In this article we will discuss about the structure of human eye.
Structure of Human Eye:
The eye is a hollow, spherical structure measuring about 2.5 cm in diameter.
Its wall is composed of three coats:
1. The outer fibrous coat— sclera, cornea.
2. The middle vascular coat— choroid, ciliary body, iris.
3. The inner nervous coat— retina.
1. Fibrous Coat:
It is divided into the sclera and the cornea.
It covers most of the eye ball. The sclera or white of the eye contains many collagen fibres. It protects and maintains shape of the eye ball.
It is a transparent portion that forms the anterior one- sixth of the eyeball. The cornea admits and helps to focus light waves as they enter the eye. The cornea is avascular (i.e., gets no blood supply). This part of eye absorbs oxygen from the air. The cornea was one of the first organs to be successfully transplanted because it lacks blood vessels.
At the junction of the sclera and cornea there is a structure called the canal of Schlemm. From the anterior chamber the aqueous humour, which is continuously produced, is drained off into the canal of Schlemm and then into the blood.
2. Vascular Coat:
It comprises the choroid, the ciliary body and the iris.
The choroid lies adjacent to the sclera and contains numerous blood vessels that supply nutrients and oxygen to the other tissues especially of retina. It also contains pigmented cells that absorb light and prevent it from being reflected within the eyeball.
(ii) Ciliary body:
The ciliary body extends towards the inside of the eye from the choroid coat. It is composed of the ciliary muscles and the ciliary processes. The ciliary processes secrete aqueous humour.
The ciliary muscles are smooth muscles and are of two types: circular and meridional. Attached to the ciliary body are the suspensory ligaments, which are in turn attached to the capsule that surrounds the lens of the eye. The capsule and ligaments, together with the ciliary body, hold the lens in place.
The iris is a circular muscular diaphragm containing the pigment giving eye its colour. It separates the aqueous humour region into anterior and posterior chambers. It extends from the ciliary body across the eyeball in front of the lens. It has an opening in the centre called the pupil. It contains two types of smooth muscles, circular muscles (sphincters) and radial muscles (dilators), of ectodermal origin.
The iris controls the amount of light entering the eye by the radial muscles contracting in dim light and the circular muscles contracting in bright light.
Both of these sets of muscles are under the control of the autonomic nervous system. Sympathetic stimulation causes the radial muscles to contract and the pupil to dilate, or get larger. Parasympathetic stimulation causes the circular muscles to contract and the pupil to constrict.
3. Neural Coat— The Retina:
The retina is the neural and sensory layer of the eye ball. Its external surface is in contact with the choroid and its internal surface with the vitreous humour. A small oval, yellowish area of the retina lying exactly opposite the centre of the cornea is named the macula lutea or yellow spot which has at its middle a shallow depression, the fovea centralis. The fovea centralis has cone cells only.
It is devoid of rods and blood vessels. The fovea centralis is the place of most distinct vision. Here the nerve fibres from the light-sensitive cells leave the eyeball to form the optic nerve.
An artery and a vein also pass through the optic disc. This area is called the blind spot because it is devoid of receptor cells. Ora serrata (= ora terminalis) is a special structure which demarcates the sensitive part of retina from its non-sensory part.
Beginning from the external surface (choroid side), the retina consists of the following layers:
(i) Pigment Cell Layer:
This layer lies close to the choroid. It consists of a single layer of cells containing pigment. These pigment cells appear to be rectangular in vertical section, their width being greater than their height. The cells give rise to pigmented processes (projections), extending into the next layer.
(ii) Layer of Rods and Cones:
The rods are processes of rod cells and cones are processes of cone cells. The total number of rods in the human retina has been estimated at 110 to 125 million and cones at 6.3 to 6.8 million (Osterberg 1935).
The rods contain a photosensitive pigment called the rhodopsin (= visual purple). Rhodopsin is composed of opsin and retinene. The opsin is a protein and is called scotopsin in rhodopsin. The retinene is an aldehyde of vitamin A and is also called retinal.
The rods mainly enable the animal to see in the darkness, therefore, rods are present in large number in nocturnal animals. The photosensitive pigment in the cones is of three types namely: porpyrosin, iodopsin and cyanopsin which give response to red, green and blue light respectively.
The sensations of different colours are produced by various combinations of these three types of cones and their photo-pigments. When the three types of cones are stimulated equally, a sensation of white light is produced. The protein in cone pigment is called photopsin, which is different from scotopsin of rhodopsin.
(iii) External Nuclear Layer:
This layer contains the cell bodies and nuclei of rod and cone cells.
(iv) External Plexiform Layer (= Outer Synaptic zone):
This layer consists only of nerve fibres that form a plexus (network). The axons of rods and cones synapse here with dendrites of bipolar neurons. Processes of horizontal cells also take part in the formation of these synapses.
(v) Internal Nuclear Layer:
This layer contains the cell bodies and nuclei of three types of neurons:
(a) Bipolar Neurons
(b) Horizontal Neurons and
(c) Amacrine cells.
(vi) Internal Plexiform Layer. (= Inner Synaptic Zone):
This layer consists of synapsing nerve fibres of bipolar neurons, ganglion cells and amacrine cells. This layer also contains some horizontally placed internal plexiform cells and also a few ganglion cells.
(vii) Layer of Ganglion Cells:
This layer contains the cell bodies of ganglion cells. Axon of each ganglion cell gives rise to a fibre of the optic nerve.
(viii) Layer of Optic Nerve Fibres:
This layer is made up of axons of ganglion cells that form the optic nerve. The optic nerves are connected with the brain. The nerve fibres from all parts of the retina converge to leave through a blind spot (= Optic disc) which contains no rods and cones and, therefore, no image is formed at this spot.
Retinal Gliocytes (= Cells of Muller):
In addition to bipolar, horizontal neurons and amacrine cells, the internal nuclear layer also contains the cell bodies of the retinal gliocytes (= cells of Muller). These cells form numerous protoplasmic processes that extend through almost the whole thickness of the retina and form external and internal limiting membranes.
The internal limiting membrane separates the retina from the vitreous humour. Retinal gliocytes support the neurons of the retina and may en-sheath them. They also have nutritive function. Some astrocytes (other glial cells) are also present in between the retinal neurons.
Contents of the Eye Ball:
(i) The lens:
It is a transparent, biconvex, elastic structure that bends light waves as they pass through its surfaces. The lens separates the aqueous and vitreous humours. It is composed of epithelial cells that have large amounts of clear cytoplasm in the form of fibres.
Its capsule is composed of layers of intercellular protein. The lens can change shape from moment to moment and, by doing so, focuses light waves into the retina from objects at different distances from the eye. The lens can also change shape from year to year, thereby accounting for changes in vision.
(ii) Aqueous Humour:
The space between the cornea and the lens is called the aqueous chamber which contains a thin watery fluid called aqueous humour. The epithelium of the ciliary process continuously secretes a watery fluid, the aqueous humour.
The aqueous humour helps to maintain the shape of the front part of the eye and provides nutrients to the lens and cornea. As stated earlier, the aqueous humour is continuously drained off into the canal of Schlemm and then into the blood. The pressure in the eye, called intraocular pressure is produced mainly by the aqueous humour.
(iii) Vitreous Humour:
The space between the lens and retina is called the vitreous chamber which is filled with a transparent gel called the vitreous humour.
It helps to maintain the shape of the eyeball and also contributes to intraocular pressure (pressure inside the eyeball). Unlike the aqueous humour, the vitreous humour cannot be replaced in any significant quantity. Therefore, in puncture wounds of the eye it is important to prevent the escape of vitreous humour.
Extrinsic Eye Muscles and their Nerve Supply:
There are six extrinsic muscles attached to the eyeball. Four of these muscles are straight and two are oblique. These muscles are median rectus, lateral rectus, superior rectus, inferior rectus, superior oblique and inferior oblique.
The oculomotor (3rd cranial nerve) innervates the median rectus, superior rectus, inferior rectus and inferior oblique. The trochlear (4th cranial nerve) supplies the superior oblique. The abducens (6th cranial nerve) innervates the lateral rectus.
The Accessory Structures of the Eye:
These include the eyebrows, the eyelids and eyelashes, the conjunctiva and the lacrimal apparatus.
1. The Eyebrows:
These are two arched eminences of skin surmounting the supraorbital margins of the frontal bone. Numerous hairs project obliquely from the surface of the skin. The function of the eyebrows is to protect the anterior aspect of the eyeball from sweat, dust and other foreign bodies.
2. The Eyelids (Palpebrae) and Eyelashes:
The eyelids are two movable folds situated above and below front of the eye. On their free edges, there are outgrowths of hairs— the eyelashes. The third eyelid is vestigial and is called plica semilunaris (nictitating membrane). The inner surface of each eyelid and parts of the eyeball are covered with mucous membrane, called the conjunctiva.
Glands of Zeis are modified sebaceous glands which are associated with the follicles of eye lashes. They open into the follicles of eye lashes. Meibomian or tarsal glands are also modified sebaceous glands (oil glands) which are present along the edges of the eyelids.
They produce an oily secretion which serves to lubricate the corneal surface and hold a thin layer of tears over the comea. Glands of Moll are modified sweat glands at the edge of the eye lid.
It is a transparent mucous membrane, which covers the internal palpebral surfaces, and folds on to the anterior sclera and comea where it is continuous with the comeal epithelium. The conjunctiva helps to protect the eye ball and keeps it moist. It is this membrane that becomes inflamed in conjunctivitis or “pink eye”.
4. The Lacrimal Apparatus:
The lacrimal apparatus of each eye consists of a lacrimal gland and its numerous ducts, the superior and inferior canaliculi, a lacrimal sac and a nasolacrimal duct. The lacrimal gland is situated in the orbit on the superior, lateral surface of the eyeball.
The lacrimal gland secretes tears which are composed of water, salts and bactericidal protein called lysozyme. The tears flow into the superior and inferior canaliculi, then to the lacrimal sac and through the nasolacrimal duct into the nasal cavity.
The function of tears is to bathe the front of the eye, washing away any dust, grit and microorganisms. Lysozyme destroys microorganisms present on the front of the eyeball. In emotional states the secretion of tears may be increased and if the nasolacrimal duct cannot carry them all into the nasal cavity, they overflow. Gland cells in the conjunctiva also secrete a mucous substance that is a component of tears.
5. Adipose Tissue (fat):
A layer of adipose tissue surrounds the eyeball in the orbit. It serves as a soft, shockproof pad.
Mechanism of Vision:
The light rays pass through comea, aqueous humour, lens and vitreous humour and focus on retina where they generate potentials (impulses) in rods and cones. As we know, human eyes have remarkable power of accommodation by changing the convexity of the lens.
By the action of iris muscles the size of pupil can be increased or decreased. In bright light the pupil is constricted. In dim light it is dilated. Due to the action of the muscles of the ciliary body and the suspensory ligament, the focal length of the lens can be changed. Then the objects can be focused in different intensity of light from varying distances.
The photosensitive compounds (photo pigments) in the human eyes are composed of opsin (a protein) and retinal (an aldehyde of vitamin A). Light induces dissociation of retinal from opsin which changes the structure of the opsin. Thus potential differences are generated in the photoreceptor cells.
This causes action potentials in the ganglion cells through the bipolar cells. These action potentials (impulses) are transmitted by the optic nerves to the visual cortex area in the occipital lobe of the cerebral hemisphere of the brain where the neural impulses are analysed and erect image is recognised.
In the darkness, rhodopsin is resynthesized from opsin and retenene to restore the dark vision. It is called dark adaptation. It is considered that there are three different kinds of cones, each of which contains a different light-sensitive pigment,
(a) Cones that contain erythrolable are most sensitive to red light,
(b) Cones that contain chlorolable are most sensitive to green light,
(c) Cones that contain cyanolable are most sensitive to blue light.
Combinations of these three colours of light produce all the colours human can see. This is in accordance with the trichromacy theory.
When both the eyes can be focused simultaneously on a common object, as in human eyes, it is called binocular vision. It is just reverse to the monocular vision, as in many animals like rabbit, in which each eye focuses its own object and both the eyes cannot focus on one object.