The Human Eye: Structure, Function, and Common Diseases
The Eye
Introduction
The eye is an organ of vision in humans and animals. The eyes of different species range in complexity from simple structures capable of differentiating only between light and darkness, to the advanced systems present in humans and other mammals, which can distinguish very small variations of shape, color, brightness, and distance. In fact, the organ undertaking the process of vision is the brain; the function of the eye is to translate the electromagnetic vibrations of light into a specific type of nerve impulse that is transmitted to the brain.
The Human Eye
The human eye, the organ that collects light and forms impressions as visual images, works like a camera. The iris represents the diaphragm; the lens, the camera lens; and the retina, the photographic plate. The images pass through the iris and through the lens, projecting onto the retina. Outside the lens are ciliary muscles, which change the lens’s curvature, accommodating for near or far vision, so that the image is formed precisely on the retina.
Defects in the curvature of the lens result in vision problems:
- Presbyopia: When the image is formed behind the retina, corrected by a converging lens.
- Myopia: When the image is formed in front of the retina, corrected by a diverging lens.
- Astigmatism: Corrected by cylindrical lenses.
The regulation of light entering the eye is done by the iris, a circular muscle that contracts reflexively if there is too much light, or expands if there is too little. Light-sensitive cells are distributed in the retina and are of two types:
- Cones: Responsible for perceiving colors.
- Rods: Sensitive to white, black, and gray.
In both cases, light excitation causes a breakdown of substances that make up the photosensitive retinal purple, acting on the rods and cones, which transmit the sensation through the different layers of the retina and optic nerve to the brain. The highest concentration of cones occurs in the macula lutea or yellow spot, located behind the pupil, where the images are clearest.
The Formation of the Image
The formation of the image on the retina is not a simple process, much less static.
A normal eye, focused at infinity (from about 5 meters), is at rest. Besides the possible contraction of the iris to regulate the amount of light (as in a camera with the diaphragm), the other dynamic part of the optical system, the lens, is at rest. That is, the human eye does not need extra effort to see far away.
Logically, an eye focused at infinity will see blurred objects at a close distance, just like if we focus a camera and photograph at a distance closer than another, the photo will be blurred.
What varies is the thickness of the lens. When we need to focus at a close distance, the ciliary muscles spring into action and cause a thickening of the lens, thereby increasing its power (after all, it is a convex lens) and getting the correct focus. This mechanism is called accommodation, and its failure is what causes presbyopia, which we commonly call “old sight.”
The eye functions as a receptor organ of the visual apparatus. The light rays are focused to reach the cornea and lens to form an inverted, real, and smaller image of the object on the light-sensitive retinal rods and cones. The focus is implemented by altering the convexity of the lens. In the rest position, with the ciliary muscle relaxed, the lens is flattened by the elastic tension of the zonule. Ciliary muscle contraction, especially in the southern external fibers, makes the choroid and ciliary body move forward. This relaxes the tension of the zonule and allows the lens, which is elastic, to increase its convexity and thus its ability to refract.
Image on the Retina
The real image produced on the retina is reversed with respect to the observed object, which has led to the question: Why, then, do we see objects upright?
The question is meaningless because we do not look at the image on our retina itself. The phenomenon is as follows: habit teaches us that certain impressions on the organs of our senses represent certain realities in the outside world, so that when such sensations are repeated, we attribute them to the existence of those realities, and this process is independent of the mechanism that produces the impression and conveys it to the brain. The retinal image has no objective reality for us.
Diseases of the Human Eye
Myopia
Myopia is a defect or imperfection of vision characterized by defective vision of distant objects. It is caused by excessive eye refraction in which rays from objects located a great distance form the focus before reaching the retina, where and when they reach, they are divergent. There are two types of myopia, steady and progressive, and its fundamental symptom is the reduction of distance visual acuity. The treatment, apart from the optical correction by concave spherical lenses, also includes medicated hygiene and lifestyle changes. It is prescribed to prevent fatigue with prolonged visual work; maintain proper lighting and avoid straining vision when reading small print.
A nearsighted person cannot see well far away. Because the focal point of the eye is closer to the cornea than in a normal eye, the images of objects at infinity are formed in front of the retina and appear blurred. They start to look clear when they are closer (at the far point).
The far point and the near point of accommodation are closer than in the normal eye.
an in the normal eye.