There are several parts of the eye. These parts are called the irises, sclera, cornea, and cones. These parts are responsible for allowing us to see and focus light. These parts are quite complex. However, they are fairly well-understood, even though new knowledge is always being discovered through ongoing research.
Iris
The iris is a complex structure that controls the size and shape of the pupil. It is made up of nerves and muscles. A fluid called aqueous humor drains from the eye, maintaining its shape and size. This fluid also has several functions, including controlling the pressure in the eye.
The iris contains two distinct regions, the pupillary zone and the ciliary zone. The pupillary zone is located inside the eye, while the ciliary zone is on the outside. The ciliary zone is a thin layer of tissue that extends from the iris’ posterior edge to the ciliary body. The iris also has crypts, which are openings in the collarette that allow the deeper tissues of the iris to absorb aqueous humor. The iris’ color is determined by the amount of pigment in the anterior stroma. Individuals with a lack of stromal pigment will have blue eyes, while people with hyperpigmented stroma will have brown eyes.
The iris is a complex structure that consists of an iris, a stroma, and melanocytes. The amount of pigment and its distribution in the iris determines its color. Those without pigment have a blue iris, while infants have a greater proportion of melanocytes, which can lead to a darker iridal color. The iris is also equipped with two distinct layers of muscles, the dilator pupillae and sphincter pupillae. These muscles respond to the stimulation of sympathetic and parasympathetic nerves.
Sclera
The sclera of the eye is composed of a thin layer of connective tissue. This layer is highly vascularised and contains bundles of collagen and elastic fibres. It also contains fibroblasts, which help organise the collagen. It is located below the tenon’s capsule, and is separated from the retina by a membrane known as Bruch’s membrane, which is a thin potential space surrounded by collagen fibers.
The sclera is made of two parts: the anterior and posterior chambers. The anterior chamber contains aqueous humour, which supplies nourishment to the eye’s internal structures. The posterior chamber contains a jellylike fluid and connects the iris and lens.
Cornea
The cornea is the outermost layer of the eye and is composed of two layers. The central epithelium is composed of 5-7 layers, with columnar basal cells, and the peripheral epithelium contains seven to 10 layers and contains keratan sulfate and undulating extensions. These layers are connected to one another by a layer called the Descemet’s membrane.
A longitudinal section of the human cornea shows the epithelium, anterior and posterior elastic laminae, and Descemet’s membrane. The anterior chamber is made up of a thin lining, whereas the posterior chamber is made up of layers of cartilaginous tissue.
Cones
Human eyes have six to seven million cones, which are responsible for our visual acuity and for identifying colours. They are concentrated in the central part of the retina known as the fovea centralis. This area is devoid of rods and consists only of cones. Of these, red cones account for about 64%, green cones for 32%, and blue cones for 2%.
The red, green, and blue cones are used to perceive colors. They are paired with the S cones, which are responsible for detecting ultraviolet light. However, these receptors are insufficient to compensate for the deficiency in the number of cones. As a result, the final visual perception is colored based on the sensitivity of the retinal cones to different wavelengths. This means that a blue amplifier is required somewhere in the visual processing chain.
Lens
The lenses in your eyes are the structure that focuses light. The lens is a biconvex body composed of epithelial cells and fibers. There are 3 layers to the lens. Its structure depends on the regular packing of the fibers, which helps it to be transparent and provide refractive power to your eyes.
The lens is situated behind the iris and in front of the vitreous. Its posterior surface is attached to the anterior vitreous with the help of a ligament called Hyaloideo capsulare, but this is not a true ligament and the strength of the attachment gradually decreases with age. It is surrounded by the hyaloid face of the vitreous, which is called the retrolental space. In addition, the lens can change shape with the help of the ciliary muscles. This changes the focal distance to the retina, bringing the image into sharp focus.
The flexibility of the lens is a crucial component of good vision. As we age, the lens becomes stiff, which hinders the vision. This problem is known as presbyopia and affects every individual to some degree.
Conjunctiva
The conjunctiva receives its blood supply from the palpebral arteries. These arteries are part of the tarsal arcade and lie about two millimeters in front of the eyelid margin. They branch into two smaller arteries, the lateral and medial, and enter the conjunctiva. The lateral and medial arteries supply the palpebral portion of the conjunctiva.
The conjunctiva is made up of three distinct layers. The outer layer is the sclera, while the inner layer, or conjunctiva, covers the iris. The cornea, or clear layer in front of the eyelid, is covered with a layer of mucus and other tissue. Together, these tissues help protect the eye, providing lubrication and a protective barrier.
Conjunctiva lines the lids
The conjunctiva lines the lids, and is a mucous membrane that connects the eyeball to its orbit. The conjunctiva also allows the eyeball to rotate within the orbit. In addition, it contains sweat glands and hairs. Near the skin-conjunctiva junction, these hairs change to form eyelashes.
The conjunctiva lines the eyelids and bends over the eyeball’s surface, forming an outer covering for the anterior part of the eye. Its two main types are palpebral and bulbar, with palpebral conjunctiva covering the lids and bulbar conjunctiva covering the eyeball’s white part. There are also redundant portions of the conjunctiva, called fornices, located toward the equator. These regions form recesses, or sacs, in the conjunctiva that allow the eyeball to move freely.
Conjunctiva bends back over the surface of the eyeball
The eyeball’s surface is exposed to a variety of external influences. As a result, it is prone to a wide variety of disorders, including dryness, allergies, chemical irritation, trauma, and infections. It may also be affected by systemic diseases, such as sickle-cell anemia and type II diabetes mellitus.
The eyelid’s conjunctiva is a thin, translucent mucous membrane that covers the anterior surface of the eyeball, the anterior portion of the eyelid, and the anterior sclera. The conjunctiva also forms an important barrier to infection. The conjunctiva is separated into three zones: the basal, palpebral, and bulbar.
Muscles
There are several muscles within the eye and they all play a vital role in the eye’s structure. These muscles are located on the inside of the eye and originate from the tendinous band surrounding the optic nerve. This tendinous band provides holes for the eye’s nerves and anchors the eye muscles to the orbit. The superior rectus muscle is the longest and forms the muscular band between the eye and the annulus of Zinn. The inferior, medial, and lateral rectus muscles are similar to the superior rectus muscle but insert on the medial, inferior, and lateral surfaces of the eye.
Muscles of the eye are controlled by nerve impulses from the base of the brain to the brain stem. Three of these nerves enervate the muscles within the eye, including the lateral rectus, superior oblique, and lateral rectus. These muscles control the eye’s movement and function. These muscles act together and individually to keep the eye steady and track objects.
Ciliary body
The ciliary body is a smooth muscle that controls the shape and zonule tension of the crystalline lens. When the ciliary muscle is relaxed, it causes the lens to flatten out slightly. This results in a change in the focus of the lens. This change in focus is known as accommodation.
Damage to the ciliary body can cause serious problems. Acute trauma, such as a concussion or a laceration, can result in transient iritis, cell growth in the anterior chamber, and a flare in the anterior vitreous. More serious injuries may result in pupillary sphincter rupture or iridodialysis. Severe trauma can also lead to choroidal detachment or cyclodialysis.
There are many genes that are expressed in the ciliary body of the eye. One study identified 62 genes in the eye and 7 were expressed in the developing ciliary body. None of these genes were expressed in the same way in different clones of the ciliary epithelium. This suggests that the ciliary epithelium is composed of distinct populations of genes, which may not be related.
