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Lecture notes, cheat sheets
Eye diseases. Structure of the eye (part II) (lecture notes)
Directory / Lecture notes, cheat sheets Table of contents (expand) LECTURE No. 2. The structure of the eye (part II) 1. The structure of the eyeball The eyeball has an irregular spherical shape. Its anterior section is more convex. The anteroposterior size of the eye is on average 16 mm in a newborn, 19 mm by one year of life, 20 mm by three, 21 mm by seven, 22,5 mm by fifteen, and 23 mm by twenty years. The weight of the eyeball of a newborn is about 3,0 g, and that of an adult is 8,0 g. The eyeball has three shells: outer (represented by the cornea and sclera), middle (represented by the vascular tract) and inner (represented by the retina). Inside the eyeball are aqueous humor, lens, vitreous body, blood vessels. 2. Cornea and sclera The cornea is the anterior transparent part of the capsule of the eye. Its horizontal size in a newborn is 9,0 mm, by one year 10,0 mm, by three years 10,5 mm, by five years 11,0 mm, and by nine years it acquires the same dimensions as in adults, 11,5 .0,5 mm. The vertical dimension of the cornea is 78 mm smaller. The radius of curvature of the cornea is 1,12 mm. The thickness of this shell in the center in a child is 0,8 mm, in an adult 85 mm. The cornea contains up to XNUMX% water. The cornea normally has transparency, specularity, luster, sensitivity, sphericity. The cornea is the strongest refractive medium in the eye (60,0 diopters in newborns and 40,0 diopters in adults). The cornea is nourished by diffusion of nutrients from the marginal looped network and moisture from the anterior chamber. Sensitive innervation of the cornea is carried out by the trigeminal nerve, and trophic innervation is also due to the branches of the facial and sympathetic nerves. The sclera is a dense opaque fibrous membrane, occupies 5/6 of the entire outer shell of the eye and anteriorly passes into the transparent cornea, and the surface layer of the sclera passes into the transparent shell later than the middle and deep ones. Thus, at the transition point, a translucent limbus border is formed. In the posterior pole of the eye, the sclera becomes thinner and has a large number of holes through which the fibers of the optic nerve exit. This area of the sclera is called the lamina cribrosa and is one of its weak points. The plate under the influence of increased pressure can stretch, forming a deep excavation of the optic disc. Outside, the sclera is covered with episclera, which forms the inner wall of Tenon's space. All oculomotor muscles are attached to the sclera. It has openings for the blood vessels and nerves of the eye. In newborns and children of the first years of life, the sclera is thin, elastic, the choroid is visible through it, so the sclera has a bluish tint. With age, it becomes white, and by old age it turns yellow due to the degeneration of its tissue. Thin, elastic sclera in children of the first years of life under the influence of high intraocular pressure can stretch, which leads to an increase in the size of the eye (hydrophthalmos, buphthalmos). The outer shell is the main optical medium, it gives the eye a shape, maintains a constant volume, which is associated with the turgor of the eye, performs the function of protecting the thinner and more delicate inner shells of the eye. 3. Vascular tract of the eye The vascular tract, consisting of the iris, ciliary body and choroid, is located medially from the outer shell of the eye. It is separated from the latter by the suprachoroidal space, which is formed in the first months of a child's life. The iris (anterior part of the vascular tract) forms a vertically standing diaphragm with a hole in the center of the pupil that regulates the amount of light entering the retina. The vascular network of the iris is formed by branches of the posterior long and anterior ciliary arteries and has two circles of blood circulation. The iris can have different colors: from blue to black. Its color depends on the amount of melanin pigment it contains: the more pigment in the stroma, the darker the iris; in the absence or small amount of pigment, this shell has a blue or gray color. Children have little pigment in the iris, so in newborns and children of the first year of life it is bluish-grayish. The color of the iris is formed by the age of ten to twelve. On its anterior surface two parts can be distinguished: narrow, located near the pupil (the so-called pupillary), and wide, bordering the ciliary body (ciliary). The boundary between them is the pulmonary circulation of the iris. There are two muscles in the iris that are antagonists. One is placed in the pupillary area, its fibers are located concentrically with the pupil, and when they contract, the pupil narrows. Another muscle is represented by radially running muscle fibers in the ciliary part, with the contraction of which the pupil dilates. In infants, the muscle fibers that dilate the pupil are poorly developed; parasympathetic innervation predominates, so the pupil is narrow (22,5 mm), but dilates under the influence of mydriatics. By one to three years, the pupil acquires dimensions characteristic of adults (33,5 mm). The ciliary body consists of a flat and thickened coronal parts. The thickened coronal part consists of 70 to 80 ciliary processes, each of which has vessels and nerves. The ciliary, or accommodative, muscle is located in the ciliary body. The ciliary body is dark in color and covered with retinal pigment epithelium. Zinn ligaments of the lens are woven into it in the interprocesses. The ciliary body is involved in the formation of intraocular fluid that nourishes the avascular structures of the eye (cornea, lens, vitreous body), as well as in the outflow of this fluid. In newborns, the ciliary body is underdeveloped, the accommodative muscle is in a spastic state. The vessels of the ciliary body depart from the large arterial circle of the iris, which is formed from the posterior long and anterior ciliary arteries. Sensitive innervation is carried out by long ciliary fibers, motor parasympathetic fibers of the oculomotor nerve and sympathetic branches. The choroid, or the choroid itself, is composed mainly of short posterior ciliary vessels. In it, with age, the number of pigment cells of chromatophores increases, due to which the choroid forms a dark chamber that prevents the reflection of rays entering through the pupil. The basis of the choroid is a thin connective tissue stroma with elastic fibers. Due to the fact that the choriocapillary layer of the choroid is attached to the retinal pigment epithelium, a photochemical process is carried out in the latter. 4. Retina and optic nerve The retina contributes to the lining of the entire inner surface of the vascular tract. It is also a peripheral part of the visual analyzer. Under microscopic examination, ten layers are distinguished in it. At the place corresponding to the transition of the choroid itself into the flat part of the ciliary body (the region of the dentate line), only two layers of epithelial cells are preserved from its ten layers, passing to the ciliary body, and then to the iris. In the region of the dentate line, as well as at the exit of the optic nerve, the retina is tightly fused with the underlying formations. For the rest of its length, it is held in a constant position by the pressure of the vitreous body, as well as the connection between the rods and cones and the retinal pigment epithelium, which is genetically related to the retina, and anatomically closely related to the choroid. There are three types of neurons in the retina: rods and cones, bipolar cells, and multipolar cells. The most important area of the retina is the yellow spot, located at the posterior pole of the eyeball. The macula has a central fossa. In the area of the central fovea of the macula, instead of ten layers, only three or four layers of the retina remain: the outer and inner border plates and the layer of cones and their nuclei located between them. However, newborns have all ten layers in the macula area. This, along with other reasons, explains the child’s low central vision. In the central zone of the retina, predominantly cones are located, and towards the periphery the number of rods increases. Nerve cell fibers (about 100) form the optic nerve, passing through the lamina cribrosa of the sclera. The inner part of the optic nerve is called the disc (nipple). It has a somewhat oval shape, its diameter in newborns is 000 mm, in adults it reaches 0,8 mm. In the center of the disc are the central retinal artery and vein, which branch and participate in feeding the inner layers of the retina. Topographically, in addition to the intraocular one, the intraorbital, intracanalicular and intracranial parts of the optic nerve are distinguished. In the cranial cavity, the optic nerve forms a partial decussation of the nerve fibers of the chiasm. The optic tracts emerge from the chiasm in the form of two separate trunks, ending in the primary visual centers (external geniculate bodies, visual tuberosities). Through the internal capsule in the form of a bundle, the optic fibers go to the cortical visual centers, ending in the occipital lobe, in the region of the avian spur furrow (field seventeen to nineteen according to Brodmann). 5. Lens and vitreous body The transparent contents of the eyeball are represented by aqueous humor, the lens and the vitreous body. Aqueous moisture is contained in the anterior and posterior chambers of the eye. Its quantity in children does not exceed 0,2 cm3, and in adults it reaches 0,45 cm3. The anterior chamber is the space bounded by the posterior surface of the cornea in front, the iris behind, and in the region of the pupil by the lens. The chamber has the greatest depth in the center, to the periphery it gradually decreases. In a newborn, mainly due to the greater sphericity of the lens, the anterior chamber is smaller than 1,5 mm. The place where the cornea passes into the sclera, and the iris into the ciliary body, is called the angle of the anterior chamber of the eye. Through the angle of the anterior chamber, aqueous and anterior ciliary veins, aqueous humor is drained. The posterior chamber is the space bounded anteriorly by the iris and posteriorly by the anterior surface of the lens. Through the region of the pupil, the posterior chamber communicates with the anterior one. The lens is a transparent elastic body, has the shape of a biconvex lens. In newborns, the lens is almost spherical in shape. With age, the lens flattens somewhat, the radius of curvature of the anterior surface increases from 6 to 10 mm, and the posterior surface from 4,5 to 6 mm. The anteroposterior size of the lens of a newborn is 4 mm, and the diameter is 6 mm; the lens of an adult is 44,5 and 10 mm, respectively. The lens has anterior and posterior surfaces, anterior and posterior poles, sagittal axis, and equator. The lens is held in place by the ciliary body by the ligament of zon. The lens contains a capsule and lenticular, or cortical, fibers. In children, the fibers are elastic; with age, the center of the lens becomes denser, and from the age of twenty-five to thirty, a core begins to form, which gradually increases in size. 65% of the lens consists of water. It performs a refractive function; in relation to the average refractive power of the eye, it accounts for up to 40 out of 7780 diopters in newborns, and by the age of fifteen, 20 out of 60 diopters. The vitreous body is the main supporting tissue of the eyeball. Its weight in a newborn is 1,5 g, in an adult 67 g. The vitreous body is a formation of a gelatinous consistency, 98% consisting of water, containing an insignificant amount of protein and salts. In addition, it has a thin connective tissue skeleton, thanks to which it does not blur, even if taken out of the eye. On the anterior surface of the vitreous body there is a recess, the so-called plate-shaped fossa, in which lies the posterior surface of the lens. The vitreous body, being a transparent medium, provides free passage of light rays to the retina, protects the inner membranes (retina, lens, ciliary body) from dislocation. 6. Blood supply and innervation of the eye The blood supply to the eye is provided by the ophthalmic artery, a branch of the internal carotid artery. The outflow of venous blood is carried out by the whirlpool and anterior ciliary, and then by the ophthalmic veins of the upper and lower. The superior vein leaves through the superior orbital fissure and flows into the cavernous sinus, the inferior orbital vein passes through the inferior orbital fissure with its second branch, opens into the deep veins of the face and the venous plexus of the pterygopalatine fossa. The sensory nerves of the eye are mainly branches of the first branch of the trigeminal nerve. The main nerve plexus for the eye is the ciliary ganglion (2 mm). It is adjacent to and external to the optic nerves. The node is formed by a sensitive branch from the nasociliary nerve, parasympathetic from the oculomotor nerve and sympathetic from the plexus of the internal carotid artery. Four to six short ciliary nerves depart from the ciliary ganglion, which penetrate the sclera at the posterior pole and are joined by branches of the sympathetic nerve (which dilates the pupil). Short ciliary nerves provide all tissues of the eye with sensitive, motor and sympathetic innervation. Parasympathetic fibers innervate the sphincter of the pupil and the ciliary muscle. Motor innervation is provided by cranial nerves. Author: Shilnikov L.V. << Back: Structure of the eye (part I) (Structure of the orbit. Muscles and soft tissues of the eye. Connective membrane of the eye. Lacrimal organs) >> Forward: Technique for examining the condition of the eye (part I) (External examination of the eye in natural light. Side lighting method. Examination using a combined method)
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