Which muscle encircles the eye

The sclera is the white of the eye and covers the entire eyeball except for the cornea. It protects its inner parts, gives shape to the eyeball, and makes it more rigid. The vascular tunic, also known as the uvea, is the middle layer of the eyeball.

It is composed of three parts: the choroid, the ciliary body, and the iris. The choroid is the posterior portion of the vascular tunic, and it lines the internal surface of the sclera. Its numerous blood vessels provide nutrients to the retina. Also, the choroid contains melanocytes that produce the pigment melanin. Melanin in the choroid absorbs excess light, which prevents reflection and scattering of light within the eyeball. Albinos lack melanin in all parts of their body; therefore, they often need to wear sunglasses because bright light is perceived as a bright glare.

In the anterior portion of the vascular tunic, the choroid becomes the ciliary body, which is composed of ciliary processes and ciliary muscle.

The ciliary processes contain blood capillaries that secrete aqueous humor. Furthermore, the zonular fibers that attach to the lens extend from the ciliary processes. The ciliary muscle contracts and relaxes in order to change the tightness of the zonular fibers which in turn alters the shape of the lens, adjusting it for near or far vision.

The iris is the colored portion of the eye. It is located between the cornea and the lens and it attached to the ciliary processes. The amount of melanin in the iris determines eye color. The main function of the iris is to regulate how much light passes through the pupil, the hole in the center of the iris. The third and innermost layer of the eyeball is the retina.

It consists of a pigmented layer and a neural layer. The pigmented layer is a sheet of melanin-containing epithelial cells that is located between the choroid and the neural part of the retina. The neural, or sensory, layer is an outgrowth of the brain that processes visual data extensively before sending electrical impulses to the axons of the optic nerve.

The three distinct layers of the retinal neurons are the ganglion cell layer, the bipolar cell layer, and the photoreceptor layer. Light passes through both the ganglion and bipolar cell layers before it reaches the photoreceptor layer. Photoreceptors are specialized cells that convert light rays into nerve impulses.

The two types of photoreceptor cells are rods and cones. Each retina contains about 6 million cones and more than million rods. Cones produce color vision, while rods enable us to see in dim light. Color vision results from the stimulation of different combinations of blue, green, and red cones. Eventually, the visual information reaches the optic disk, also called the blind spot because it contains no rod or cone cells. The macula lutea is a flat spot that is located in the exact center of the posterior portion of the retina.

The fovea centralis is located in the center of the macula lutea and it contains only cones. It is the area of highest visual acuity or resolution with highest density of cone photoreceptors and exclusion of rod receptors. This is the part of the eye we use to focus on something we are looking at. The lens is located in the cavity of the eyeball, behind the pupil and iris. It is suspended from ciliary body muscles via zonule fibers. Ciliary body muscles provideaccomodation during the near vision.

The lens divides the interior of the eyeball into two cavities: the anterior cavity and the vitreous cavity. The anterior cavity, the space anterior to the lens, is divided into the anterior chamber and the posterior chamber. The anterior chamber is located between the cornea and the iris. The posterior chamber is located behind the iris and in front of the zonular fibers and lens. Both chambers of the anterior cavity are filled with aqueous humor, a watery fluid that nourishes the cornea and lens.

The aqueous humor is produced by epithelium of the ciliary body and is replaced every 90 minutes. The final destination of this fluid inside of the eye is schlemm's canal which drain the fluid into the body circulation. Within the vitreous chamber is the vitreous body, a transparent jellylike substance that holds the retina against the choroid. Unlike the aqueous humor, the vitreous body does not undergo rapid replacement. It contains phagocytic cells that remove debris to maintain unobstructed vision.

Occasionally, some debris may cast a shadow on the field of vision and these are known as vitreal floaters. They are much more common in older individuals. Formation of the eye in the human embryo begins approximately three weeks into embryonic development and continues through the tenth week. Cells from both the ectodermal and the mesodermal tissues contribute to its development. Specifically, the eye is derived from the neuroepithelium, the surface ectoderm, and the extracellular mesenchyme which consists of both the neural crest and mesoderm.

Neuroepithelium forms the retina, ciliary body, iris, and optic nerves. Surface ectoderm forms the lens, corneal epithelium, and eyelid. The extracellular mesenchyme forms the sclera, the corneal endothelium, stroma, blood vessels, muscles, and vitreous. All eye structures, except for the eyelids and conjunctiva, are supplied by branches of the internal carotid artery.

The external carotid artery supplies the eyelids and conjunctiva. The internal carotid artery has many branches; one is the ophthalmic artery. This artery branches into the posterior ciliary, retinal, and muscular arteries. These arteries supply blood to the ciliary bodies, retina, and muscles, respectively.

The central retinal artery is a branch of ophthalmic artery which runs within the optic nerve. The optic nerve is the sensory nerve that transmits impulses from the retina to the brain.

The oculomotor , trochlear, and abducens nerve innervate the extraocular muscles. The oculomotor nerve has The ophthalmic nerve branches into the lacrimal, frontal, and nasociliary nerves.

The lacrimal nerve runs to the lacrimal glands and branches off to other areas. The frontal nerve supplies sensation to the superior eyelid and forehead. The only function of the medial rectus is to bring the pupil closer to the midline of the body. This muscle shares an origin with several other extrinsic eye muscles, the anulus tendineus, or common tendon. It is the largest of the extraocular muscles and its only action is adduction of the eyeball.

The inferior rectus is also a muscle of the orbit. This muscle has multiple functions, mainly helping to extort the eye. Here are more details of the inferior rectus muscle:. The inferior rectus muscle is the only muscle that is capable of depressing the pupil when it is in a fully abducted position. The superior rectus is mostly in charge of elevation, which means it helps you look up. It has other functions but that is the primary one. Again, the superior rectus comes from Latin roots.

The superior oblique muscle is on the upper medial side of the eye. That means it is closer to the nose. The primary job of this muscle is to turn the eye inward.