Place in the visual analyzer
The visual organ (organum visus) is a complex system—an analyzer. To understand its function, it is important to divide its structure into three functional parts:
- Peripheral part is the receptor apparatus itself, primarily formed by the eyeball. It is supported by accessory structures: eyelids, conjunctiva, lacrimal apparatus, and extraocular muscles.
- Conduction part includes the optic nerve and pathways through which impulses travel to the brain.
- Central part comprises the cortical visual centers where final image processing takes place.
The eyeball itself is a sphere whose walls are formed by three layers (tunics), and whose internal space is filled with specialized light-refracting media.
Outer fibrous tunic
The outer layer is called the fibrous tunic (tunica fibrosa bulbi). It acts as a sturdy framework, maintaining the shape of the eye and protecting its delicate internal structures. It consists of two parts:
- Sclera (sclera) is a dense, opaque tissue formed of connective fibers. It comprises the greater part 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) is the anterior, completely transparent part of the outer tunic. It is entirely devoid of blood vessels, yet it is extremely rich in nerve endings. The structure of the cornea includes several layers: the proper substance (substantia propria corneae), anterior and posterior limiting laminae, and the outer epithelium.
The transition zone where the dense, opaque sclera meets the transparent cornea is called the 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 a high amount of pigment. The main functions of this layer are the nutrition (trophism) of the eyeball and the regulation of the amount of entering light. Anatomically, it is divided into three parts:
- Choroid proper (choroidea). Closely adheres to the inner surface of the sclera. It contains a dense vascular network, which is particularly well-developed in the region contacting the retina.
- Ciliary body (corpus ciliare). Occupies an intermediate position between the choroid and the iris. Structurally, it comprises the ciliary ring (orbiculus ciliaris) and the ciliary crown (corona ciliaris), which bears ciliary folds (plicae ciliares). The ciliary body is connected to the lens by the zonular fibers (fibrae zonulares).
- Iris (iris). The most anterior part of the vascular tract. There is an aperture in the center of the iris called the pupil (pupilla). The stroma of the iris (stroma iridis) contains, in addition to vascular and connective tissue components, two smooth muscles that control pupil size: the sphincter (m. sphincter pupillae), which constricts it, 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 the inside and has a complex multi-layered structure. It comprises a photoreceptor layer, nuclear layers, and neurofibrillar layers.
Topographically, the retina is divided into two zones:
- Optic part (pars optica retinae) contains specialized cells (rods and cones) that perceive light.
- Blind part includes the ciliary part (pars ciliaris retinae) and the iridial part (pars iridica retinae), which lack light-sensitive elements.
The optic part of the retina features several clinically important landmarks:
- Macula lutea (macula lutea) is the area responsible for highest visual acuity. At its center lies the fovea centralis (fovea centralis).
- Optic disc (discus nervi optici) is the site where nerve fibers converge and leave the eyeball. It contains neither rods nor cones, thereby forming the physiological "blind spot".
Briefly: answers to topical questions
What anatomical structures constitute the light-refracting media of the eyeball?
The light-refracting (dioptric) media of the eye ensure the conduction and focusing of light on its path to the retina. These transparent structures and media include:
- Cornea (cornea), the anterior transparent part of the fibrous tunic, which possesses the highest refractive power.
- Aqueous humor of the eye chambers (humor aquosus), the fluid filling the anterior and posterior chambers of the eyeball.
- Lens (lens), a transparent biconvex body with high refractive power and the ability to change its curvature.
- Vitreous body (corpus vitreum), a transparent gel-like mass filling the cavity of the eyeball between the lens and the retina.
What layers are distinguished in the histological structure of the cornea?
The histological structure of the cornea comprises five main layers arranged sequentially:
- Anterior epithelium (epithelium anterius), a stratified squamous non-keratinizing epithelium of ectodermal origin.
- Anterior limiting membrane (membrana limitans anterior, Bowman's layer), a thickened basement membrane beneath the anterior epithelium.
- Stroma of cornea (substantia propria corneae), the bulk of the cornea formed by dense fibrous connective tissue with parallel collagen lamellae.
- Posterior limiting membrane (membrana limitans posterior, Descemet's membrane), the membrane underlying the posterior epithelium.
- Posterior epithelium (epithelium posterius, endothelium), a simple squamous epithelium lining the inner surface of the cornea.
What structures are included in the anatomical composition of the ciliary body?
The ciliary body (corpus ciliare) is the anterior thickened part of the vascular tunic and includes the following main structures:
- Ciliary crown (corona ciliaris) with ciliary folds and processes (plicae ciliares, processus ciliares).
- Ciliary ring (orbiculus ciliaris), the posterior segment of the ciliary body continuous with the choroid proper.
- Ciliary muscle (musculus ciliaris), a smooth muscle structure with meridional and circular bundles.
- Connected to the ciliary body are the zonular fibers (fibrae zonulares), which link it to the lens.
What smooth muscles are located in the iris and how are they innervated?
The iris (iris) contains two smooth muscles of neuroglial origin that regulate pupil diameter:
- Sphincter pupillae muscle (musculus sphincter pupillae), a circular muscle innervated by parasympathetic fibers within the oculomotor nerve (n. oculomotorius); pupil constriction is accompanied by miosis.
- Dilator pupillae muscle (musculus dilatator pupillae), a radial muscle innervated by sympathetic fibers, whose contraction leads to pupil dilation (mydriasis).
Which blood vessels supply blood to the retina?
Blood supply to the retina is provided by branches of the ophthalmic artery (a. ophthalmica). The main source is:
- Central retinal artery (a. centralis retinae), which enters the optic nerve 1 cm prior to the eyeball, penetrates the optic disc region, and divides into superior and inferior branches that branch into retinal arterioles to nourish its inner layers.
What layers of cells and fibers are distinguished in the optic part of the retina?
The optic part of the retina (pars optica retinae) comprises ten layers arranged from outside to inside:
- Layer of pigment epithelium (stratum pigmentosum), the outermost layer bordering the vascular tunic.
- Layer of rods and cones — dendrites of photoreceptor neurons.
- Outer limiting membrane (membrana limitans externa) — formed by the processes of Müller cells.
- Outer nuclear layer — cell bodies of photoreceptor cells.
- Outer plexiform layer — area of contact between photoreceptor axons and processes of local association neurons.
- Inner nuclear layer — cell bodies of local association neurons and gliocytes.
- Inner plexiform layer — area of contact between local association neurons and with ganglion neurons.
- Ganglion cell layer — cell bodies of ganglion neurons.
- Nerve fiber layer — axons of ganglion cells running toward the blind spot.
- Inner limiting membrane (membrana limitans interna) — formed by the processes of Müller cells.
To remember the tunics, use the mnemonic «FiSoSet»: Fibrous (outer), Vascular (middle), Retina (inner).
Class and exam questions
Why is the optic disc called the blind spot?
In the region of the optic disc (discus nervi optici), where nerve fibers exit the eye, the retina completely lacks photoreceptors (rods and cones); therefore, this area does not perceive light.
How does the cornea differ from the sclera?
Unlike the dense and opaque sclera, the cornea is transparent, blood vessel-free, and contains a vast number of nerve endings.
What muscles are located in the iris and what is their function?
The iris contains two muscles: m. sphincter pupillae, which is responsible for pupil constriction, and m. dilatator pupillae, which provides its dilation.
What else to study on this topic
- Accessory organs of the eye (eyelids, conjunctiva, lacrimal apparatus)
- Light-refracting media of the eyeball
- Histological structure of the corneal layers
- Conduction pathway: optic nerve and visual pathways
- Extraocular muscles and their innervation
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