The Anatomy of the Human Eye
The eyes are housed in the bony orbits that are formed by eight different bones and covered in periorbita. The orbits protect the eyeballs and the structures they need to function. A bit of orbital fat takes up any space not occupied by other parts. The walls of the orbit are made up of bones.
The superior wall of the orbit is formed in large part by the orbital part of the frontal bone and a small amount from the lesser wing of the sphenoid bone. The fossa for the lacrimal gland is found in this orbital part of the frontal bone.
The medial wall is formed mostly by the ethmoid bone and parts of the frontal, lacrimal, and sphenoid bones. The medial wall contains the lacrimal groove and the fossa for the lacrimal sac.
The lateral wall is made up of the frontal processes of the zygomatic bone and the greater wing of the sphenoid bone.
The inferior wall is formed mostly by the maxilla with parts of the zygomatic and palatine bones. It’s separated from the lateral wall by the inferior orbital fissure.
The eyes are covered with eyelids that open and close, and help keep your eyes moist.
Dense bands of connective tissue called superior and inferior tarsi strengthen the eyelids. The tarsi are connected to the medial and lateral margins of the orbit by medial and lateral palpebral ligaments. The orbital septum is a weak membrane running between the tarsi and the edges of the orbit. Tarsal glands secrete fluids that prevent the edges of your eyelids from sticking together when they’re closed.
Eyelashes grow at the edges of the eyelids near modified sweat glands called ciliary glands.
Located at the medial angle of the eye is a pinkish red mound of tissue called the lacrimal caruncle. The lacrimal gland is located in the fossa for the lacrimal gland. It secretes tears that are conveyed to the conjunctival sac by the lacrimal ducts. Tears cover the eyeball to keep it moist and to flush away bacteria and foreign materials. The tears drain into the lacrimal lake, where they enter openings into tiny canals on their way to the lacrimal sac. From there, the tears drain through nasolacrimal duct into the nasal cavity.
The eyeball holds the structures that allow you to see things. Its posterior portion is covered by a layer of connective tissue that connects the eyeball to the orbit called the fascial sheath of the eyeball. The episcleral space lies between the fascial sheath and the rest of the eyeball. This little space allows your eyeball to move around.
The eyeball has three layers surrounding its inner chambers:
Fibrous layer: This external layer gives shape to the eyeball. It includes the sclera, or the white of the eye. The cornea is the transparent part of the fibrous layer. A corneoscleral junction is formed at the intersection of the cornea and sclera.
Vascular layer: This middle layer is called the uvea. It includes the choroid, which is a dark reddish-brown layer that lines the sclera and contains a dense vascular bed. This layer has several important structures:
The ciliary body is a thickened muscular part of the uvea found behind the corneoscleral junction. It serves as the attachment point for the lens. It also controls the thickness of the lens by contraction and relaxation of the ciliary muscle contained within.
Ciliary processes lie on the internal surface of the ciliary body. They secrete aqueous humor, a fluid that fills the anterior segment of the eyeball.
The iris is the colored portion of the eye that surrounds the pupil, an opening through which light is transmitted.
The muscles that control the iris include the sphincter pupillae (parasympathetically innervated), which decreases the diameter of the pupil, and the dilator pupillae (sympathetically innervated), which increases the size of the pupil.
Retina: This innermost layer has an optic part that’s sensitive to light and a nonvisual part that extends over the ciliary body and the posterior part of the iris. The optic part contains photoreceptors called rods and cones that are sensitive to light. The ora serrata is the border between the optic part and the nonvisual part. The posterior part of the retina is what you see when you view the eye through an ophthalmoscope. The fundus (the part of the retina you see) includes the following parts:
Optic disc: This blind spot is the location where the sensory fibers that form the optic nerve exit the eyeball. It has no photoreceptors.
Macula: This small oval area is equipped with cells specialized for visual acuity.
Fovea centralis: This small depression is in the middle of the macula.
The lens lies between the posterior chamber and the vitreous body. The lens is transparent and biconvex (bends outward) on both front and back. The suspensory ligaments anchor the lens and its elastic capsule.