Human Anatomy - H. I. Koliadenko 2009

Sense Organs (Analyzers)
The Organ of Vision

The visual system consists of the Eyeball, the Optic nerve, and the Accessory structures of the eye.

The eyeball in a child is spherical in shape, whereas in an adult it is slightly elongated in the anteroposterior direction. The eyeball is located within the bony Orbit of the Skull. The anterior surface of the eyeball is slightly convex and is referred to as the anterior pole of the eye, while the posterior surface is known as the posterior pole. A straight line passing through the center in the anteroposterior direction is called the external axis of the eyeball. The eyeball consists of a core and three tunics. The core is composed of the refractive media of the eye: the lens, the vitreous body, and the aqueous humor. The outer tunic of the eyeball is the sclera (Fig. 184), the middle tunic is the vascular tunic (uvea), and the inner tunic is the retina.

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Fig. 184. Diagram of The Structure of the eyeball:

1 — cornea; 2 — iris; 3 — ciliary zonule; 4 — ciliary body; 5 — sclera; 6 — lateral rectus Muscle; 7 — choroid; 8 — retina; 9 — visual axis of the eyeball; 10 — central fovea of the macula; 11 — optic nerve; 12 — optic disc; 13 — vitreous body; 14 — medial rectus muscle; 15 — conjunctiva of the eyeball; 16 — posterior chamber of the eye; 17 — anterior chamber of the eye; 18 — lens

The outer tunic is divided into a larger posterior portion, the sclera, and a smaller anterior portion, the cornea. The sclera is composed of dense, opaque, white Collagen fibers. The thickness of the sclera increases from the posterior pole toward the cornea. At the junction between the sclera and the cornea lies the scleral venous sinus (canal of Schlemm), through which fluid drains from the anterior chamber of the eye. The cornea constitutes about 1/5 to 1/6 of the outer tunic of the eye; it is convex and consists of transparent Connective Tissue fibers and Stratified Epithelium. The thickness of the cornea is approximately 1.0–1.1 mm at the center and 0.8–0.9 mm at the periphery. The cornea consists of five layers. It lacks Blood Vessels and receives Nutrition via diffusion, partly from vessels located at the sclera-cornea junction, as well as from the aqueous humor of the anterior chamber of the eye.

The vascular tunic (uvea) of the eyeball lies beneath the sclera, has a thickness of 0.1–0.22 mm, and contains a branching network of blood vessels. The vascular tunic consists of three parts: the posterior part is the choroid (located behind the ciliary body), the middle part is the ciliary body, and the anterior part is the iris.

The choroid is thin and composed of dark-pigmented connective tissue that reflects all colors of the spectrum. Its base is formed by tightly intertwined Arteries and Veins, interspersed with loose connective tissue containing large pigment Cells. A system of clefts, known as the perichoroidal space, is situated between the choroid and the sclera. Anteriorly, the choroid transitions into the thickened, structurally complex ciliary body. Its bulk is formed by the ciliary muscle and loose Connective tissue with pigment cells. The ciliary muscle is circular in shape and closely adheres to the sclera at the transition point into the cornea. Extending from the inner surface of the ciliary body are 70–80 ciliary processes, which form the ciliary crown to which the fibers of the ciliary zonule (suspensory ligament of Zinn) attach, extending toward the lens. Each ciliary process is a dense network of venous vessels that secrete a fluid known as the aqueous humor. This fluid fills the Chambers of the eyeball.

The ciliary Muscle consists of meridional (longitudinal), circular, and radial fibers. Contraction of the ciliary muscle causes relaxation of the ciliary zonule, which in turn facilitates an increase in the curvature of the lens.

The iris is a continuation of the ciliary body, forming a thin membrane with a central opening called the pupil. Its inner margin is uneven and serrated, while its outer margin connects to the ciliary body (Fig. 185). The iris consists of five layers. Its connective tissue stroma contains blood vessels, pigment, and smooth muscle fibers. The color of a human eye—ranging from light blue to black—depends on the amount and depth of the pigment. In albinos, pigment is absent, making blood vessels visible through the cornea. Circular and radial Muscles are located within the stroma of the iris. Contraction of the circular muscles constricts the pupil, whereas contraction of the radial muscles dilates it.

Fig. 185. Ciliary body (inner surface):

1 — ciliary body; 2 — sclera; 3 — ciliary processes; 4 — retina; 5 — ciliary zonule; 6 — lens

The retina (Fig. 186) is the inner tunic of the eye, continuous with the vascular tunic. It consists of two parts. The posterior part lies directly against the choroid and is called the optic part of the retina. It contains photoreceptors—rods and cones. The anterior part of the retina lacks photoreceptors. In the posterior region of the retina lies the site where the optic nerve exits, known as the optic disc; an area right at the disc completely lacks photoreceptors and is called the blind spot. At a distance of 4–6 mm from the blind spot lies an oval depression known as the macula (yellow spot), the area of highest visual acuity. The optic part of the retina has a complex, multi-layered microscopic structure comprising 10 distinct layers. The outermost layer of the retina is the pigmented layer, which is closely associated with the choroid. Directly beneath the pigmented layer lies the layer of photoreceptor cells, which are specialized bipolar neurocytes. Their processes connect them to the pigmented layer. The processes of these bipolar neurocytes function as the light-sensitive receptors—rods and cones—which perceive and process light stimuli. The central region of the retina contains 6–7 million cones, which are responsible for Color Vision across the full spectrum. There are 120–130 million rods, located mainly in the peripheral Regions of the retina, which provide dim-light (scotopic) vision.

Fig. 186. Human retina:

1 — middle (vascular) tunic of the eyeball; 2 — retinal pigment epithelium; 3 — rod visual cells; 4 — cone visual cells; 5 — nuclei of visual cells; 6 — bipolar neurocytes; 7 — ganglion neurocytes; 8 — astrocyte; 9 — axons of ganglion neurocytes; 10 — nerve fiber layer; 11 — ganglion Cell layer; 12 — inner nuclear layer of the retina; 13 — outer nuclear layer of the retina; 14 — layer of rods and cones

Processes from the photoreceptors extend into the deeper layers of the retina, connecting with the bipolar neurocytes of the inner nuclear layer. The cells of this layer synapse with the ganglion Cells of the multipolar layer. The axons of the ganglion cells converge to run along The surface of the retina, forming the optic nerve.

The inner surface of the retina is in contact with the vitreous body. The line connecting the anterior and posterior poles is called the visual axis of the eyeball. At the posterior pole, the visual axis passes through the central part of the fovea centralis.

The core of the eyeball consists of the lens, the vitreous body, and the aqueous humor of the anterior and posterior chambers of the eyeball.

The lens is a transparent biconvex lens. Its posterior surface is more convex than the anterior surface. The lens is composed of a transparent, colorless substance devoid of blood Vessels and nerves, enclosed on all sides by an acellular capsule. Its equatorial surface is attached to the ciliary zonule. The ciliary zonule connects to the ciliary body via fine connective tissue fibers that insert internally into the lens capsule and externally into the ciliary body. The lens performs the crucial function of light refraction. This refractive ability changes through alterations in the curvature of the lens, which occur As a result of the contraction of the ciliary muscles. During this process, the ciliary zonule relaxes, the convexity of the lens increases, and its refractive power increases accordingly. When the ciliary muscles relax, the ciliary zonule tightens, the curvature of the lens decreases, and it becomes flatter. The refractive power of the lens ensures that the image of an object (whether positioned near or far) is focused precisely onto the retina. This phenomenon is known as accommodation. The refractive power of the lens is 1.43 diopters. With age, accommodation weakens due to the loss of elasticity in the lens and its reduced ability to change shape. This age-related decline in accommodation is known as presbyopia and typically manifests after 40–45 years of age.

The vitreous body occupies the major portion of the cavity of the eyeball. It is enveloped externally by a delicate, transparent membrane known as the hyaloid membrane. The vitreous body consists of a proteinaceous fluid intertwined with delicate microscopic fibers. Its anterior surface is concave, facing the posterior surface of the lens, and forms a depression—the hyaloid fossa—in which the posterior pole of the lens rests. The greater part of the vitreous body is in contact with the retina of the eyeball and maintains a convex shape.

The anterior and posterior chambers of the eye are filled with aqueous humor, which is secreted by the Blood vessels of the ciliary processes and the iris. The aqueous humor has minimal refractive properties. The anterior chamber of the eye is larger and is situated between the cornea and the iris, whereas the posterior chamber lies between the iris and the lens.

The accessory structures of the eye include the eyelids, eyelashes, eyebrows, Lacrimal Apparatus, conjunctiva, and extraocular muscles.

The eyelids (upper and lower) are folds of Skin. Each eyelid features an anterior and a posterior surface, as well as two margins that enclose the palpebral fissure. The anterior surface of the eyelids is convex, and its skin contains A large number of sebaceous and Sweat Glands. The upper eyelid is bounded superiorly by the arched eyebrow, which is covered with short hairs. The inner surfaces of the eyelids are lined with a mucous membrane known as the conjunctiva. When the eyelids are closed, the conjunctiva forms a potential space called the conjunctival sac, where a small amount of lacrimal fluid accumulates. The eyelids provide movable protection for the eyeball. The structural framework of the eyelids consists of a Cytology/practical/45.html">Dense connective tissue plate (tarsus) containing tarsal glands. The free margins of the eyelids are fringed with hairs known as eyelashes. At the medial angle of the eye, between the margins of the eyelids, lies the lacrimal lake, at the floor of which is a small elevation called the lacrimal caruncle. Along the margins of both eyelids in this region is a small opening—the lacrimal punctum—which marks the beginning of the lacrimal canaliculus.

The lacrimal apparatus of the eye (Fig. 187) consists of the lacrimal gland, lacrimal canaliculi, lacrimal sac, and nasolacrimal duct. The lacrimal gland is located in the upper-outer corner of the orbit. It features 10–14 excretory ducts that open into the superior conjunctival fornix. Washing over the eyeball, the lacrimal fluid flows toward the inferomedial corner of the eye into the lacrimal lake. Through the lacrimal puncta, it enters the lacrimal canaliculi and flows into the lacrimal sac, which tapers inferiorly into the nasolacrimal duct; this duct lies within the corresponding bony canal and opens into the inferior nasal meatus beneath the Inferior nasal concha.

Fig. 187. The lacrimal apparatus:

1 — lacrimal gland; 2 — excretory ducts; 3 — conjunctiva; 4 — lacrimal puncta (entrance to the lacrimal duct); 5 — lacrimal canaliculus; 6 — valve

The extraocular muscles ensure the mobility of the eyeball. These include four rectus muscles, two oblique muscles, and the levator palpebrae superioris. The superior, inferior, lateral, and medial rectus muscles originate from the common tendinous ring located in the orbit around the optic nerve. The rectus muscles insert at the equator of the eyeball. Together, the ocular muscles move the eyeball upward, downward, laterally, and medially. The superior oblique muscle originates from the same site as the rectus muscles (the common tendinous ring), extends toward the medial angle of the orbit, and loops through the trochlea. It inserts posterior to the equator of the eyeball, about 18 mm from the corneal limbus. Upon contraction, this muscle rotates the eyeball around its sagittal axis.

The inferior oblique muscle originates in the medial region of the orbit from the lacrimal crest of the Maxilla and the infraorbital margin, inserting into the lateral surface of the eyeball posterior to the equator. It rotates the eyeball to direct the pupil upward and outward.

Fig. 188. Muscles of the eyeball:

a — superior view; b — lateral view;

1 — tendon of the superior oblique muscle; 2 — trochlea; 3 — frontal sinus; 4 — superior oblique muscle; 5 — medial rectus muscle; 6 — superior rectus muscle; 7 — lateral rectus muscle; 8 — origin tendon of the levator palpebrae superioris muscle; 9 — common tendinous ring; 10 — optic nerve; 11 — optic chiasm; 12 — dorsum sellae; 13 — Internal Carotid Artery; 14 — Oculomotor nerve; 15 — broad aponeurosis of the levator palpebrae superioris muscle; 16 — levator palpebrae superioris muscle; 18 — lateral plate of the pterygoid process; 19 — maxillary sinus; 20 — inferior oblique muscle; 21 — lower eyelid; 22 — upper eyelid

The eyeball and its surrounding muscles are embedded in adipose tissue, which is separated from the eyeball by a fascial sheath (Tenon's capsule) lining the orbital cavity internally.

The conjunctiva of the eye is a mucous membrane that lines the inner surface of the eyelids and reflects onto the anterior surface of the eyeball.

The Blood supply to the eyeball is provided by the central retinal artery, which is an orbital branch of the internal carotid artery (Fig. 188). Blood drains via the central retinal vein, which runs parallel to the artery.

Nervous Regulation of the eyeball is carried out by the optic nerve and Branches of the Trigeminal nerve connected to the ciliary ganglion. The muscles of the eyeball are innervated by the oculomotor and trochlear nerves.

Optic nerves. Light rays pass through the refractive media of THE EYE AND strike the retinal receptors—rods and cones. Under the action of light, nerve impulses are generated in the rods and cones and transmitted to bipolar cells, and from them to ganglion cells, whose axons form the optic nerve. The excitation then proceeds to the Brain and emerges through the optic canal at the Base of the brain, where the right and left optic nerves form the optic chiasm; only the nerve fibers originating from the Medial surface of the retina cross over. After the chiasm, each nerve forms the optic tract, which contains fibers carrying impulses from the medial surface of the retina of the opposite eye and the lateral surface of the eye on the same side. The fibers of the optic tract run to the thalamus (optic thalamus), where the third neuron of the visual pathway is located. Some nerve fibers pass to the superior colliculi of the tectal plate, where they switch to corresponding Neural Pathways that coordinate HEAD movements in response to visual stimuli. Processes of the Neurons that reach the lateral geniculate bodies and the thalamus continue as thalamic radiation to the calcarine sulcus of the occipital lobe of the Cerebral Cortex, which houses the cortical nuclear zone of the visual analyzer.

Fig. 189. Optic nerves and visual pathways:

1 — eyeball; 2 — olfactory bulb; 3 — optic nerve; 4 — olfactory tract; 5 — optic chiasm; 6 — anterior perforated substance; 7 — optic tract; 8 — oculomotor nerve; 9 — cerebral peduncle; 10 — lateral geniculate body; 11 — medial geniculate body; 12 — cerebral aqueduct; 13 — corpora quadrigemina (colliculi); 14 — red Nucleus; 15 — substantia nigra; 16 — posterior perforated substance; 17 — mamillary body; 18 — tuber cinereum; 19 — infundibulum; 20 — Pituitary Gland (hypophysis); 21 — lateral sulcus of the cerebral hemisphere; 22 — longitudinal fissure of the cerebral hemisphere

Depending on the site of light refraction within the eye's media, three categories of visual refraction states of the eyeball are distinguished: emmetropic (normal), myopic (nearsighted), and hyperopic (farsighted). In a normal eyeball, light rays are refracted so that their focus falls directly on the retina. In nearsighted individuals, the anteroposterior diameter of the eyeball is larger than that of a normal eye, and the focal point of light falls short of the retina, lying instead within the vitreous body. Such a visual defect is corrected using biconcave lenses, which reduce the focal length so that the image of the object falls onto the retina. In a farsighted eye, conversely, the anteroposterior diameter of the eyeball is smaller than that of a normal eye, and light rays are focused behind the retina; the visual axis is corrected using biconvex lenses, which increase the effective focal length of the eyeball so that the light focus is refracted precisely onto the retina (Figs. 190, 191).

Fig. 190. Light path in a myopic eyeball:

1 — retina of a normal eye; 2 — retina of a myopic eye; 3 — parallel rays from a distant object; 4 — focus of rays within the vitreous body at THE POSITION OF a normal retina; 5 — divergent rays reaching the retina of the myopic eye; 6 — biconcave diverging lens; 7 — focus of light rays on the retina after passing through the lens

Fig. 191. Light path in a hyperopic eyeball:

1 — retina of a normal eye; 2 — retina of a hyperopic eye; 3 — parallel rays from a distant object; 4 — focus behind the retina of the hyperopic eye; 5 — biconvex converging lens; 6 — focus of light rays on the retina of the hyperopic eye after passing through the lens

Development of the eyeball. In the third week of embryonic development, optic vesicles originate at the anterior pole of the neural tube. By the end of the 4th week of development, the optic vesicles flatten, and in the 5th week of development, their anterior walls invaginate to form two-layered cup-shaped structures, between the layers of which slit-like cavities are formed. The retina develops from the primary layers. Later, the lens forms from the ectoderm adjacent to the optic cup. The optic cup is connected by a thin stalk to the cerebral vesicle, from which the Diencephalon develops. The stalk of the optic cup first elongates, then its wall invaginates to form a groove whose edges fuse, transforming the groove into the optic nerve. A dense capsule originates from the mesenchyme surrounding the optic cup. Its inner surface transforms into the choroid, and the outer surface into the sclera. Later, the anterior wall of the sclera transforms into the transparent cornea.