Handbook of Optometry and Eye Disorders Prakash Nathan
INDEX
×
Chapter Notes

Save Clear


2

Eye AnatomyONE

Eye is an organ of sight. The eye consists of structures that focus an image on to the retina at the back of an eye and nerve cells. Retina is like a screen and is connected into electrical pulses that are carried by the optic nerve and sent to the occipital lobes that lie at the back to the brain where the picture is interprated.
The two eyes work in co-ordination under the control of the brain, aligning themselves on an object so that clear image is formed on each retina if necessary the eyes sharpen images by altering focus in an automatic process known known as accomodation.
The pattern of light falling on the retina stimulates a complex flow of impulses along the optic nerves to the brain. The two optic nerves pass into the skull meet partially cross-over and run back, at first on the underside of the brain and then through its substance to the visual cortex the area of the back surface of the brain concerned with the vision.
 
STRUCTURE (FIG. 1.1)
The eyeballs lie in pads of fat within the orbits, that has four walls namely the upper wall or roof, lower wall or floor, lateral or temporal wall and medial or nasal wall. The bony eyesockets provides protection from external injury. Each eyeball is moved by six delicate muscles, the action of these muscles for both eyes being co-ordinated by a nerve network in the brainstem.
The eyeball has a tough coutercoat the sclera (white of the eye). The anterior part of the membrane which is circular part of the outer coat is the cornea, transparent and slightly protruding.4
zoom view
Fig. 1.1: Anatomy of the eye
The cornea serve as the main lens of the eye and performs most of the focussing. Just behind the cornea is a shallow chamber full of aqueos humour (watery fluid), at the back of which is the Iris (coloured part) with its pupil (central hole) which appears blacks. Tiny muscles alter the size of the pupil with change in light intensity to actually control the amount of light entering the eye. Immediately behind the iris and in contact with it is the crystalline lens suspended by delicate fibres from a circular muscle ring called the ciliary body. Contraction of the ciliary body alters the shape of the lens providing the eye with some focussing power additional to that provided by the cornea. Behind the lens is the main cavity of the eyeball filled with clear gel known as vitreous humour.
Inside of the back of the eye is the retina, a complex structure of nerve tissue on which the image formed by the cornea and the crystalline lens falls. The retina needs to have a constant supply of oxygen and glucose. To meet this requirement a thin network of blood vessels the choroid plexus lies immediately under it. The choroid is continuous at the front with the ciliary body and iris thus forming the uveal tract. The eyeball is sealed off from the outside by a flexible membrane known as conjunctiva which is firmly secured around the margin of the cornea but lies freely on the sclera over the front third of the globe.
5
zoom view
Fig. 1.2: Lacrimal gland
zoom view
Fig. 1.3: Eyelid and the lacrimal glands
6
It is attached to the skin at the corners of the eye and forms the inner lining of the lids with a deep cul-de-sac above and below. This arrangement provides a permenant seal while allowing free mobility of the eyeball. In the conjunctiva there are many small and tiny tear secreting and mucous producing glands. They along with an oil secreting from the meibomian glands in the eyelids provide the important, three layer tear film that must constantly cover the cornea and conjunctiva to protect them from damage due to drying out of the cells (Fig. 1.2).
Each eyelid contains about 30 meibomian glands with their openings along the lid margin just behind the roots of the lashes (Fig. 1.3). The glands secrete an oil to prevent adhesion of the lid margin during sleep and forms the outer layer of the tear film, a layer that retards evoparation and helps maintain the continuity of the tear film. The blink reflex is protective and help to spread the tear film evenly over the cornea.
 
CORNEA
The front part of the tough outer side of the eyeball. It is transparent and is shaped like a thin-walled cap. It is about 12 mm in diameter and less than 1 mm thick with a convex front surface like the front of the camera lens. At its circumference the cornea joins the sclera which is easily seen. On the other hand, cornea itself being transparent is less obvious whereas the black pupil and the coloured iris are visible beneath it.
The two main function of the cornea is to focus light-rays to the retina at the back of the eye and to protect the front of the eye. The surface of the cornea is highly-sensitive, a small scratch or foreign bodies are very painful giving you the warning of any possible damage without neglecting. The cornea must be kept moist always by tears to remain healthy. This function is performed by the lacrimal gland and the mucous, and mucous and fluid secreting cells in the eyelids and conjunctiva. The cells that form 7the inner layer of the cornea in adults cannot reproduce themselves. If there is a severs damage permenant clouding of cornea takes place because one of the functions of the cell is to pump excess water out of the cornea to keep it transparent.
The cornea is composed of five layers:
  1. Epithelium
  2. Bowman's membrane
  3. Substantia propria or stroma
  4. Descemet's membrane
  5. Endothelium
Cornea is richly supplied with nerve fibres and is very sensitive with no blood vessels.
 
Aqueous Humour
The anterior chamber just behind the cornea is filled with a clear fluid called the aqueous humour. This fluid is secreted by the ciliary body into the posterior chamber that is located between iris and the anterior portion of the crystalline lens. The aqueous humour circulates from the posterior chamber through the pupillary opening into the anterior chamber and then drains out though the duct under the side of the cornea at the limbus. This duct is called the Canal of Schlemp.
 
Iris
The coloured part of the eye that lies behind the cornea. The iris is connected at its outer edge to the ciliary body and has a central perforation called the pupil, through which light enters the eye and fall on the retina.
The iris is a loose framework of transparent collagen and muscle fibres, which constantly contracts and dilates to alter the size of the pupil and to control the amount of light that passes through the pupil and reaches the retina.8
 
Pupil
The circular opening in the centre of the iris. In bright conditions, the pupil constricts (narrows) in order to reduce the amount of light recieved by the eye; in dim light, the pupil dilates (widens) to allow more light to reach the retina. Contriction and dilation are controlled by muscles in the iris. Several drugs affect the size of the pupil, e.g. Atrophine eyedrops dilate the pupil while pilocarpine eyedrops constrict it.
 
Lens
The crystalline lens is the internal optical component of the eye, responsible for adjusting the focus, it is one of the two lenses in each eye the other is the cornea which provides most of the converging power needed to form an image on the retina.
The crystalline lens is situated behind the iris and is held in place by the suspensory ligaments or Zonules of Zinn. These zonules are not membrane but consists of bundles of strands from the ciliary body. It is elastic, transparent and slightly less convex on the front surface than on the back. It is about 4 mm in thickness in the centre, the lens grows throughout one's life but has more in the first year after the birth. The nucleus continues to grow in layers on one over the other as life progresses. In the later part of life the nucleus grows further and becomes harder, loosing the elasticity and finally interfering with the free functioning of the lens in senile life. Changing the curvature of the lens alter the focus so that near or distant objects can be seen sharply (accommodation), that means the refractive power of the eye can voluntarily be increased from 18 diopter to somewhere about 30 diopter.
Opacification of the crystalline lens by injury or by any cause is called Cataract. These lens can even be dislocated from its original position.9
 
VITREOUS HUMOUR
The transparent jelly-like body that fills the large rear compartment of the eye between the crystalline lens and the retina. The vitreous humour consists almost entirely of water. Under certain conditions, it can exert sufficient pull on the retina to cause retinal tears and retinal detachment.
Sometimes, blackspeck float before the eye and are seen by the normal person. These are various kind of opacities that throws shadow on the retina then appearing as dark sports in the field of vision. These are considered normal unless these opacities when in large number increase interferes with the vision.
Extraocular muscles: Six muscles work together on the eyeball to give a range of smooth and precise movements. One end of the muscles are attached to the bony orbit, while the other end is attached to the sclera, the outer coat of the eye. The eye can move through an arc of about 100 degrees horizontally and about 80 degress vertically. Both eyes though separate, yet work simultaneously and with co-ordination. These team of muscle that control the movement of each eye are (Fig. 1.4).
  1. Superior rectus
  2. Inferior rectus
    zoom view
    Fig. 1.4: Extraocular muscles
    10
  3. Medial rectus
  4. Lateral rectus
  5. Superior obligue
  6. Inferior oblique
The four rectus eye muscles run directly from the eyeball, while the oblique muscle are attached to and pull on the eyeball at an angle.
 
RETINA
The light-sensitive membranous layer that lines the inside of the back of the eye, on which images are cast by the cornea and the lens. The retina contains specialised nerve cells (the rods and cones) (Fig. 1.5) that convert light energy into nerve impulses. Having a wide network of connecting and integrating cells, some with very long fibres the retina conveys the impulses back along the optic nerve to the brain.
zoom view
Fig. 1.5: Retinal receptors in the eye
11
The rods are exceptionally sensitive, responding to very dim light. Cones are less-sensitive but are responsible for colour perceptions, producing impluses that vary in strength with the colour of the light striking them. Near the centre of the retina is the fovea. Here, retinal blood vessels are absent and the light-sensitive cells are packed so that vision in this area has the highest resolution. In short, after recieving an image the retina transfer the light impulses into electrical impulses and transmit it to the brain. Despite its small size the retina is subject to a variety of disorders, many of which seriously affect the vision or, in some cases, produce blindness.
 
Optic Nerve
The optic nerve consists of about one million nerve fibres that transmit impulses from the retina to the brain. The two optic nerves converge to a junction behind the eyes, where fibres from the inner halves of the retina cross-over. Nerve fibres from the right halves of both the retina pass to the rightside of the occipital lobes at the back of the brain, while those from the left halves go to the left-side. Each optic nerve is a bundle of long fibres originating from nerve cells in the retina and passing to the back of the brain. Because of the arrangement of the nerve fibres, disease or injury at any point causes a unique pattern of vision loss. Charting the pattern of visual loss allows accurate location of damage to the nerve. Disorders of the optic nerve includes optic neuritis and pappilloedema, caused by pressure on the nerve from disease withinthe orbit or a brain tumour.