Anatomical components: tunics of the eye
The eyeball is a complex spherical structure whose wall is formed by three concentric layers arranged from outside to inside.
The outermost layer is the fibrous tunic (Tunica fibrosa). It performs protective and supporting functions. Its posterior, main part is represented by the opaque white sclera (Sclera), the thickness of which is 0.3–0.6 mm. Anteriorly, the sclera passes into the thickened (0.9 mm), completely transparent cornea (Cornea), which resembles a convex watch glass. The transition zone from the cornea to the 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), richly supplied with blood vessels, and contains many pigment cells. The pigment absorbs excess light, preventing its reflection from the walls, which eliminates glare and increases contrast. The vascular tract is divided into three zones:
- Choroid (Chorioidea) — the largest posterior section. It can move relative to the sclera due to a slit-like lymphatic space (this is important during 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 part in the form of a disk with an aperture (pupil) in the center. It contains muscles: the sphincter (narrows the pupil) and the dilator (widens it).
The innermost is the retina (Retina). It consists of two layers. The outer pigmented layer continues the function of light absorption. The inner layer includes glial elements, photoreceptors, and neuronal chains. The axons of ganglion neurons gather on the internal surface at a single point — the blind spot (optic disc), pierce the tunics, and form the optic nerve. In the region of the blind spot, photoreceptors are absent.
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 is located between the cornea and the iris, the posterior chamber is behind the iris (bounded by the iris, ciliary body, and lens). Both chambers are filled with fluid and communicate with each other through the pupil.
- Lens. A transparent biconvex lens held in place on the ciliary body by the zonule of Zinn. The lens has high elasticity, which allows it to change its curvature to converge light rays. In addition, it acts as an optical filter: it completely absorbs UV rays and partially blocks the blue spectrum (which is why it has a yellowish tint).
- Vitreous body. A transparent jelly-like mass occupying the largest cavity of the eye between the lens and the retina.
Functional apparatuses
All the structures discussed (with the exception of the sclera and choroid) are functionally combined into three coordinated systems:
- Dioptric (light-refracting) apparatus. Conducts and refracts light. The light ray passes sequentially: cornea $\rightarrow$ aqueous humor $\rightarrow$ lens $\rightarrow$ vitreous body. The greatest refractive power is possessed by the cornea and the lens.
- Accommodation apparatus. Responsible for image focusing and light diaphragming. Anatomically provided by the ciliary body and the iris.
- Receptor apparatus. Represented by the retina, which directly perceives light stimulation.
Mechanism of accommodation and function of the iris
The eye is capable of clearly seeing objects at different distances due to changes in the curvature of the lens — accommodation.
- When looking into the distance (rays travel in parallel), the ciliary muscle is relaxed. The diameter of the muscular ring is maximal, the zonule of Zinn is tightly stretched, causing the lens to be mechanically flattened. The focus falls precisely on the retina.
- When looking at near objects (rays diverge), it is necessary to increase the refractive power of the eye. Parasympathetic innervation is triggered: the ciliary muscle contracts, and the diameter of its ring narrows. The zonule of Zinn relaxes, and the lens, due to its natural elasticity, becomes more convex.
The role of the light diaphragm is played by the iris. In bright light, the parasympathetic center is stimulated, the sphincter contracts, and the pupil narrows. In low light, the sympathetic nervous system is activated, the dilator contracts, and the pupillary aperture widens, letting in more light.
Briefly: answers to topical questions
What type of epithelium covers the anterior and posterior surfaces of the cornea?
The anterior surface of the cornea is covered by stratified squamous non-keratinizing epithelium, and the posterior surface is covered by simple squamous epithelium.
- Anterior epithelium — stratified squamous non-keratinizing epithelium of ectodermal origin, covering the external surface of the cornea.
- Posterior epithelium ( endothelium) — simple squamous epithelium of mesenchymal origin, covering the internal surface of the cornea.
What layers does the retina consist of?
The retina consists of two layers: the outer pigmented layer and the inner layer, the retina proper.
Ten layers are distinguished in the retina, arranged from outside to inside:
- Layer of pigmented epithelium
- Layer of rods and cones
- Outer limiting membrane
- Outer nuclear layer
- Outer plexiform layer
- Inner nuclear layer
- Inner plexiform layer
- Ganglion cell layer
- Nerve fiber layer
- Inner limiting membrane
What cells are part of the inner layer of the retina?
The inner layer (retina proper) contains photoreceptors, neurons, and glial cells.
It includes:
- Photoreceptor neurons (light-sensitive cells).
- Local associative neurons — bipolar, horizontal, and amacrine cells.
- Ganglion neurons, whose axons form the optic nerve.
- Glial cells — Müller cells (radial gliocytes), astroglia, and microglia.
What elements does the ciliary body consist of?
The ciliary body (Corpus ciliare) consists of an epithelial lining, stroma, and ciliary muscle.
- Epithelial lining (Pars ciliaris retinae) — a double-layered epithelium of glial origin (superficial secretory non-pigmented layer and deep pigmented layer).
- Stroma — loose fibrous connective tissue containing blood vessels and an abundance of pigment cells (melanocytes).
- Ciliary muscle (Musculus ciliaris) — bundles of smooth myocytes running in meridional and circular directions.
It also bears about 70 ciliary processes (processus ciliares).
To avoid confusing the mechanism of accommodation when viewing near objects, use the rule of three "N"s (in Russian): when looking Near (Vblizi), the muscle Contracts (Napryagaetsya), the ring narrows, and the tension of the ligament Conversely drops (l relaxes / rasslablyaetsya).
Class and exam questions
What do the accessory structures of the eye consist of?
It includes structures that ensure the movement and protection of the eyeball: extraocular muscles, eyelids, and the lacrimal apparatus.
Why does the lens have a yellowish tint?
This is due to its protective filtration function: the lens completely absorbs ultraviolet radiation and partially blocks blue spectrum rays.
What is the blind spot and where is it located?
This is the optic disc on the inner surface of the retina, the area where axons of ganglion cells converge and exit. Photoreceptors are completely absent in this zone.
What else to study on this topic
- Histological structure of the cornea and limbus
- Retinal layers: pathway of impulse transmission
- Microscopic structure of the ciliary body and mechanism of aqueous humor secretion
- Lens histology and age-related changes in its elasticity (presbyopia)
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