 |
| Mechanism of Vision |
|
| The eye works on the same principle as that of the camera. |
| |
 |
| |
| The light rays from the object pass through the conjuctiva, cornea, aqueous humour, lens and vitreous humour in that order. All these structures refract the light such that it falls on the retina. This is called focussing. Maximum focussing is done by the cornea and the lens. The light then falls on the retina. |
| |
| This light is received by the photoreceptors - rods and cones, on the retina. The absorbed light activates the pigments present in the rods and cones. The pigments are present on the membranes of the vesicles. Thus, the light is then converted into action potentials in the membranes of the vesicles. These travel as nervous impulses through the rod or the cone cell and reach the synaptic knobs. From here the impulses are transmitted to the bipolar nerve cells, then to the ganglions and then to the optic nerves. Thus the nervous impulses generated in the retina are carried to the brain by about a million neurons of the optic nerve. The vision is controlled by the occipital lobe at the back of the brain. The information received is processed and we are able to see the image. The image formed on the retina is inverted. However, the brain makes us see the image erect. So, though the eyes are essential for vision, any damage to the optic nerves also results in impairment of vision. |
| |
| Accommodation |
| |
| Accommodation is a reflex action of the eye to focus the light from an object on the retina. |
| |
| The adjustment to the distance of the object is done by the ciliary muscles. The ciliary muscles contract and expand to make the lens thin and thick, respectively. This changes the focal length of the lens. If the object is far, then the focal length is increased and if the object is near, then the focal length is decreased. The optimal focal length is 6 metres or 20 ft. This adjustment can be shown in the following manner: |
| |
 |
| |
 |
| |
 |
| |
| Muscular control of the eyes |
| |
| There are three types of movements associated with the eye that are due to the action of muscles. They are the movement of the eye, the change in size of the pupil and the change in thickness of the lens. The movement of the eye is controlled by six eye muscles that attach the sclera to the bones lining the optic cavity. These six muscles are superior and inferior rectus, internal and external rectus and superior and inferior oblique muscles. |
| |
 |
| |
| Binocular and stereoscopic vision |
| |
| In man, the eyes are frontal and thus the vision is binocular. This means that the image finally perceived is the result of the information received from both the eyes. Binocular vision results in stereoscopic vision that gives a 3-dimensional image. Thus, this capability of stereoscopic vision gives more information about the object. This vision is, therefore, present in predatory animals like the hawks, eagles and the members of the cat family. The other animals like the herbivores (horse, cow, etc.) have laterally place eyes. These animals have limited stereoscopic vision. The region of the environment from which each eye collects light is called the visual field. |
| |
| Associated structures |
| |
| There are certain structures associated with the eyes and carry out, mainly, the function of protection. These structures are- |
| |
| Eye brows |
| |
| Along the arched upper ridges of the eye sockets are hairs that form the eye brows. They prevent the dust, rain, sweat, etc from entering the eyes. |
| |
| Eye lids |
| |
| Each eye in man is protected by two eyelids - upper and lower. The upper lid moves and covers the eye at regular intervals. This is called blinking. This action protects the eye from foreign particles. In certain animals, like the fishes and frogs, there is a third eyelid called the nictitating membrane. In man, it is very small and vestigial. Embedded in each tarsal plate of the eyelids is a row of elongated modified sebaceous glands, known as Meibomian glands. Their oily secreation helps keep the eyelids from adhering to each other. |
| |
| Lacrymal glands |
| |
| They are also called the tear glands as they produce secretion called tears. |
| |
 |
| |
| The lacrymal glands are present one on the outer upper border of each eye. The lacrymal secretion is watery, alkaline and carries out the following functions |
| |
| i) cleans the eyes |
| |
| ii) keeps the eyes moist |
| |
| iii) keeps the eyes free of bateria as it contains bacteriolytic lysozyme |
| |
| iv) provides nutrition to the cornea. |
| |
