much power of the lens system of the eye.
Cataract
A cataract is a clouding of the lens or its capsule so that it becomes opaque or milky .white. The two basic processes involved in cataract formation are the breakdown of the normal lens protein and an influx of water into the lens. As a result, the light from an object, which normally passes directly through the lens to produce a sharp image, produces only a degraded image. If the cataract is severe enough, no image at all is produced. This problem is associated with ageing, but may also be caused by injury, exposure to radiation and certain medications.
Glaucoma
It is a most common cause of blindness, especially in the elderly. In fact, it is a group of disorder characterised by an abnormally high intraocular pressure owing to a build up of aqueous humor inside the eyeball. This aqueous humor does not return into the blood-stream and accumulates there. By compresing the lens into the vitreous body, it puts pressure on the neurons of the retina, resulting in degeneration of the optic disc, visual field defects and blindness.
Nightblindness
Nightblindness occurs in severe vitamin A deficiency. When the total quantity of vitamin A in the blood becomes greatly reduced, the quantities of vitamin A, retinal and rhodopsin in the rods, as well as the colour photosensitive chemicals in the cones, are all depressed, thus decreasing the sensitivities of the rods and cones. This condition is called nightblindness because at night the amount of available light is far too little to permit adequate vision, though in daylight, sufficient light is available to excite the rods and cones despite their reduction in photochemical substances.
Preksha — Yoga management
Shatkriyas – Jal neti
Special yogic exercises
• Of head and neck
• Palming — for local relaxation and strengthening of eye muslces
• Candle gazing — for relaxation of eye muscles and steadying the mind
• Fucusing exercises
(i) Shoulder gazing
(ii) Centre gazing
(iii) Up-and-down gazing
All these exercises are to train eye muscles for quicker adjustment capacity and strength
• Water cleansing — Take a wide-mouthed bowel, full of clean and cold water. Keep this pot on a table of shoulder height. Now tilt the head downward putting the right eye in the water with eyelid open. Roll the eyeball around in the same condition for a few seconds. Repeat the process with left eye also. Repeat the exercise four-five times a day.
Asanaa – Simhasana, Sarvangasana, Bhujangasana, Vajrasana, Ushtrasana, Matsyasana
Pranayama – Nadi shodhan and sheetali
Kayotsarga – 30 minutes — thrice a day
Preksha – Perception of long-deep breathing with the concentration on eyes along with green colour
Anupreksha – Contemplation for strengthening the eye muscles
Dietary Recommendation
• To consume light vegetarian diet, providing maximum quantity of vitamin A
• To avoid chilli, spicy and stimula-‘ tory food items
• To avoid going in strong sunlight, flood light and reading in inappropriate light intensity