- Anatomy of the visual apparatus
- Eye Anatomy and Function - Made Easy
- Human Eye Diagram:
- Human Eye - Structure And Functioning
- The Eye Definition Biology:
- Structures auxiliary to the eye
- Structure of Human Eye (With Diagram) | Human Body
- Human eye structure and function pdf converter
- The Human Eye Facts, Functions, Structure and Problems
- Structure and Function of the Human Eye
The human eye, the organ containing visual receptors, provides vision, with the assistance of accessory organs. These accessory organs include the eyelids and lacrimal apparatus, which protect the eye, and a set of extrinsic muscles, which move the eye.
The eye, lacrimal gland, and associated extrinsic muscles are housed in the orbital cavity, or orbit, of the skull. Each orbit is lined with the periosteum of various bones, and also contains fat, blood vessels, nerves, and connective tissues. Each eyelid, both upper and lower, has four layers—skin, muscle, connective tissue, and conjunctiva. The eyelids are moved by the orbicularis oculi muscle Figure 4 , which acts as a sphincter and closes the lids when it contracts, and by the levator palpebrae superioris muscle, which raises the upper lid and thus helps open the eye.
The conjunctiva is a mucous membrane that lines the inner surfaces of the eyelids palpebral conjunctiva and folds back to cover the anterior surface of the eyeball bulbar conjunctiva , except for its central portion cornea. The lacrimal apparatus consists of the lacrimal gland, which secretes tears, and a series of ducts that carry tears into the nasal cavity Figure 3.
The lacrimal gland is located in the orbit and secretes tears continuously through tiny tubules. The tears flow downward and medially across the eye.
Anatomy of the visual apparatus
Two small ducts the superior and inferior canaliculi collect tears, which then flow into the lacrimal sac, located in a deep groove of the lacrimal bone.
From there the tears flow into the nasolacrimal duct, which empties into the nasal cavity. Secretion by the lacrimal gland moistens and lubricates the surface of the eye and the lining of the lids.
Tears also have an enzyme lysozyme that kills bacteria, reducing the risk of eye infections. Six extrinsic muscles move the eye in different directions Figure 5.
Any given eye movement may utilize more than one extrinsic muscle, but each muscle is associated with a primary action. Figure 5 illustrates the locations of these extrinsic muscles and Table 1 lists their functions, as well as the functions of the eyelid muscles. The eye is a hollow, spherical structure about 2. Its wall has three distinct layers—an outer fibrous layer, a middle vascular layer, and an inner nervous layer.
The spaces within the eye are filled with fluids that help maintain its shape. Light is often described as moving in straight lines called rays of light. The anterior sixth of the outer layer bulges forward as the transparent cornea, the window of the eye. The cornea helps focus entering light rays. The cornea is composed largely of connective tissue with a thin surface layer of epithelium. It is transparent because it contains few cells and no blood vessels, and the cells and collagenous fibers form unusually regular patterns.
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The cornea is continuous with the sclera , the white portion of the eye. The sclera makes up the posterior five-sixths of the outer layer of the wall of the eye. It is opaque due to many large, disorganized, collagenous and elastic fibers.
The sclera protects the eye and serves as an attachment for the extrinsic muscles. In the back of the eye, the optic nerve and certain blood vessels pierce the sclera. The middle layer of the wall of the eye includes the choroid coat , ciliary body, and iris. The choroid coat, in the posterior five-sixths of the globe of the eye, is loosely joined to the sclera and is honeycombed with blood vessels, which nourish surrounding tissues.
The choroid coat also has many pigment-producing melanocytes. The melanin that these cells produce absorbs excess light, which helps keep the inside of the eye dark.
The ciliary body is the thickest part of the middle layer of the wall of the eye. It extends forward and inward from the choroid coat and forms a ring inside the front of the eye. Within the ciliary body are many radiating folds called ciliary processes and groups of smooth muscle cells that constitute the ciliary muscle.
Human Eye Diagram:
Many strong but delicate fibers, called suspensory ligaments, extend inward from the ciliary processes and hold the transparent lens in position. The distal ends of these fibers attach along the margin of a thin capsule that surrounds the lens. The body of the lens lies directly behind the iris and pupil and is composed of highly specialized epithelial cells called lens fibers.
The cytoplasm of these cells is the transparent substance of the lens. The ciliary muscle and suspensory ligaments, along with the structure of the lens itself, enable the lens to adjust shape to facilitate focusing, a phenomenon called accommodation.
The lens is enclosed by a clear capsule composed largely of elastic fibers. This elastic nature keeps the lens under constant tension, and enables it to assume a globular shape.
Human Eye - Structure And Functioning
The suspensory ligaments attached to the margin of the capsule are also under tension. When they pull outward, flattening the capsule and the lens inside, the lens focuses on distant objects.
However, if the tension on the suspensory ligaments relaxes, the elastic lens capsule rebounds, and the lens surface becomes more convex—focused for viewing closer objects.
The ciliary muscle controls the actions of the suspensory ligaments in accommodation. One set of ciliary muscle cells extends back from fixed points in the sclera to the choroid coat.
When the muscle contracts, the choroid coat is pulled forward and the ciliary body shortens. This action relaxes the suspensory ligaments, and the lens thickens in response. When the ciliary muscle relaxes, tension on the suspensory ligaments increases, and the lens becomes thinner and less convex again. These proteins, along with the absence of organelles that scatter light mitochondria, endoplasmic reticula, and nuclei , account for the transparency of the lens.
An eye disorder common in older people is cataract.
The Eye Definition Biology:
The lens or its capsule slowly becomes cloudy and opaque. Cataracts are treated on an outpatient basis with surgery to remove the clouded lens and replace it with an artificial lens.
Without treatment, cataracts eventually cause blindness. The iris is a thin diaphragm composed mostly of connective tissue and smooth muscle fibers.
From the outside, the iris is the colored part of the eye.
Structures auxiliary to the eye
The iris extends forward from the periphery of the ciliary body and lies between the cornea and lens. The iris divides the space anterior cavity separating these parts into an anterior chamber between the cornea and the iris and a posterior chamber containing the lens between the iris and the vitreous body. The epithelium on the inner surface of the ciliary body secretes a watery fluid called aqueous humor into the posterior chamber. The fluid circulates from this chamber through the pupil, a circular opening in the center of the iris, and into the anterior chamber.
Aqueous humor fills the space between the cornea and lens, helps nourish these parts, and aids in maintaining the shape of the front of the eye. Aqueous humor leaves the anterior chamber through veins and a special drainage canal, the scleral venous sinus canal of Schlemm.
This sinus is in the wall of the anterior chamber at the junction of the cornea and the sclera.
Structure of Human Eye (With Diagram) | Human Body
The iris controls the size of the pupil, through which light passes as it enters the eye. The contractile cells of the iris are organized into two groups, a circular set and a radial set. The circular set pupillary constrictor is smooth muscle and acts as a sphincter. When the muscle cells contract, the pupil gets smaller, and less light enters. Bright light stimulates the circular muscles to contract, which decreases the amount of light entering the eye.
The radial set pupillary dilator is composed of specialized contractile epithelial cells myoepithelial cells. Dim light stimulates the radial cells to contract, which increases the amount of light entering the eye.
The inner layer of the wall of the eye consists of the retina , which contains the visual receptor cells photoreceptors. The retina is a nearly transparent sheet of tissue continuous with the optic nerve in the back of the eye and extending forward as the inner lining of the eyeball. The retina ends just behind the margin of the ciliary body. The retina is thin and delicate, but its structure is quite complex. It has a number of distinct layers, as Figure 7 illustrates.
In the central region of the retina is a yellowish spot called the macula lutea. A depression in its center, called the fovea centralis , is in the region of the retina that produces the sharpest vision see Figure 6.
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Just medial to the fovea centralis is an area called the optic disc. Here, nerve fibers from the retina leave the eye and form the optic nerve. Because the optic disc region does not have photoreceptors, it is commonly known as the blind spot of the eye. A central artery and vein also pass through the optic disc.
These vessels are continuous with the capillary networks of the retina.
Along with vessels in the underlying choroid coat, they supply blood to the cells of the inner layer. The space bounded by the lens, ciliary body, and retina is the largest compartment of the eye and is called the posterior cavity. It is filled with a transparent, jellylike fluid called vitreous humor , which along with collagen fibers forms the vitreous body. Photoreceptors are modified neurons of two distinct kinds, as Figure 7 illustrates.
The Human Eye Facts, Functions, Structure and Problems
One group of photoreceptors, called rods , have long, thin projections at their ends, and provide vision without color, in shades of gray. The other group of photoreceptors, called cones , have short, blunt projections, and provide color vision. There is only one type of rod, but there are three types of cones. A human eye has million rods and 7 million cones.
Rods and cones are in a deep part of the retina.
Structure and Function of the Human Eye
They are closely associated with an adjacent layer of retinal pigment epithelium RPE that absorbs light the photoreceptors do not absorb. With the pigment of the choroid coat, the retinal pigment epithelium keeps light from reflecting off surfaces inside the eye. Photoreceptors are stimulated only when light reaches them. A light image focused on an area of the retina stimulates some photoreceptors, which results in impulses traveling to the brain.
However, the impulses leaving in response to each activated photoreceptor deliver only a fragment of the information required for the brain to interpret a complete scene.