Eye and vision
Anatomy of the eye
Out of the five senses, sight is the one that is used most.
The eye is the main organ of the visual system, it gets images seen by the person and turns them into an electric signal which is carried along the optic nerve to the brain.
Once the signal reaches the visual cortex, it is "translated" by the brain, to create the image so the person can interpret the environment.
The eye is a globe of around 25mm of diameter and 8 grams.
Several organs form it :
The ciliary body is the anterior part of the choroid, on which is tied the lens thanks to several fibres called the zonule of Zinn or the suspensory ligaments of the lens. It has a fundamental role in the secretion of aqueous humor and the focus of light rays to enable vision.
The iris is a round membrane, perforated in the middle by the pupil. It forms the coloured part of the eye where the colour is determined by the thickness of the epithelium. The iris is light when the epithelium is thin, and dark when it is thick.
The contraction or distension of the iris is a physiological reflex of adaptation to the light. If the light is strong, the pupil is small (myosis), if the light is dim, the pupil size increases in order to get a maximum of light (mydriasis).
The cornea is a transparent tissue, on the anterior part of the eye, which relays the light to the lens and the retina. It is formed by 5 layers (epithelium, Bowman’s Membrane, Stroma, Descemet Membrane and Endothelium), it does not have a blood supply (which is why it doesn't bleed), but is supplied with a lot of nerves. This explains its important sensitivity, and the "corneal reflex" which makes the eyelid blink as a protective mechanism when objects are close to the eye. It is fed permanently by tears and aqueous humor.
The aqueous humor is a transparent liquid which provides the nutrients intended for the cornea and pupil. Its function is to maintain the intraocular pressure and the shape of the ocular globe.
The sclera, is a strong white membrane, which constitutes the "white" of the eye.
The choroid is a tissue of the ocular globe, very vascular and is the nutritive membrane of the eye.
The retina is a thin membrane which covers a large part of the internal surface of the ocular globe. Sensitive to light, it is formed by photoreceptors (rods and cones) and neurons which transmit electrics signals to the brain. The central retina includes the macula and the fovea. Its blood supply is provided by the central retina artery and vein.
The optic nerve, which is the second cranial nerve, begins on the optic nerve head and enables the transmission of visual information from the retina to the brain.
Inside of the eye socket, the ocular globe is maintained and moved by the oculomotor muscles:
- 4 rectus
- 2 oblique
- The inferior rectus enables the movement of the eye towards the bottom : downward movement.
- The superior rectus enables the movement of the eye towards the top : upward movement.
- The medial rectus enables the movement of the eye towards the nose : inward movement.
- The lateral rectus enables the movement of the eye towards the temple : outward movement.
- The inferior oblique : It is the shorter oculomotor muscle. It enables the movement of the eye towards the temple and an elevation of the field of vision.
- The superior oblique : It is the longer oculomotor muscle. It enables the movement of the eye towards the nose and a lowering of the field of vision.
The lachrymal system corresponds to the whole organ permitting the production, repartition and secretion of tears.
The continuous circulation of tears helps to prevent the dehydration of the cornea (nutritive function), and the elimination of impurities present in the eye.
Tears are produce by the lachrymal gland, under the superior eyelid. They spread on the anterior area of the eye, on which they are spread out through the blinking action. This also enable their drainage through the lachrymal point in the internal corner of the eyelids.
The tears are drained away by the lachrymal canaliculus into nasolacrimal canal and finally into the nasal cavity.
Tears consist of 98% water, along with several substances (electrolytes, glucose, urea, proteins...).
Light beams, present in our environment, enable vision, and the different organs of the eye are behind an ensemble of mechanisms to perceive light and thus, images.
The luminous flux is firstly detected by the iris, which adapt the size of the pupil accordingly.
After what the light crosses the ocular environments, namely the lens and the vitreous body, which has to be transparent so the light can be transmitted.
Then, it reaches the retina and its photoreceptor cells;
- the cones, mainly located in the central retina (macula), are responsible for the color vision, for the details of forms, and are associated with day vision.
- the rods, mainly located in the peripheral retina, are a lot more sensitive to light and are responsible for the vision of the contours and movements. They are associated with low luminosity vision.
This ensemble of organs transforms the light in electric signals, sent to the brain via the optic nerve, so the image can be interpreted.
Alimentation and vision
A good alimentation is important to have healthy eyes, several nutrients have an essential role :
- Vitamin A (liposoluble vitamin) and beta carotene (pro-vitamin A that the body turns into vitamin A) are important for the retina and its cells.
Liver, whole milk and butter contain vitamin A. Beta carotene can be found in fruits and vegetables like sweet potato, carrot, pumpkin, spinach and broccoli.
- Vitamin C (antioxidant) enables cells to fight aging and could have a role in the prevention of eye diseases linked to old age such as Macular Degeneration Age Related or Cataract.
Citrus fruits, kiwi, green vegetables and cabbages contain vitamin C.
- Lutein and zeaxanthin are pigments that have an effect as antioxidants on the lens and the macula as they protect against cell aging and associated eye diseases.
These nutrients can be found in dark fruits and dark vegetables: kale, spinach, broccoli, orange sweet pepper, kiwi, green vegetables and egg yolk.
- Omega-3 are polyunsaturated fatty acids, and are a major component of cellular membranes and nervous cells, especially on the retina. They also contribute to the hydration of the eye and could prevent eye dryness. Their preventive role on Macular Degeneration Age Related is also important.
Salmon, sardines, mackerel, walnuts, rape and flax contains these fatty acids.