If your eyes were always open to the world via tiny circular holes you would always have good visual acuity, and you would not get pain from walking into full sunlight. However, when dusk came you wouldn't get enough light in to your retinas to be able to see things. read more
The iris reflex, iris, automatic adjustment, retina, light, protective reflex; pigment melanin, pupil, radial muscles, circular muscles, antagonistic Site author Richard Steane The BioTopics website gives access to interactive resource material, developed to support the learning and teaching of Biology at a variety of levels. read more
The iris is an adjustable diaphragm around an opening called the pupil. The tiny muscles that make up the iris, known as the circular and radial muscles, relax and contract to maintain a fairly constant level of light entering the eye. read more