Human Anatomy and Physiology (with Age-Related Features of the Child's Body) - Sapin M.R., Sivoglazov V.I. 2002

Nervous system
Sensory organs
Organ of vision

Sensory Organs are a complex of anatomical structures that perceive the energy of external stimuli, convert it into a Nerve Impulse, and transmit it to the corresponding centers of the Brain, including the Cerebral Cortex, where higher analysis takes place. Sensory organs include the organs of vision, Hearing, gravity (gravitation), taste, smell, and cutaneous sensation.

The Organ of Vision consists of the Eyeball and the Accessory structures of the eye located within the Orbit.

The eyeball is spherical in shape, with distinct anterior and posterior poles. The anterior pole is the most prominent point of the cornea, while the posterior pole is located laterally to the exit site of the Optic nerve. The imaginary line connecting both poles is called the external axis of the eye, measuring approximately 24 mm. There is also an internal, optic axis of the eye, which runs from the cornea through the center of the lens to the fovea centralis.

The eyeball consists of an inner core surrounded by three tunics: the outer fibrous tunic, the middle vascular tunic, and the inner nervous tunic (retina) (Fig. 97).

The outer fibrous tunic is divided into a posterior part, the sclera (white of the eye), and a transparent anterior part, the cornea. The sclera is composed of Cytology/practical/45.html">Dense Connective Tissue, with a thickness of 0.3–0.6 mm. The optic nerve exits the eyeball through the posterior part of the sclera. Within the anterior part of the sclera, near its junction with the cornea, lies a narrow circular channel—the scleral venous sinus (canal of Schlemm)—into which fluid drains from the anterior chamber of the eye. The transparent cornea is a meniscus (convex-concave) lens through which light enters the eye. The thickness of the cornea ranges from 0.8–0.3 mm at its center to 1.1 mm at its margin with the sclera. The cornea contains numerous nerve endings, providing high sensitivity, and is completely avascular.

The vascular tunic (uvea) of the eyeball lies beneath the sclera and consists of three parts: the choroid proper, the ciliary body, and the iris.

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Fig. 97. The eyeball. Horizontal section (diagram):

The lens during relaxation (a) and contraction (b) of the ciliary Muscle.

1 — cornea, 2 — anterior chamber of the eye, 3 — lens, 4 — iris, 5 — posterior chamber of the eye, 6 — conjunctiva, 7 — lateral rectus muscle, 8 — sclera, 9 — choroid, 10 — retina, 11 — fovea centralis, 12 — optic nerve, 13 — optic disc (blind spot), 14 — external axis of the eye, 15 — medial rectus muscle, 16 — transverse axis of the eye, 17 — ciliary body, 18 — ciliary zonule, 19 — optic axis of the eye

The choroid proper consists of a network of Blood Vessels and a small amount of connective tissue. Anteriorly, the choroid continues into the thickened, ring-shaped ciliary body.

The ciliary body, composed of smooth muscle bundles running in various directions, is involved in accommodation (the adjustment of the eye for viewing objects at various distances). Extending from the ciliary body toward the lens are 70–75 ciliary processes, which continue into the fibers of the ciliary zonule (suspensory ligament of the lens) attached to the lens. The ciliary processes are highly vascularized and secrete aqueous humor, which enters the posterior chamber of the eye. Anteriorly, the ciliary body continues as the iris.

The iris is a circular disc with a central opening (the pupil). It is positioned between the cornea anteriorly and the lens posteriorly. It separates the anterior chamber of the eye, bounded anteriorly by the cornea, from the posterior chamber of the eye, located anterior to the lens. The lateral peripheral margin of the iris merges with the ciliary body. The anterior and posterior surfaces of the iris are covered by epithelium. Within the stroma of the iris, There are two Muscles. Bundles of myocytes arranged circularly around the pupil form the sphincter pupillae (pupillary sphincter). Radially arranged myocyte bundles form the dilator pupillae (pupillary dilator). The presence of pigment Cells containing melanin in the iris determines eye color—brown or black (when pigment is abundant) or blue or greenish (when pigment is sparse).

Internal to the vascular tunic lies the inner (photosensitive) tunic of the eyeball—the retina. The retina is divided into two parts: the posterior optic part and the anterior ciliary part. The latter covers the posterior aspect of the ciliary body and contains no photoreceptor cells. The posterior optic part of the retina contains photosensitive rod and cone cells, shaped like rods and cones. The Deep Layer of the retina, adjacent to the choroid, is formed by pigment cells. The photosensitive (photoreceptor) Cells of the retina connect to retinal ganglion cells via interneurons (bipolar cells). The axons of the ganglion cells converge in the posterior part of the eyeball to form the thick optic nerve, which pierces the vascular and fibrous tunics and travels toward the apex of the orbit. The site where the ganglion Cell axons exit the retina is called the optic disc (blind spot). Rods and cones are absent in this area. The central retinal artery enters the retina at the optic disc.

Located 4 mm laterally to the optic disc is a yellowish area called the macula lutea, containing the fovea centralis. The fovea centralis is the area of highest visual acuity, where a high concentration of cones is clustered.

The internal media of the eyeball are formed by the lens, the vitreous body, and the Chambers of the eye.

The lens is a transparent, biconvex Structure about 9 mm in diameter, with anterior and posterior surfaces. It is enclosed in a transparent capsule. The substance of the lens is colorless, transparent, firm, and completely lacks Vessels and nerves. The fibers of the ciliary zonule (suspensory ligament) attach to the lens. When the ligament is tensioned during the relaxation of the ciliary muscle, the lens flattens, adjusting for distant vision. When the ligament relaxes during the contraction of the ciliary muscle, the curvature of the lens increases, adjusting for near vision. The adaptation of the lens for viewing at different distances is called accommodation of the eye.

The vitreous body fills the space between the lens anteriorly and the retina posteriorly. It consists of an amorphous intercellular substance with a gelatinous consistency. On the anterior surface of the vitreous body, There is a depression (hyaloid fossa) in which the lens rests.

The chambers of the eye are bounded anteriorly by the cornea and posteriorly by the lens, suspensory ligament, and ciliary body. There are two chambers—anterior and posterior—which are separated by the iris and communicate with each other through the pupil. The chambers contain a clear fluid, the aqueous humor, which is produced by the capillaries of the ciliary processes and secreted into the posterior chamber, from where it flows through the pupil into the anterior chamber. The posterior chamber communicates with the spaces between the fibers of the ciliary zonule extending to the lens from the ciliary processes. In the iridocorneal angle (formed by the junction of the iris and cornea), there are narrow spaces (trabecular meshwork) through which the aqueous humor drains into the scleral venous sinus, and from there into the ophthalmic Veins.

The outflow of aqueous humor maintains a balance between its production and absorption, which is essential for regulating intraocular pressure.

Accessory Organs of the eye. In humans, the eyeball can rotate so that the visual axes of both eyes converge on the object being viewed. There are six striated extraocular muscles in the orbit. These are the four rectus muscles (superior, inferior, medial, and lateral) and two oblique muscles (superior and inferior). The inferior oblique muscle originates from the floor of the orbit near the opening of the nasolacrimal canal. The remaining muscles originate deep within the orbit around the optic canal. All rectus muscles insert into the sclera anterior to the equator. The tendon of the superior oblique muscle passes through a pulley (trochlea) in the anteromedial angle of the orbit, turns backward and laterally, and inserts into the sclera posterior to the equator. The inferior oblique muscle also inserts posterior to the equator. The rectus muscles rotate the eyeball in their respective directions, while the oblique muscles rotate the eye around the sagittal axis. Due to the coordinated action of the extraocular muscles, the movements of both eyeballs are synchronized.

Behind the eyeball lies the orbital fat body, which serves as an elastic cushion for the eye.

The eyelids protect the eyeball anteriorly. They are folds of Skin that bound the palpebral fissure and close it when shut. When the eyes open, the lower eyelid drops slightly due to gravity. The levator palpebrae superioris muscle, which originates along with the rectus muscles, inserts into the upper eyelid. Within the eyelids lie branched sebaceous (Meibomian) glands, opening near the roots of the eyelashes. The posterior surface of the eyelids is lined with conjunctiva, which continues onto the eyeball as the bulbar conjunctiva. The conjunctiva is a thin connective tissue membrane covered by Stratified Epithelium. At the points of transition from the eyelids to the eyeball, the conjunctiva forms narrow recesses—the superior and inferior conjunctival fornices.

The Lacrimal Apparatus of the eye includes the lacrimal gland, lacrimal canaliculi, lacrimal sac, and nasolacrimal duct.

The lacrimal gland is located on the anterolateral roof of the orbit, in the lacrimal fossa. Its 5 to 12 excretory ducts open into the superior conjunctival fornix. Lacrimal fluid bathes the eyeball and moistens the cornea. Blinking Movements of the eyelids sweep the lacrimal fluid toward the medial angle of the eye, where the lacrimal canaliculi begin at the margins of the upper and lower eyelids. The superior and inferior lacrimal canaliculi drain into the lacrimal sac, which has a blind upper end. The lower part of the lacrimal sac continues as the nasolacrimal duct, which opens into the inferior nasal meatus. The lacrimal part of the orbicularis oculi muscle, fused with the wall of the lacrimal sac, expands the sac upon contraction, facilitating the aspiration of tears into the lacrimal sac through the canaliculi.

Age-related Features of the organ of vision

The eyeball in a newborn is relatively large, with an anteroposterior diameter of 17.5 mm and a mass of 2.3 g. The visual axis of the eyeball runs more laterally than in an adult. The eyeball grows faster During the first year of a child's life than in subsequent years. By age 5, the mass of the eyeball increases by 70%, and by age 20–25, it increases threefold compared to that of a newborn.

The cornea in a newborn is relatively thick, and its curvature remains almost unchanged throughout life; the lens is nearly spherical, with the radii of its anterior and posterior curvatures being approximately equal. The lens grows particularly rapidly during the first year of life, after which its growth rate declines. The iris is convex anteriorly, contains little pigment, and the pupil diameter is 2.5 mm. As the child grows older, the thickness of the iris increases, its pigment content increases by age two, and the pupil diameter becomes larger. At the age of 40–50 years, the pupil constricts slightly.

The ciliary body in a newborn is poorly developed. The growth and Differentiation of the ciliary muscle occur quite rapidly. The capacity for accommodation is established by age 10. The optic nerve in a newborn is thin (0.8 mm) and short. By age 20, its diameter nearly doubles.

The extraocular muscles in a newborn are fairly well developed, except for their tendinous portions. Therefore, Eye Movements are possible immediately after birth, but coordination of these movements begins In the second month of life.

The lacrimal gland in a newborn is small, and its excretory ducts are thin. During the first month of life, a baby cries without tears. Lacrimation begins in the second month of life. The orbital fat pad is poorly developed. In elderly and senile individuals, the orbital fat pad decreases in size and partially atrophies, causing the eyeball to recede deeper into the orbit.

The palpebral fissure in a newborn is narrow, and the medial angle of the eye is rounded. Subsequently, the palpebral fissure increases rapidly. In children up to 14–15 years of age, it is wide, making the eye appear larger than in an adult.

Optical system and accommodative apparatus of the eye

The Optical System of the eye. Visual Perception begins with the projection of an image onto the retina and the excitation of its photoreceptor cells—rod and cone photoreceptors (rods and cones). The PROJECTION OF THE image onto the retina is provided by the Optical System of the eye, which consists of the refractive and accommodative apparatuses.

The refractive apparatus includes the cornea, aqueous humor, lens, and vitreous body. These are transparent structures that refract light as it passes from one medium to another (air–cornea–fluid–lens). The cornea has a high refractive power.

The accommodative apparatus is formed by the ciliary body with its muscle, the iris, and the lens. These structures focus light rays coming from observed objects onto the optic part of the retina. The primary mechanism of accommodation (adaptation) is the lens, which is capable of changing its refractive power. The change in the curvature of the lens is regulated by the complex ciliary muscle. When the ciliary muscle contracts, the tension of the ciliary zonule fibers attached to the lens capsule decreases. Consequently, the lens, freed from the pressure of its capsule, bulges and becomes more convex, increasing its refractive power. When the ciliary muscle relaxes, the fibers of the ciliary zonule are stretched, flattening the lens and decreasing its refractive power. With the help of the ciliary muscle, the lens constantly changes its curvature, adapting the eye for clear vision of objects at various distances. This property of the lens is called accommodation. Meanwhile, the refractive power of the cornea, aqueous humor, and vitreous body remains constant. The transparent media of THE EYE AND its accommodative apparatus optimally refract parallel light rays, focusing them precisely on the retina. If the refractive power of the cornea or lens is weak (the lens is flattened), light rays converge at a focal point behind the retina. This phenomenon is called farsightedness (hyperopia). In this case, a person sees distant objects well and near objects poorly. With an increase in the refractive power of the transparent media of the eye (the lens is more convex), light rays converge at a point in front of the retina. This leads to nearsightedness (myopia), where near objects are seen clearly, while distant ones are blurry. Both hyperopia and myopia are corrected using glasses with biconvex or biconcave lenses.

Conducting pathway of the visual analyzer

The peripheral component of the visual analyzer consists of photosensitive elements—rods and cones. The central component, or the core of this analyzer, is the visual cortex on the Medial surface of the occipital lobe of the cerebral hemispheres, in the region of (along the banks of) the calcarine sulcus.

On its way to the photosensitive retina, light passes through all the transparent media of the eye. The pupil, acting as a Diaphragm under the action of its muscles, either constricts or dilates, letting a smaller or larger beam of light into the eye. The refractive media (cornea, aqueous humor of the anterior and posterior chambers, lens, and vitreous body) direct the light beam to the most sensitive area of the retina—the macula lutea with its fovea centralis. Extraocular muscles turn the eyes toward the object being viewed.

Light entering the eye penetrates into the deepest layers of the retina, where it stimulates rod and cone photoreceptors (rods and cones). The conversion of light energy into nerve impulses occurs As a result of chemical processes in the rods and cones. Under METABOLISM/18.html">The Influence of light, Chemical Reactions take place in the outer segments of the photoreceptor cells, during which visual pigments (rhodopsin) break down into simpler chemical substances. These substances act on the rods and cones, exciting them. Once the light stimulus ceases, rhodopsin is regenerated. Consequently, chemical reactions lead to the generation of a receptor potential in the photoreceptor cells, which produces a nerve impulse.

Rod photoreceptors (rods) are unable to distinguish colors; they are used primarily in twilight and night vision to recognize objects by their shape and illumination. Cone photoreceptors (cones) function during the day and are responsible for Color Vision. Depending on their structural and chemical characteristics, some cones perceive blue, others green, and others red, corresponding to different wavelengths of light.

The nerve impulse generated in the rods and cones is transmitted to the bipolar cells located within the retina, and then to the ganglion cells, which are elements of the visual pathway. The axons of the ganglion cells converge at the optic disc (blind spot) to form the optic nerve, which extends into the cranial cavity. On the Inferior surface of the brain, the right and left optic nerves undergo a partial decussation. In this optic chiasm, not all nerve fibers of the optic nerve cross to the opposite side, but only those originating from the medial (nasal) part of the retina. Thus, past the optic chiasm, the optic tract contains nerve fibers from the lateral (temporal) part of the retina of the ipsilateral eye and the medial (nasal) part of the retina of the contralateral eye. The nerve fibers then proceed to the subcortical visual centers—the lateral geniculate body and the superior colliculi of the Midbrain tectum. In these centers, the impulse from the retinal ganglion cell fibers is transmitted to the next Neurons, whose processes project to the cortical visual center—the cortex of the occipital lobe of the brain, where the highest analysis of visual perceptions takes place. The partial decussation of the visual pathways provides binocular vision.

Binocular, black-and-white, and color vision

Vision with two eyes (binocular vision) makes it possible to perceive three-dimensional images of objects, the depth of their Location, and to estimate the distance at which they are situated. When looking at an object, the right eye sees more of its right side, and the left eye sees more of its left side. At the same time, a person perceives these two images as a single, three-dimensional one. Binocular vision is possible because the image is formed on identical, corresponding areas of the retina of the right and left eyes. Working together and combining visual information, both eyes provide stereoscopic vision, which allows for a more accurate understanding of the shape, volume, and depth of objects.

Adaptation of the eyes to light. When moving from a dark room into the light or from a bright room into the dark, some time is needed for adjustment, or adaptation. Adjusting to bright light (light adaptation) occurs quickly, within 4–6 minutes. The eyes adjust to the dark much more slowly. When moving from a bright room into the dark, dark adaptation lasts up to 45 minutes or more. During this process, the sensitivity of rod photoreceptors (rods) increases dramatically.

Color vision is provided by cone photoreceptors (cones). In the dark, only rods function, and they do not distinguish colors. The perception of colors involves not only the cone photoreceptors of the eye (cones) but also the visual centers of the brain.

Color vision deficiency {color blindness) occurs in approximately 8% of men and 0.5% of women. In such cases, there is a lack of perception of either red, green, or blue colors. Complete color blindness (achromatopsia) is rare.

Review and Self-Assessment Questions:

1. List the Sensory Organs and provide a functional description of each.

2. Describe The structure of the Tunics of the eyeball.

3. Name the structures that make up the transparent media of the eye. What is the purpose of each of these media?

4. List the organs that belong to the Accessory apparatus of the eye. What Functions does each of these accessory organs perform?

5. Describe the STRUCTURE AND FUNCTIONS of the accommodative apparatus of the eye.

6. Describe The pathway of the visual analyzer, from the light-sensitive receptors to the cerebral cortex.

7. Describe light adaptation of the eye and color vision.