Anatomical Components: Tunics of the Eye
The eyeball is a complex spherical structure whose wall is formed by three concentric layers arranged from superficial to deep.
The outermost layer is the fibrous tunic (Tunica fibrosa). It performs protective and supportive functions. Its posterior, main part is represented by the opaque white sclera (Sclera), with a thickness of 0.3–0.6 mm. Anteriorly, the sclera transitions into the thickened (0.9 mm), completely transparent cornea (Cornea), which resembles a convex watch glass. The transition zone between the cornea and sclera is called the limbus (Limbus).
The middle layer is occupied by the vascular tunic (Tunica vasculosa). It is significantly thinner than the sclera (0.1–0.3 mm), highly vascularized, and contains many pigment cells. The pigment absorbs excess light, preventing reflection from the walls, which eliminates glare and increases image contrast. The vascular tract is divided into three zones:
- Choroid (Chorioidea): The largest posterior segment. It can shift relative to the sclera due to a slit-like lymphatic potential space (important for accommodation).
- Ciliary body (Corpus ciliare): A thickening at the level of the limbus. It contains muscular elements and ciliary processes, from which the suspensory ligament of the lens (zonule of Zinn) extends to the lens.
- Iris (Iris): The anterior disc-shaped part with an opening (pupil) in the center. It contains smooth muscles: the sphincter pupillae (constricts the pupil) and dilator pupillae (dilates the pupil).
The innermost layer is the retina (Retina). It consists of two layers. The outer pigmented layer continues the function of light absorption. The inner neural layer includes glial elements, photoreceptors, and neuronal circuits. The axons of ganglion cells converge on the inner surface at a single point—the blind spot (optic disc)—pierce the tunics, and form the optic nerve. Photoreceptors are absent in the region of the blind spot.
Internal Core of the Eyeball
The space enclosed by the tunics is filled with transparent light-refracting media that form the internal core of the eye:
- Chambers of the eye. The anterior chamber lies between the cornea and the iris; the posterior chamber lies behind the iris (bounded by the iris, ciliary body, and lens). Both chambers are filled with aqueous humor and communicate with each other through the pupil.
- Lens. A transparent biconvex lens held in place on the ciliary body by the zonular fibers (suspensory ligament). The lens has high elasticity, allowing it to change its curvature to focus light rays. Additionally, it acts as an optical filter: it completely absorbs UV rays and partially filters the blue spectrum (giving it a yellowish tint).
- Vitreous body. A transparent gel-like mass occupying the largest volume cavity of the eye between the lens and the retina.
Functional Apparatuses
All the structures discussed (except the sclera and choroid) are functionally combined into three coordinated systems:
- Dioptric (refractive) apparatus. Conducts and refracts light. Light passes sequentially: cornea $\rightarrow$ aqueous humor $\rightarrow$ lens $\rightarrow$ vitreous body. The cornea and lens possess the greatest refractive power.
- Accommodation apparatus. Responsible for image focusing and light diaphragming. Anatomically provided by the ciliary body and iris.
- Receptor apparatus. Represented by the retina, which directly perceives light stimuli.
Mechanism of Accommodation and Function of the Iris
The eye is capable of clearly seeing objects at various distances due to changes in the curvature of the lens—accommodation.
- When viewing distant objects (parallel light rays), the ciliary muscle is relaxed. The diameter of the muscular ring is maximal, the suspensory ligament is tightly stretched, which mechanically flattens the lens. The focal point falls precisely on the retina.
- When viewing near objects (diverging rays), the refractive power of the eye must increase. Parasympathetic innervation is triggered: the ciliary muscle contracts, narrowing the diameter of its ring. The suspensory ligament relaxes, and the lens, due to its natural elasticity, becomes more convex.
The iris acts as a light diaphragm. In bright light, the parasympathetic center is stimulated, the sphincter contracts, and the pupil constricts. In dim light, the sympathetic nervous system is activated, the dilator contracts, and the pupillary opening enlarges to let in more light.