Parts of The Human Eye and Their Functions
The adult human eyeball is a spherical globe. The longest diameter is 24mm. The eye can be divided into fibrous, vascular, and nervous layers. The fibrous layer is the outermost layer that consists of the cornea and sclera. The cornea is the major refractive component.
The sclera covers the rest of the eye. The vascular layer is composed of the ciliary body, iris, and choroid.
Parts of the Eyes
The sclera is the white part of the eye. It is composed of collagen and elastic fibers, which provide a tough, opaque protective posterior coating for the eye. The sclera and cornea are actually composed of fibrous tissue. If the tissue is more dehydrated, it will be more transparent like the cornea.
The dehydration of the cornea is maintained by the corneal endothelium. If the cornea is more hydrated, it will be opaque like the sclera. The region where the sclera comes into contact with the cornea is called the corneal limbus.
Stem cells required for the repair of damage to the corneal epithelium have been found in the basal membrane of the corneal limbus. The sclera is largely an avascular structure and derives its nutrients from the episclera and the choroid.
The cornea serves as the entry point of the eye for the light to reach the retina. It is a transparent structure composed of epithelium, stroma- made up of thick fibrous structure and a single layer of endothelium.
The cornea is the protecting body and keeps the eye from germs, dust, and other harmful substances.
As well as it filters the ultraviolet radiation from the sun and is the primary focusing system of the eye. It has a refractive index greater than air. Cornea by the means of its transparency helps in the minimum scattering of light.
Upon entry into the eye, the image is refracted and converged on the retina to form the final image.
Iris and Pupil
By the time light has passed the cornea and aqueous humor, it moves through the iris and pupil. Both the structures regulate the amount of light entering the system. The iris is composed of the pigmented layer that lies in front of the lens.
It has the ability to dilate and contract with the help of sphincter and dilator muscles in order to regulate the pupil. The aperture of the eye. The iris tends to decrease the aperture in presence of too much light and vice versa is true for dim light.
The ciliary body
It is a circular structure surrounding the lens. It consists of the ciliary muscle, ciliary zonule, and ciliary epithelium. The ciliary body has thin suspensory ligaments that hold the lens in place.
The epithelium of the ciliary body tends to secrete and synthesize aqueous humor. The inner layer of the ciliary epithelium is non-pigmented that is continuous with neural retinal tissue.
The outer layer is pigmented and is continuous with pigmented epithelium. The vitreous humor is a gel-like substance that occupies the space between the lens and the retina. The vitreous humor accounts for approximately two-thirds of the volume of the entire eye.
It is composed of 99% of water, with a small amount of collagen. The vitreous humor is clear and avascular structure. It is a transparent structure. It is in contact with the retina but only adherent to ora Serrata. The attachment helps to hold the retina in place.
The vitreous ends anteriorly and is attached to the lens and ora Serrata. By the time vitreous humor has developed, it becomes stagnant. As the patient ages, vitreous gel shrinks and dilutes the vitreous.
The process is called vitreous syneresis. If the vitreous is detached from the eye’s posterior region due to trauma / high myopia, the occurrence of floaters in vision is likely. Aging, along with other retinal disorders, cause the development of small holes in places where the retina has thinned.
Vitreous humor can leak through those holes and cause retinal detachment from the underlying support tissue, which is detrimental to visual acuity and can lead to blindness.
It is the vascular layer of the eye containing connective tissue that surrounds the globe. it is thickest at the extreme posterior eye (0.2 mm), and thinnest in the anterior surface (0.1 mm). Located between the retina and sclera, the choroid is separated from retinal nervous tissue by two structures, the Bruch’s membrane, and the RPE.
Bruch’s membrane serves to mediate the passage of nutrients into the retina, and filter out retinal debris. The choroid provides the greatest blood flow to the retina (65–85%of total blood supply) and allows an adequate supply of oxygen and nutrients to the photoreceptors in the outer layers of the retina.
The aqueous humor
Aqueous humor is a transparent liquid present between the cornea and the lens. It is formed by the ciliary epithelium of the ciliary body positioned in the posterior chamber. The aqueous humor is continuously formed and it flows through the pupil into the anterior chamber.
From the anterior chamber, a major portion of the aqueous humor leaves the eye through the trabecular meshwork into schlemm’s canal and the episcleral venous system. The rest drains into uveoscleral route through interstitial tissue of the ciliary muscle into supra choroidal space and leaves through the sclera.
The constant production of aqueous humor regulates the intraocular pressure of the eye. It also plays a major role to maintain the optical clarity of the cornea and anterior chamber. It is suggested that nutrients and oxygen supply is also provided to the anterior portion of the eye and the waste products are removed by the same.
The aqueous humor also has a role in the local immune response by dispensing ascorbate, an antioxidant concentrated by the ciliary epithelium, throughout the eye.
The lens is a crystalline structure. As the light enters through the pupil, it focuses on the lens. The lens is composed of an epithelium layer, that is made up of proteins called crystallin which further refines the light from the cornea.
The molecules inside the lens are densely and uniformly packed which is the main characteristic required for its transparency. Lens has the second greatest refractive power. The refractive power can be changed to some extend with the help of ciliary muscles and ciliary zonular fibers.
The process is called accommodation. The eye focusing the object at a distance of 6 meters, and the lens changes its shape to a flatter shape while the eye focusing on the object at a near distance causes the lens to adopt a more spherical shape, which increases the optic power in order to create a clear image.
The Retina as a Part of the Central Nervous System
Retina serves as the light-processing center of the eye. It transforms the visual signals into neural signals that is processed by the brain.
The Fovea and Macula
The fovea and macula are the most sensitive part of the retina that provides sharp central vision. The cones of the eye are responsible for minute details. The highest concentration of cones is found in the fovea, a small pit at the center of the retina while no rods are present in this area.
The image is focused on this area with the help of the cornea and lens. Surrounding the fovea in the central retina is the macula which is a highly pigmented, yellow spot with a diameter of 5 mm. The yellow pigment of the macula is derived from two types of xanthophylls, lutein, and zeaxanthin.
The optic nerve and optic disc:
The optic nerve connects the retina to the brain’s visual processing center. The optic nerve crosses through the posterior part of the fundus called the optic disc, also termed the optic nerve head or optic disc It is 1.5mm in diameter.
The central retinal artery enters the eye through the optic canal. As the optic disc has no photoreceptors, it creates a blind spot on the retina.
There are seven extraocular muscles, four rectus muscles, 2 oblique muscles, and levator-palpebrae-superioris. The four rectus mussels are medial rectus, lateral rectus, superior rectus, and inferior rectus. The obliques are superior and inferior obliques.
All rectus muscles originate at the annulus of Zinn and course anteriorly to reach its insertion points. These muscles insert at varying distances from limbs making a spiral known as the spiral of Tillaux.
The medial rectus inserts at 5.5mm from the limbus, the inferior inserts at 6.5mm from the limbus, the lateral rectus inserts at 6.9 mm from the limbus, and the superior rectus at 7.7 mm from the limbus.
The superior oblique travels through the trochlea and inserts posterior to the insertion of the superior rectus the inferior oblique inserts near the macula.
The levator- palpebrae- superioris originates from the lesser wing of the sphenoid and travels over the superior rectus towards the eyelid where the connective tissue from levator-palpebrae- superiors connects with tissue from the superior rectus to form the Whitnall ligament.
Extraocular muscles are specialized skeletal muscles with slow tonic fibers and also rapid-type muscle fibers.myopia