Role in the Visual System
The visual organ (organum visus) is a complex system—an analyzer. To understand its function, it is important to divide the structure into three functional parts:
- Peripheral part — the receptor apparatus itself, centered on the eyeball. It is supported by accessory visual structures: eyelids, conjunctiva, lacrimal apparatus, and extraocular muscles.
- Conduction pathway — includes the optic nerve and tracts that transmit impulses to the brain.
- Central part — the cortical visual centers where final image processing occurs.
The eyeball itself is a sphere whose walls are formed by three layers (tunics), while the internal cavity is filled with specialized refractive media.
Outer Fibrous Tunic
The outer layer is called the fibrous tunic (tunica fibrosa bulbi). It serves as a sturdy structural framework, maintaining the shape of the eye and protecting its delicate internal structures. It consists of two regions:
- Sclera (sclera) — a dense, opaque tissue formed of connective fibers, making up the majority of the fibrous layer. At the posterior pole of the eye, where the optic nerve exits, the sclera thins out. This perforated area is known as the lamina cribrosa of the sclera (lamina cribrosa sclerae).
- Cornea (cornea) — the anterior, completely transparent part of the outer tunic. It is entirely devoid of blood vessels, though it is exceptionally rich in nerve endings. The corneal structure includes several layers: the proper substance (substantia propria corneae), anterior and posterior limiting laminae, and the outer epithelium.
The transitional zone where the dense, opaque sclera meets the transparent cornea is called the corneal limbus (limbus corneae).
Middle Vascular Tunic
Beneath the fibrous layer lies the vascular tunic (tunica vasculosa bulbi). It is richly supplied with blood vessels and contains abundant pigment. The primary functions of this layer are nourishment (trophics) of the eyeball and regulation of the amount of light entering the eye. Anatomically, it is divided into three parts:
- Choroid (choroidea). It closely apposes the inner surface of the sclera, featuring a dense vascular network that is especially pronounced in the zone contacting the retina.
- Ciliary body (corpus ciliare). Occupies an intermediate position between the choroid and the iris. Structurally, it is divided into the ciliary ring (orbiculus ciliaris) and the ciliary crown (corona ciliaris), which bears ciliary folds (plicae ciliares). The ciliary body connects to the lens via zonular fibers (fibrae zonulares).
- Iris (iris). The most anterior part of the vascular tract. In the center of the iris is an aperture called the pupil (pupilla). Within the stroma of the iris (stroma iridis), aside from vascular and connective tissue components, lie two smooth muscles that control pupil size: the sphincter (m. sphincter pupillae), which constricts the pupil, and the dilator (m. dilatator pupillae), which dilates it.
Inner Sensory Tunic
The innermost tunic is the retina (tunica interna, or retina). It lines the vascular tunic from within and features a complex multilayered structure comprising a photoreceptor layer, nuclear layers, and neurofibrillary layers.
Topographically, the retina is divided into two zones:
- Optic part (pars optica retinae) — contains specialized cells (rods and cones) that perceive light.
- Non-visual (blind) part — includes the ciliary (pars ciliaris retinae) and iridial (pars iridica retinae) parts, which lack light-sensitive elements.
Within the optic part of the retina, several clinically important landmarks exist:
- Macula (macula lutea) — the region providing the sharpest vision, containing the central fovea (fovea centralis) at its center.
- Optic disc (discus nervi optici) — the site where nerve fibers converge and leave the eyeball. It contains neither rods nor cones, thereby forming the physiological "blind spot".