Human Anatomy - Kotsan I. Y. 2009

Sense Organs
Organ of Vision (Eye)
Nucleus of the Eyeball

The core of the Eyeball is composed of transparent light-refracting media: the lens, the vitreous body, and the aqueous humor, which fills the anterior and posterior Chambers of the eye.

The lens is a major light-refracting medium of the eyeball. The refractive index of the lens is 1.32 in the superficial layers and 1.42 in the central region. It is located behind the iris within the hyaloid fossa of the vitreous body and is secured to the ciliary body by the suspensory ligament. The lens is a biconvex Structure measuring 9—10 mm in diameter and approximately 4 mm in thickness. It features an anterior surface and a posterior surface (facies anterior et facies posterior). The peripheral margin where these two surfaces meet is called the equator of the lens (equator lentis). The posterior surface is more convex than the anterior one. The most prominent central points on the anterior and posterior surfaces are designated as the anterior and posterior poles (polus anterior et polus posterior). The line connecting both poles forms the axis of the lens (axis lentis), with its length varying between 3.7 and 4.4 mm depending on the degree of accommodation. The substance of the lens (substantia lentis) is colorless, transparent, and dense, completely lacking Blood Vessels and nerves. The lens receives its nourishment through the diffusion of nutrients from the aqueous humor of the posterior chamber. Any impairment in the metabolic processes of the lens leads to opacity, known as a cataract. The inner portion—The Nucleus of the lens (nucleus lentis)—is significantly denser than the peripheral portion—the cortex of the lens (cortex lentis). Externally, the lens is enclosed by a thin, transparent, and elastic capsule (capsula lentis), which anchors it to the ciliary body via the suspensory ligament. When the ciliary Muscle contracts, the vascular tunic shifts forward, moving the ciliary body closer to the equator of the lens; this relaxes the suspensory ligament (zonule of Zinn), allowing the lens to spring back and increase its anteroposterior dimension, thereby becoming more convex and enhancing its refractive power. Conversely, when the ciliary muscle relaxes, the ciliary body moves away from the equator, the zonule tightens, and the lens flattens, reducing its refractive power. This adaptive mechanism that enables clear Vision at varying distances is called accommodation, and the ciliary muscle is also referred to as the muscle of accommodation. With advancing age, accommodation diminishes as the lens gradually loses its elasticity and ability to change shape.

The vitreous body (corpus vitreum) lies posterior to the lens, filling the entire back portion of the cavity of the eyeball. It is transparent, gel-like in consistency, devoid of Blood Vessels and nerves, and its light-refraction power is approximately equal to that of the aqueous humor. The vitreous body consists of delicate, interwoven Collagen fibers—the vitreous stroma (stroma vitreum)—and the vitreous humor distributed between these fibers. The vitreous humor (humor vitreus) is composed of 98% Water with dissolved salts (NaCl) and glycosaminoglycans. The density of the vitreous stroma fibers increases toward the periphery of the vitreous body, where they form the hyaloid membrane (membrana vitrea) on its surface. The anterior surface of the vitreous body features the hyaloid fossa (fossa hyaloidea), which accommodates the lens.

The anterior chamber of the eyeball (camera anterior bulbi) is situated between the posterior surface of the cornea and the anterior surface of the iris. Along its circumference, where the margins of the cornea and iris meet, the chamber is bounded by the pectinate ligament, among the fiber bundles of which lie the spaces (spaces of Fontana) of the iridocorneal angle (angulus iridocornealis). Through these spaces, the aqueous humor drains from the anterior chamber into the venous sinus (sinus venosus) (canal of Schlemm), and subsequently into the anterior ciliary Veins. Obstruction of this aqueous outflow pathway leads to elevated intraocular pressure (glaucoma). The anterior chamber communicates with the posterior chamber via the pupil.

The posterior chamber of the eyeball (camera posterior bulbi) is narrower than the anterior one. It is located posterior to the iris, bounded posteriorly by the lens and laterally by the ciliary body. The posterior chamber also encompasses the spaces between the fibers of the suspensory apparatus. The zonular spaces (spatia zonularia) form a circular cleft (canal of Petit) situated along the periphery of the lens. Both chambers of the eye are filled with aqueous humor.

The aqueous humor (humor aquosus) is a transparent fluid with a refractive index of 1.336. It consists of 98% water, containing primarily sodium chloride and Glycoproteins. The aqueous humor is secreted by the epithelium of the ciliary processes and folds, with the involvement of blood capillaries embedded within them. Its total volume reaches 0.2—0.3 ml. The outflow of the fluid occurs partly from the posterior chamber through the canal of Petit into the vitreous body, while the major portion passes through the pupil into the anterior chamber, from which it is absorbed into the venous bloodstream via the venous sinus (canal of Schlemm). Through the continuous Circulation of the aqueous humor, a balance is maintained between its secretion and absorption, which acts as the primary factor in stabilizing intraocular pressure. In addition to its light-refracting function, it also serves as a nutritive medium for both the lens and the cornea.