Human Anatomy - Kotsan I. Y. 2009

Sense Organs
Organ of Vision (Eye)
Tunics of the Eyeball

I. The fibrous tunic of the Eyeball (tunica fibrosa bulbi) serves a protective function. Its anterior part is transparent and is known as the cornea, while its larger posterior part is white in color (resembling the appearance of boiled egg white) and is called the tunica albuginea or sclera. The boundary between the cornea and the sclera is marked by a shallow circular groove, the scleral sulcus (sulcus sclerae).

1. The cornea (cornea) is a transparent plate, convex anteriorly and concave posteriorly, fitted by its edge (the limbus) like a watch Glass into the concave anterior part of the sclera. The diameter of the cornea is 12 mm, and its thickness is about 1 mm (1.2 mm at the edge and 0.9 mm at the center). The cornea consists of five layers: the anterior epithelium, the anterior limiting lamina, the proper substance (substantia propria of the cornea), the posterior limiting lamina, and the posterior epithelium (corneal endothelium). The cornea contains no Blood Vessels, is rich in nerve endings, and is nourished by the diffusion of essential substances from the fluid of the anterior chamber of THE EYE AND the Vessels of the fibrous tunic adjacent to the corneal margin.

2. The sclera (sclera) is a direct continuation of the cornea and occupies 5/6 of The surface of the eyeball. The thickness of the sclera ranges from 1.0 mm (in the posterior part) to 0.5 mm (in the anterior part). It is built of Cytology/practical/45.html">Dense Connective Tissue formed by bundles of Collagen fibers, between which lie flattened fibroblasts with a small number of elastic fibers. Externally, in its anterior part, the sclera is covered by the conjunctiva, while internally it is entirely lined with endothelium. The area of the sclera through which the bundles of Optic nerve fibers pass is called the lamina cribrosa sclerae. At the border with the cornea, within the thickness of the sclera, lies a narrow circular canal filled with venous blood—the venous sinus of the sclera (sinus venosus sclerae), or Canal of Schlemm.

II. The vascular tunic of the eyeball (tunica vasculosa bulbi) is located immediately behind the fibrous tunic. It contains A large number of blood vessels and pigment Cells, which give it a dark color. The vascular tunic comprises three parts: the choroid proper, the ciliary body, and the iris.

1. The choroid proper (choroidea) constitutes the largest posterior section of the vascular tunic, situated beneath the sclera. The basis of the choroid proper is formed by the vascular lamina (lamina vasculosa)—a dense network of interwoven Arteries and Veins, interspersed with loose Fibrous connective tissue containing large pigment cells. The vascular lamina is covered by the suprachoroid lamina (lamina suprachoroidea), which is formed by loose fibrous connective tissue dominated by elastic fibers and containing a large number of pigment cells. Between the suprachoroid lamina and the sclera lies the perichoroid space (spatium perichoroideale)—a complex of canal-like cavities communicating with each other and with Lymphatic vessels. The presence of the perichoroid space allows a certain mobility of the vascular tunic relative to the sclera. Beneath the vascular lamina lies the choriocapillary lamina (lamina choroidocapillaris), which is constructed of a connective tissue layer devoid of pigment cells but rich in capillaries.

2. The ciliary body (corpus ciliare) is the anterior, thickened part of the vascular tunic, located as a circular ridge in the region where the sclera transitions into the cornea. The ciliary body consists of the ciliary ring and the ciliary crown.

The ciliary ring (orbiculus ciliaris) is the posterior part of the ciliary body, appearing as a thickened band (4 mm wide) that transitions posteriorly into the choroid proper. The ciliary body contains the ciliary Muscle (m. ciliaris), which is composed of intricately interwoven smooth muscle cells; their contraction brings about accommodation—the adjustment of the eye for clear Vision of objects located at various distances. The ciliary muscle comprises meridional, circular, and radial bundles of smooth muscle cells. The meridional (longitudinal) fibers (fibrae meridionales) run parallel to the meridians of the eyeball, originate from the edge of the posterior limiting lamina of the cornea, and weave into the anterior part of the choroid proper. When they contract, this tunic shifts anteriorly, resulting in a reduced tension of the ciliary zonule (zonula ciliaris) that suspends the lens; the lens capsule relaxes, the lens changes its curvature, becoming more convex, and its refractive power increases. The circular fibers (fibrae circulares), located deeper than the preceding fibers, also originate from the posterior limiting lamina of the cornea and run in a circular direction. Upon contraction, they narrow the ciliary body, bringing it closer to the lens, which likewise promotes relaxation of the lens capsule. The radial fibers (fibrae radiales) are situated radially among the preceding fibers. They originate from the posterior limiting lamina of the cornea and the inner surface of the sclera in the region of their margins and interweave between the meridional and circular fibers, drawing them closer together upon contraction. Elastic fibers located among the aforementioned muscle cells facilitate the expansion of the ciliary muscle when it relaxes.

The ciliary crown (corona ciliaris) is formed by 70–80 ciliary processes. The ciliary processes (processus ciliares) are radially arranged folds of the ciliary body, 3 mm long, 0.1–0.2 mm wide, and up to 1 mm high, containing a dense capillary plexus. The epithelium covering the ciliary processes produces aqueous humor.

3. The iris (iris) appears as a round, vertically positioned plate (about 0.4 mm thick and 10–12 mm in diameter) with a round aperture in the center called the pupil (pupilla). The pupil diameter is variable; it constricts under bright light and dilates in the dark, acting as the Diaphragm of the eyeball (regulating The amount of light entering the eye).

The iris has two margins—pupillary and ciliary. The pupillary margin (margo pupillaris) is free and bounds the pupillary aperture; the ciliary margin (margo ciliaris) is connected to the ciliary body and sclera by means of the pectinate ligament.

The anterior surface (facies anterior) of the iris faces the anterior chamber of the eye and the cornea. On the anterior surface of the iris, low radial folds (plicae iridis) are distinguished, as well as two rings: the outer—major, and the inner—minor.

The posterior surface (facies posterior) of the iris faces the posterior chamber of the eye and the lens. This surface is covered by the pigmented epithelium (epithelium pigmentosum), which consists of two layers of pigment cells (melanocytes). The color of the eye's iris depends on the amount of pigment in the melanocytes. If pigment is abundant, the eyes are brown to black, and conversely, if pigment is sparse, the eyes appear mixed— greenish-gray or blue. In the absence of pigment (in albinos), the iris has a reddish hue due to the translucent blood vessels.

The stroma of the iris (stroma iridis) is composed of Connective tissue with a lattice architecture, into which blood vessels running radially from the periphery to the pupil are embedded. These vessels, which are the sole carriers of elastic elements (since the connective tissue of the stroma lacks elastic fibers), together with the connective tissue form the elastic Skeleton of the iris, enabling it to easily change in size. Within the thickness of the stroma lies a network of smooth muscle cells forming two Muscles: the sphincter pupillae and the dilator pupillae. The sphincter pupillae (m. sphincter pupillae) consists of circularly arranged bundles of smooth muscle cells, whereas the dilator pupillae (m. dilatator pupillae) consists of a group of cells oriented radially (from the ciliary margin of the iris to its pupillary margin). Both muscles are interconnected and influence one another: the sphincter stretches the dilator, and the dilator straightens the sphincter. Consequently, each muscle returns to its initial position, thereby ensuring the speed of the iris movements. The sphincter pupillae is innervated by parasympathetic fibers of the Oculomotor nerve, while the dilator pupillae is innervated by Branches of the internal carotid plexus belonging to the sympathetic Nervous system.

III. The internal tunic of the eyeball (tunica interna bulbi), or retina (retina), adheres internally to the vascular tunic throughout its entire extent, from the exit site of the optic nerve to the margin of the pupil. Unlike the other tunics of the eyeball, the retina develops not from mesenchyme but from ectoderm (from the two layers of the optic cup). According to its origin, it consists of two parts: the outer pigmented part (pars pigmentosa), made up of pigment cells, and the inner nervous part (pars nervosa), composed of photoreceptor and Nerve Cells. The pigmented part is more firmly bound to the vascular tunic than to the nervous layer of the retina; in histological preparations, it remains attached to the vascular tunic, detaching from the retina. Functionally and structurally, the nervous part is further divided into two sections: a smaller anterior one and a larger posterior one. The anterior part contains no light-sensitive elements and is therefore often referred to as the "blind part" (pars caeca) of the retina. It occupies the anterior Regions of the internal tunic, lining the iris and ciliary PARTS OF THE vascular tunic from the inside. The posterior part lines the choroid proper, contains light-sensitive receptors (rods and cones), and is thus termed the optic part of the retina (pars optica retinae). It is almost entirely transparent and becomes turbid only post-mortem. The boundary between the anterior (blind) and posterior (optic) parts of the internal tunic is marked by the ora serrata, which runs at the level where the choroid proper transitions into the ciliary ring of the ciliary body.

Using an ophthalmoscope, one can observe the internal surface of the living human retina—the Fundus of the eye. The fundus has a red color, which is explained by the translucency of the vascular tunic capillaries through the thin internal tunic. A whitish spot (1.6–1.7 mm in diameter) can be observed on the fundus—the optic disc (discus nervi optici), with raised ridge-like margins and a small central depression known as the optic cup (excavatio disci). The disc is the site where the fibers of the optic nerve exit the eyeball; the central retinal artery and vein pass through the optic cup. Due to the absence of light-sensitive elements, the disc area is called the blind spot. Lateral to the optic disc (by approximately 4 mm) lies the macula (macula), which is the site of highest visual acuity. It has a round or oval shape with a small central depression—the fovea centralis (fovea centralis), 1–2 mm in diameter, where all layers of the neural retina are thinned. In the middle of the central pit lies an even smaller depression (about 0.3 mm in diameter), the foveola (foveola), which serves as the site of maximal visual acuity.

The retina is a part of the Brain that separated from it during early Selection/3.html">Stages of development, yet remains connected to it by a bundle of fibers—the optic nerve. Like many other structures of the Central Nervous System, the retina has the form of a sheet about 1/4 mm thick.

Morphologically, the retina comprises 10 layers: 1) the retinal pigment epithelium, adjacent to the inner surface of the vascular tunic; 2) the layer of neuroepithelial receptors—rods (photoreceptive) and cones (color-receptive elements); 3) the outer limiting lamina; 4) the outer nuclear layer, formed by the portions of the rods and cones containing the nuclei; 5) the outer plexiform layer; 6) the inner nuclear layer; 7) the inner plexiform layer; 8) the ganglion Cell layer containing multipolar Neurons; 9) the optic nerve fiber layer; 10) the inner limiting lamina, adjacent to the vitreous body.

From a functional standpoint, the principal pathway is a radially oriented three-neuron chain consisting of the outer photoreceptive, middle associative, and inner ganglionic neurons.

Photoreceptive visual cells (first-order neurons), whose peripheral ends resemble rods and cones, are located in the outer nuclear layer. Each rod consists of an outer and an inner segment. The outer segment is light-sensitive and is formed by double membrane discs, which represent folds of The Plasma Membrane. Visual purple—rhodopsin, located in the membranes of the outer segment—undergoes changes under the action of light, leading to the generation of an impulse. The inner segment contains numerous Mitochondria, Ribosomes, elements of The Endoplasmic reticulum, and the Golgi apparatus. Cones differ from rods in their larger size and The Structure of their discs. In the outer segment of cones, invaginations of the plasma membrane form half-discs that maintain connection with the membrane. Three types of cones are distinguished, each sensitive to light of a specific wavelength. Unlike rods, the outer segment of one cone type contains iodopsin, which is sensitive to red light. The number of cones in the human retina reaches 6–7 million, while the number of rods is 10–20 times greater. The proportions of rods and cones vary markedly across different regions of the retina. Cones are present throughout the retina but are most concentrated in the fovea centralis, where our visual acuity for fine details is maximal. Rods detect dim light operating in the dark and shut down under bright illumination. Cones contain no visual purple, do not respond to dim light, and are responsible for perceiving fine details and Color Vision. Color vision is explained by the presence of three types of cones in the retina: some are excited by red light, others by green, and still others by blue. Sensations of other colors arise from the combined excitation of these cones in varying proportions.

Associative (bipolar) neurons (second-order neurons) are located in the inner nuclear layer of the retina. They transmit excitation from photoreceptive cells to large ganglion neurons (third-order neurons) residing in the ganglion cell layer of the retina. The axons of the ganglion cells are long and directed toward a single site on the retina, where they exit together, piercing the eye tunics to form the optic nerve.