Human Anatomy, Part 2 - K. A. Dyubenko, A. K. Kolomiysev, Yu. B. Chaykovsky 2008

Special Part
Sense Organs, organa sensuum
Organ of Vision

The Organ of Vision, organum visus, or the eye, oculus*, consists of the Eyeball and Accessory structures of the eye (ocular Muscles AND FASCIAE, eyelids with eyelashes, conjunctiva, Lacrimal Apparatus, Blood Vessels, and nerves).

* Eye — (Greek ophtalmos, hence ophthalmology).

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Fig. 212. The eyeball, bulbus oculi (in horizontal section)

The eyeball, bulbus oculi, (Fig. 212) is shaped like an irregular sphere (with the anterior convexity slightly greater). The anterior and posterior poles, polus anterior et posterior, of the eyeball are distinguished. The line connecting them is called the external axis of the eyeball, axis bulbi externus. Its length averages 24 mm. The segment of this axis between the posterior surface of the cornea and the retina is called the internal axis of the eyeball, axis bulbi internus, and is 21.3 mm long. In nearsighted (myopic) individuals, this axis is longer—light rays are focused in front of the retina, whereas in farsighted (hyperopic) individuals, the opposite is true. In addition, the eyeball features the optical axis, axis opticus, which extends from the viewed object through the nodal point to the fovea centralis of the retina. The nodal point lies in the center of the crystalline lens, where the line of sight intersects with the optical axis.

The eyeball also has an equator, equator, which corresponds to an imaginary line drawn in the frontal plane at the level of the greatest transverse diameter of the eyeball; and meridians, meridiani, which are imaginary lines drawn from the anterior pole to the posterior pole across The surface of the eyeball, perpendicular to its equator.

The eyeball consists of a core and a capsule. The core is divided into the lens, aqueous humor, and vitreous body. The capsule consists of the outer, middle, and inner tunics (layers) (Figs. 213, 214 A, B).

Outer Tunic

The outer fibrous tunic, tunica fibrosa bulbi, comprises two parts. The anterior transparent part is smaller and is called the cornea, cornea. The posterior opaque part constitutes at least 4/5 of the entire fibrous tunic and is called the sclera, sclera (Figs. 213, 214).

At the junction of the cornea and the sclera, due to discrepancies in their curvature, thickness, and the partial overlapping of the sclera onto the cornea, a distinctive shallow groove is formed—the scleral sulcus, sulcus sclerae.

The cornea is a convex plate that participates in the transmission of light rays and performs a protective function. Its thickness is 0.7 mm at the periphery and 0.5 mm at the center; the horizontal diameter is 11–12 mm, and the vertical diameter is 10–11 mm. Optically speaking, the cornea acts as a unique lens, accounting for about 70% of the total refractive power of the dioptric apparatus.

The transparency of the cornea is due to the Specific features of its histological Structure. Nutrition of the cornea, which lacks its own blood vessels, is provided by diffusion from the Vessels of the sclera and from the aqueous humor.

The sclera is formed by Cytology/practical/45.html">Dense Connective Tissue of a white color resembling the boiled white of a chicken egg, hence its name (Fig. 214 A). The sclera contains numerous Collagen and elastic fibers and little ground substance. In the anterior part of the sclera, bundles of collagen fibers run parallel to the equator; in the posterior part, they curve toward the posterior pole, and near the exit of the Optic nerve, they again become parallel to the equator. This difference in fiber topography must be considered during sclerotomies, as incisions running along the fibers diverge less and adapt more easily.

When suturing scleral incisions and injuries, its thickness must be taken into account. The latter reaches 1 mm in the region of the posterior pole, 0.4 mm near the equator, and 0.6 mm anteriorly. The thinnest part of the sclera is at the site of the optic nerve exit, where it forms the lamina cribrosa, through the openings of which the fibers of the optic nerve pass.

The sclera contains few blood vessels. Veins are located in its deep layers. At the border with the cornea, within the thickness of the sclera, lies the venous sinus of the sclera, sinus venosus sclerae, which projects onto the surface of the eye along the scleral sulcus.

Middle Tunic

The middle tunic—the vascular tunic, tunica vasculosa bulbi—is divided into three parts: 1) the posterior, largest part, adjacent to the inner side of the sclera—the choroid proper, choroidea; 2) the middle, thickened part—the ciliary body, corpus ciliare; 3) the anterior part—the iris, iris (Fig. 213).

The choroid proper, choroidea, is loosely connected to the sclera over most of its area, allowing it to easily shift during the act of accommodation. There is an opening in this tunic corresponding to the optic nerve disc. The outer surface of the choroid contains endothelial and pigment Cells, which, together with the inner surface of the sclera, bound the perichoroidal space, spatium perichoroideale.

Fig. 213. Sagittal section of the Orbit and eyeball

Fig. 214 A, B. Tunics of the eyeball

The choroidea consists of elastic fibers and A large number of Blood and Lymphatic vessels.

The ciliary body, corpus ciliare, is located in the transition zone from the sclera to the cornea (Fig. 213). In transverse section, it has a triangular shape. It comprises the ciliary Muscle, m. ciliaris, the ciliary crown, corona ciliaris, and the ciliary ring, orbiculus ciliaris. The ciliary muscle is formed by Smooth Muscle tissue. In its outer layers, myocytes are arranged predominantly meridionally, whereas in the inner layers, they are arranged partly radially and partly circularly. The ciliary ring is a thickening of the vascular tunic about 4 mm wide that surrounds the posterior surface of the iris. The inner surface of the ciliary ring is heavily pigmented and gathered into small meridional folds, plicae ciliares, which merge anteriorly to form 70–80 ciliary processes, processus ciliares. Together, these processes form the ciliary crown.

The iris has the shape of a frontally positioned disc with a circular opening in the center—the pupil, pupilla (Fig. 214 A, B). The iris features an anterior and posterior surface, facies anterior et posterior, a pupillary (free) margin, margo pupillaris, and a ciliary (attached to the corpus ciliare) margin, margo ciliaris. In newborns, the diameter of the iris is 3.3 mm and its thickness is 0.17 mm; in adults, these values are 12 mm and 0.5 mm, respectively.

Depending on The amount of pigment, the iris exhibits significant individual variations in color, ranging from pale blue to almost black. Albino eyes have a pinkish hue because their iris completely lacks pigment, allowing the rich underlying blood vessels to show through the anterior surface.

The connective tissue stroma of the iris contains smooth muscle fibers: closer to the pupillary margin lie circular bundles of the sphincter pupillae muscle (m. sphincter pupillae), while radially arranged bundles of the dilator pupillae muscle (m. dilator pupillae) are located in the periphery.

The angle where the ciliary margin of the iris meets the ciliary body and the fibrous tunic (angulus iridocornealis) is filled with the pectinate ligament (lig. pectinatum). It consists of trabeculae covered with flat epitheliocytes and separated by a system of lymphatic clefts known as the spaces of the iridocorneal angle (spatia anguli iridocornealis).

Internal Tunic

The internal (sensory) tunic—the retina (retina)—is divided into the optic part (pars optica), which contains highly differentiated neural elements, and the ciliary and iridial parts (partes ciliaris et iridica), where Nerve Cells are absent. The former directly adjoins the vascular tunic from the inside and matches it in surface area. It consists of ten layers that can be grouped into two main components: the outer pigmented layer (pars pigmentosa) and the inner neural layer (pars nervosa).

The retina should be regarded as the peripheral part of the visual analyzer, containing photoreceptor, conducting, and supporting elements (Fig. 215).

Fig. 215. Diagram of the microscopic STRUCTURE OF THE retina

Fig. 216. Fundus of the eye (posterior region of the retina) as seen during ophthalmoscopy

In the ciliary and iridial regions, the retina loses its ten-layered structure and is essentially represented by just two layers: an outer pigmented layer and an inner epithelial layer. The boundary between the optic and ciliary PARTS OF THE retina is clearly marked by the serrated margin (ora serrata), which lies anterior to the equator.

Particular attention should be paid to the optic disc (discus n. optici, Fig. 216). Located about 4 mm medial to the posterior pole, it appears as a pale pink, rounded elevation (1.5–2 mm in diameter) with a small depression at its apex known as the optic cup (excavatio disci). This corresponds to the entry point of the retinal blood vessels. At this site, known as the blind spot, light-sensitive elements are entirely absent. The axons of retinal ganglion cells converge within the disc and pierce the sclera to form the optic nerve.

About 3–4 mm lateral to the optic disc lies the macula (macula), an oval, red-yellow-brown area featuring a small central depression called the fovea centralis (fovea centralis). The fovea is approximately 2–3 mm in diameter and is surrounded by a zone of thickened retina. At its very center lies a deeper pit, the foveola (foveola), where the retina thins down to 0.5–0.1 mm. Cone neurosensory cells are densely concentrated here, providing the highest visual acuity of the retina. Visual acuity and color discrimination decrease toward the periphery of the macula.

It should be noted that the pigmented layer of the retina is more firmly attached to the vascular tunic than to the neural layer. Consequently, certain pathological conditions can cause a so-called retinal detachment—the Separation of the neural layer from the pigmented layer, leading to disrupted Blood supply to the former.

From a clinical perspective, a crucial part of the eyeball is the ocular fundus (fundus oculi). This is the inner surface of the eyeball wall visible during ophthalmoscopy, encompassing the optic disc, macula, fovea centralis, and the central retinal artery and vein. When examined with an ophthalmoscope in a living person, the retina appears bright red due to the light shining through the Blood vessels of the choroid. When retinal pigmentation is sparse and choroidal pigmentation is abundant, the fundus presents a mottled appearance because the underlying choroidal vessels show through in certain areas. This type of fundus is known as a tigroid or tessellated fundus (fundus tabulatus).

The ocular fundus undergoes changes in vascular pathologies, inflammatory states, blood disorders, and other diseases.

Core of the Eyeball

Aqueous humor is secreted by the ciliary processes, their covering epithelium, and the microvasculature of the ciliary body. It fills the Chambers of the eyeball. The anterior chamber (camera anterior) is a space (roughly shaped like a spherical segment) situated between the posterior surface of the cornea and the anterior surface of the iris, extending to the anterior surface of the lens in the pupillary region.

The posterior chamber (camera posterior) is bounded by the posterior surface of the iris, the equatorial region of the lens, and the inner surface of the ciliary body. The anterior and posterior chambers communicate freely through the opening between the lens and the iris.

The volume of aqueous humor in an adult eye is 0.20–0.35 ml, with 14–20% of the total volume located in the posterior chamber. The half-turnover time of aqueous humor is 45 minutes. It nourishes the avascular lens and cornea and also participates in the refraction of light rays. Aqueous humor drains through the lymphatic cleft system in the region of the iridocorneal angle (angulus iridocornealis) into the venous sinus of the sclera (canal of Schlemm) and further into the anterior ciliary veins.

The lens (lens) is a transparent, elastic biconvex body. It features anterior and posterior (more convex) surfaces (facies anterior et posterior) and anterior and posterior poles (polus anterior et posterior). The line marking the transition from the anterior to the posterior surface is called the equator of the lens (equator lentis), while the imaginary line connecting the poles is the axis of the lens (axis lentis).

The equatorial diameter is about 7.5 mm in a child under one year of age and 9–10 mm in an adult. The axial length does not exceed 2.5 mm in newborns, reaches 3.7 mm in adults with a relaxed ciliary muscle, and increases to 4.4–5 mm during accommodation.

The lens is shaped like a biconvex lens (Figs. 212, 213). It is composed of transparent fibers and epitheliocytes that form a central core, the lens Nucleus (nucleus lentis), and a peripheral zone, the lens cortex (cortex lentis).

The lens is enclosed in a transparent, elastic capsule (capsula lentis). Numerous fine fibers originate from the equatorial region of the capsule, connecting the lens to the inner posterior surface of the ciliary body. These fibers form the suspensory ligament of the lens, known as the ciliary zonule (zonula ciliaris). Clefts communicating with the posterior chamber of the eye—the zonular spaces (spatia zonularia)—form between its anterior and posterior fibers. Due to its elasticity, the lens readily changes its curvature, enabling near vision (accommodation).

Nutrition of the avascular lens is supplied via the diffusion of nutrients from the aqueous humor. When The chemical composition of the aqueous humor alters, the METABOLISM of the lens is disrupted, which can lead to a loss of transparency (cataract). In recent decades, the standard Surgical Treatment for cataracts has involved removing the opaque lens and replacing it with an artificial intraocular lens. In Ukraine, one of the pioneers to introduce this technique into widespread clinical practice was corresponding member of the AMS of Ukraine M. M. Serhiienko.

The vitreous body, corpus vitreum, consists of a transparent jelly-like substance containing about 98% Water. It is spherical in shape, with the greater part of its surface adjacent to the retina. In the anterior part, There is a depression that accommodates the lens. The total volume of the vitreous body in an adult is 3.9 ml.