The human eye is a remarkable optical device, and its function is essentially to see. This is accomplished by utilizing a number of different structures, including the retina, the choroid, and the iris. The eye is roughly spherical, with the white part, or sclera, sitting on the outside of the eye and the dark, pigmented choroid on the inside. Together, these structures keep the eye essentially light-tight, except for its optic axis.
The iris is the transparent part of the eye, while the crystalline lens sits behind it. Together, they work to focus light onto the retina, allowing us to see objects up close and far away. This lens also has ciliary muscles attached to it, which help it to maintain its shape and function properly. The retina is the light-sensitive part of the eye, and connects to the brain via the optic nerve. The other components of the eye are necessary for vision, and help make it possible to see in bright sunlight.
The iris controls the amount of light entering the eye. It contracts or expands according to the amount of light entering the eye. When we see a train moving past us, we stabilize it on our retina and then move it out of our field of vision. Our eyes automatically adapt to the light by adjusting the size of the pupil. Once we adjust to near-sightedness, we see a train that is only slightly further away. As the train moves away from us, the eye returns to its original position.
The iris is the outermost section of the eye. This circular structure covers the lens, and is connected to iliary muscles. The iris regulates the size of the pupil, and contains pigments that give our eyes their colour. The retina is the innermost layer of the eye, and is responsible for the perception of images. If we can see the retina, it is a sign of health and wellness. And with these structures, the eye can see the world in colour.
The human eye is composed of three layers, or coats, that cover its various anatomical structures. The outermost layer is composed of the sclera and cornea, and the middle layer is the choroid, ciliary body, and iris. The innermost layer is the retina, which gets its blood supply from vessels in the choroid. So, how do these components work together? By studying their anatomy and functions, we can gain better insight into how they work.
When we move our head, our eyes move. This is called vestibulo-ocular reflex. The vestibulo-ocular reflex stabilizes retinal images during head movement. This reflex makes the eyes move in the opposite direction of the head movement, so that an image remains in the center of the visual field. When we move to the right, our eyes move to the left, and vice versa. These movements, called saccades, ensure that the individual photosensitive cells in each eye are constantly stimulated.