ANATOMY OF THE EYE:
The eye is a special sense organ responsible for sight. It is spherical in shape, about 2.5 cm in diameter, and located within the orbital cavity. The orbit, formed by several bones, protects the eyeball from injury.
The wall of the eyeball is divided into three distinct layers:
- Outermost layer (Fibrous coat): Composed of the sclera and cornea.
- Middle layer (Vascular coat): Includes the choroid, ciliary body, and iris.
- Innermost layer (Nervous tissue): Consists of the retina.
The eye is controlled by six external muscles—four rectus muscles (superior, inferior, lateral, and medial) and two oblique muscles (superior and inferior)—which move the eyeball within the orbital cavity.
Eyelids and Conjunctiva:
The eyelids protect the eyes from dust and intense light. Eyelashes at the eyelid margins further guard the eye. The conjunctiva is a mucous membrane lining the inner surface of each eyelid and covering the exposed part of the eyeball.
Lachrymal Apparatus:
The lachrymal apparatus includes the lachrymal gland, canaliculi, lachrymal sac, and nasolachrymal duct. These structures produce and drain tears into the nasal cavity. The lachrymal glands, located in the frontal bone depression, secrete antibacterial fluid that protects the eye from infection and foreign particles.
The Sclera and Cornea:
The sclera is the tough, outer fibrous layer that maintains the shape of the eye and provides attachment for external muscles. The anterior part of the sclera becomes the transparent cornea, which allows light rays to enter the eye. The cornea’s convex shape helps bend (refract) light rays toward the retina for clear vision.
The Choroid:
The choroid forms the middle vascular layer of the eye, lining the sclera for about five-sixths of its area. It is dark brown due to melanin pigment produced by melanocytes. The choroid supplies nutrients to the posterior part of the retina and prevents internal reflection of light.
Ciliary Body:
The anterior part of the choroid forms the ciliary body, which contains ciliary muscles and epithelial cells. The ciliary processes secrete aqueous humor, a watery fluid filling the space in front of the lens. The ciliary muscle alters the lens shape for near and distant vision. Its nerve supply comes from the parasympathetic branch of the oculomotor nerve.
Iris and Pupil:
The iris extends from the ciliary body and lies between the cornea and lens. It contains circular and radial muscles that control the size of the pupil (central opening). In bright light, circular muscles contract to constrict the pupil; in dim light, radial muscles contract to dilate it. These are autonomic reflexes.
The Retina:
The retina is the innermost, light-sensitive layer lining the inner surface of the choroid. It contains photoreceptor cells—rods and cones—that convert light into nerve impulses. The retina connects with the optic nerve, which transmits visual signals to the brain’s occipital lobe.
At the center of the retina lies the macula lutea, containing the fovea centralis—a region with only cone cells responsible for sharp central vision. The point where the optic nerve exits the eyeball lacks photoreceptors and is known as the optic disc or blind spot.
Blood Supply:
The retina receives oxygenated blood from the ciliary and central retinal arteries, branches of the ophthalmic artery. Venous drainage occurs via the central retinal vein.
Rods and Cones:
- Rods: Sensitive to dim light; essential for night vision. Contain rhodopsin, a pigment made from vitamin A and a protein. Deficiency of vitamin A causes night blindness.
- Cones: Responsible for daylight and color vision. Three types of cones respond to red, blue, and green light. A combination of their stimulation produces color perception. Color blindness occurs when one or more cone pigments are deficient.
Fluids of the Eye:
The space between the cornea and lens is divided into anterior and posterior chambers filled with aqueous humor. The area behind the lens contains vitreous humor, a jelly-like substance that maintains intraocular pressure and supports the retina.
Physiology of Vision:
Light entering the eye is focused on the retina, where it stimulates photoreceptors. These transmit signals through bipolar and ganglion cells to the lateral geniculate body of the thalamus and then to the primary visual cortex in the occipital lobe.
Photoreceptors contain light-sensitive pigments that change structure upon light absorption, generating receptor potentials. In rods, the photopigment rhodopsin (vitamin A derivative) plays a vital role. Continuous renewal of photoreceptor discs maintains optimal vision.
Optic Nerve and Pathway:
The optic nerve originates from the retina and exits through the optic disc. It passes through the optic foramen of the sphenoid bone to form the optic chiasma, where fibers from the nasal sides of both eyes cross to the opposite side. From here, fibers continue as the optic tract to the lateral geniculate bodies of the thalamus and then project to the visual cortex in the occipital lobes.
Accommodation:
Accommodation is the process by which the lens changes shape to focus on objects at various distances. When viewing near objects, the ciliary muscles contract, reducing tension on the suspensory ligaments and allowing the lens to become more convex. For distant vision, the muscles relax, flattening the lens. This flexibility is due to the elastic nature of the lens.
Proper accommodation ensures that light rays from an object converge accurately on both retinas. Any disturbance in this process may result in double vision.
Detailed Notes:
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