Renu Jogi Basic Ophthalmology Fourth Edition PDF
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Renu Jogi Basic Ophthalmology Fourth Edition
Pupil size influences accommodation by controlling the amount of light entering the eye. In a bright light the pupils are constricted. In a dim light they are dilated.
If the pupils were dilated in a bright light, too much light would enter the eye and damage the retina. In a dim light, if the pupils were constricted, insufficient light would enter the eye to activate the photosensitive pigments in the rods and cones which stimulate the nerve endings in the retina.
The iris consists of one layer of circular and one of radiating smooth muscle fibres. Contraction of the circular fibres constricts the pupil, and contraction of the radiating fibres dilates it. The size of the
pupil is controlled by the nerves of the autonomic nervous system. Sympathetic stimulation dilates the pupil and parasympathetic stimulation causes contraction of the pupil.
2. Movements of the Eyeballs-convergence
Light rays from objects enter the two eyes at different angles and for clear vision they must stimulate corresponding areas of the two retinae. Extraocular muscles move the eyes and to obtain a clear image they rotate the eyes so that they converge on the object viewed. This co-ordinated muscle activity is under autonomic control. When there is voluntary movement of the eyes both eyes move and convergence is maintained. The nearer an object is to the eyes the greater the eye rotation
needed to achieve convergence.
If convergence is not complete there is double vision, i.e. diplopia. After a period of time during which convergence is not possible, the brain tends to ignore the impulses received from the divergent eye.