January 30, 2025 Author : Rui Andrês Each eye constantly adjusts the amount of light it lets in, focuses on objects and generates continuous images, which are transmitted to the brain instantly for processing. The eyeball is located in the orbit , which is a bony cavity that also contains the extraocular muscles, which allow the eyes to move in different directions, nerves and blood vessels. The wall of the eyeball is formed by 3 layers: The outermost one has two parts : The sclera (white of the eye), which is opaque, tough and occupies most of the surface; The cornea , which is transparent, curves and extends with the sclera at the level of the limbus, in the anterior region of the eye. It occupies about one sixth of the surface and covers the iris and pupil. It is the most powerful refractive structure in the eye that helps focus light on the retina. The uvea (or uveal tract) corresponds to the intermediate, vascular layer and has three parts, from the posterior to the anterior zone: The choroid , a layer of vascular connective tissue, which supplies oxygen and nutrients to the outermost layer of the retina; The ciliary body , which produces aqueous humor, which feeds the lens and cornea (which have no blood vessels); The iris , the colored area of the eye, which has a central opening, the pupil (the black dot in the middle of the eye), through which light enters the eye. The muscles in the iris control the size of the pupil. Mydriasis is the dilation of the pupil. It happens when the light intensity is lower (dark). Miosis is the contraction of the pupil. It happens when we are in an environment with greater light intensity. The retina corresponds to the inner layer. It is a thin and delicate tissue, where visual stimuli are captured and converted into nerve impulses, which then travel to the brain through the optic nerve, to form visual images. There are two types of photoreceptors or specialized cells in the neurosensory retina : Cones are responsible for central, detailed and colored vision, and are grouped mainly in the macula (central area of the retina); Rods are responsible for peripheral (side) vision and night vision. They are more numerous than cones and are grouped together, mainly, in the peripheral areas of the retina. Inside the eyeball there are: The lens , which is transparent and is located posterior to the iris. It transmits and focuses light onto the retina to form sharp images of objects observed at different distances. It has the shape of a biconvex lens and its curvature varies with the proximity of objects; The vitreous body , which is the largest structure in the eye. It is gelatinous and occupies all interior, from the posterior face of the lens to the surface of the retina (posterior segment of the eye). The cornea, lens, aqueous humor and vitreous humor are the refractive media of the eye. They change the direction of light entering the eye to focus it on the retina. Eye protection and lubrication The two eyelids , upper and lower, protect the eye, help regulate the entry of light and contribute to eye lubrication, distributing the tear film when blinking. Internally, the eyelids are lined by the conjunctiva (palpebral conjunctiva), which extends over the anterior surface of the eye (bulbar conjunctiva), except in the region of the cornea. The conjunctiva allows the eyelids to move smoothly over the eyes, maintains lubrication, and protects the eyes. The tear film lubricates the eyes and protects them. One part is produced by the lacrimal glands, located under the upper outer rim of the orbit and the remainder is produced by the conjunctiva and the Meibomian glands. Meet the ophthalmology clinical team from Hospital da Luz network The content published in the Health Guide is intended for the general public and is provided for informational and educational purposes only. It does not replace individual medical assessment, diagnosis, or personalized clinical advice from a qualified healthcare professional. The content is produced or reviewed by physicians and other healthcare professionals at Hospital da Luz and is based on the knowledge available at the time of publication or the most recent update, as indicated at the beginning of each item.
