Eyeball

The eyeball meaning can be given as, it is a spheroidal structure that includes vision sense receptors found in all vertebrates and is built similarly to a simple camera. The retina’s eyeball is a metabolically active layer of nerve tissue made up of millions of light receptors (also called photoreceptors) and all of the structures needed to concentrate light onto it, which is housed in the eyeball.

Parts and Structure of Eyeball

The sclera, or the tough protective outer shell of the eyeball, is formed with a dense fibrous tissue that covers completely four-fifths of the eyeball and also serves as the attachment point for the eye muscles. The conjunctiva, a translucent mucous membrane that keeps the human eye from drying out, covers the sclera from the front. The tear film completely covers the translucent cornea, also known as the "window" through which light enters the eye, at the front of the eye.

The cornea, in conjunction with the aqueous humour behind it, offers the eye's highest focusing power. However, unlike the lens, the cornea focusing power and eyeball shape are not adjustable. A few other important structures in the eyeball are given as the iris and lens. Much of the eyeball is filled with transparent material, which is a gel-like one, called the vitreous humour, that helps in maintaining the spheroidal shape.

The sclera is an underlying vascular layer immediately beneath, called the uvea, that supplies nutrients to several eye parts. A ciliary body, a muscular structure located behind the iris that affects the lens shape during focusing and creates the aqueous humour that bathes the anterior chamber, is a component of the uvea. At the same time, the other components of the uvea are given as the iris and choroid. The choroid is a highly vascular tissue layer that supplies blood to the outer layers of the retina that lie on top of it.

The cornea, in which the focusing process begins, can be curved to a much greater extent than the remaining eyeball part. Defects in the corneal curvature cause a distortion of vision, which is called astigmatism. Behind it, the cornea is an anterior chamber that posteriorly extends to the plane of the pupil and iris. It is also filled with a watery fluid, which is known as aqueous humour. The iris is a doughnut-shaped, muscular curtain that shuts and opens to control the amount of light that enters the eye through the pupil, which is the opening in the iris's centre.

The aqueous humour flows via the pupil from the posterior chamber (which is a small space between the iris and its lens) to the anterior chamber and also out of the eye via the trabecular meshwork and the Schlemm’s canal, which encircles the peripheral iris. Also, a few aqueous humour exits the eye directly with the help of the ciliary body. The ciliary muscle attachments, including the lens, separate the aqueous humour in front behind from the vitreous humour.

The Shape of the Lens

The shape of the lens can be controlled by the ciliary body action, altering the focusing power of the lens as required. Both cornea and lens focus an image onto the retina at the backside of the eye. If the particular image is projected too far in front of the retina, then it causes a visual defect, which is called either nearsightedness or myopia. Whereas, if the image is theoretically focused “behind” the retina, the result is said to be farsightedness or hyperopia.

And, if no deformation of the lens exists, the image is projected onto the fovea, which is a structure near the retina’s centre that contains a large number of cone photoreceptors, and that also provides the sharpest vision. When it is stimulated by light, retinal photoreceptor cells send signals to the neighbouring cells in the retina that then relays the signals with the help of an optic nerve to the brain’s visual centres.

Fascial Sheath (Tenon’s capsule)

Retrobulbar Fat (Corpus Adiposum Orbitae)

The Tenon’s capsule is defined as a fascial sheet that forms a socket around our eyeball. It connects to the sclera in the front and fuses with the meninges that surround the optic nerve in the back. The fascia's inner surface is smoother and can be distinguished from the sclera's surface by a potential space known as the episcleral space.

Tenon’s outer surface capsule provides the attaching points to the extraocular muscles. The tendon of every muscle penetrates the fascial sheath that reflects on their tendons by forming a short sleeve around them. These sleeve-like projections are more essential as they attach to the orbit’s surrounding structures, thereby limiting the extraocular muscle’s actions. The two specifically important tendon sleeves are the ones around the tendons of the lateral and medial rectus muscles. The former is known as the medial check ligament, and it attaches to the lacrimal bone. In contrast, the latter is defined as the lateral check ligament, where it attaches to the zygomatic bone.