Parts of the Visual Analyzer
The visual analyzer is morphologically and functionally divided into several levels:
- Peripheral part: includes retinal photoreceptors, the optic nerve (n. opticus), and the optic tract.
- Central part: represented by subcortical and brainstem centers (lateral geniculate body, pulvinar of the thalamus, superior colliculus), as well as the optic radiation. The highest center is the visual cortex (areas 17, 18, and 19), where final information processing takes place.
These structures interact closely with other parts of the brain, forming the basis for complex visual behavior.
Geometry and Tunnics of the Eyeball
The eyeball is spherical, and specific landmarks are used to describe it. The anterior pole is located at the center of the cornea, and the posterior pole is at the center of the fovea centralis (macula lutea). The line passing through these points is called the visual (principal) axis. The widest part in the frontal plane is the equator, and the lines passing through the poles are called meridians.
The wall of the eye is formed by three layers:
- Outer (fibrous) layer: consists of the cornea and sclera.
- Middle (vascular) layer (uvea): includes the iris, ciliary body, and choroid.
- Inner layer: represented by the retina containing the receptor apparatus.
Light-Refracting Structures
The internal media of the eye that provide light refraction include the aqueous humor, lens, and vitreous body.
Cornea (Cornea) — the anterior transparent part of the fibrous tunic. It lacks blood vessels (obtaining nutrition from the marginal capillary net), which ensures its transparency, and possesses high refractive power (2.5 times higher than that of the lens). It is innervated by long ciliary nerves.
Lens (Lens) — a biconvex lens, the main element of the accommodation apparatus. Located behind the iris, it has an equatorial diameter of 9–10 mm and a sagittal thickness of 3.7–5 mm. Its radius of curvature changes during accommodation.
Iris and Ciliary Body
Iris (Iris) functions as an optical diaphragm, regulating light entry through an opening called the pupil. It contains two smooth muscles:
- Sphincter pupillae — constricts the pupil (circular fibers).
- Dilator pupillae — dilates the pupil (radial elements including myoepithelial cells).
The pupillary response depends on illumination and object distance, while the color of the iris is determined by pigment volume.
Ciliary body (Corpus ciliare) — a thickened part of the vascular tunic, consisting of an outer muscular layer and an inner vascular layer. It is involved in accommodation.