Human Anatomy (with the Fundamentals of Dynamic and Sports Morphology) - Ivanitsky M. F. 2008

The Doctrine of Sense Organs
The Organ of Vision

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

The eyeball is located within the Orbit, which is formed by the BONES OF THE neurocranium and viscerocranium (Fig. 134). It is spherical in shape and more convex anteriorly. Its anterior and posterior poles are distinguished. The anterior pole corresponds to the center of the cornea, i.e., its most convex part, while the posterior pole lies slightly lateral to the entrance of the Optic nerve into the eyeball. The straight line passing through the anterior and posterior poles is called the visual axis of the eye. This axis intersects the straight line connecting the center of the cornea with the point of clearest vision, which is located in the region of the so-called macula (yellow spot) situated at the Cytology/practical/108.html">Fundus of the eyeball. The eyeball consists of tunics and light-refracting media. The outer tunic is called the fibrous tunic, the middle one is the vascular tunic (uvea), and the inner one is the sensory (nervous) tunic.

Class="center">

Fig. 134. Horizontal section of the eyeball (diagram, after M.S. Gracheva)

The fibrous tunic, in turn, is subdivided into two parts: the posterior and larger part, known as the white of the eye or sclera, and the anterior and smaller part, known as the Cornea of the eye. The sclera is white in color and is constructed from Dense Connective Tissue containing a large amount of Collagen and elastic fibers. It becomes somewhat thicker posteriorly. The sclera continues into the Sheath of the optic nerve (see Fig. 134).

The cornea is transparent and in appearance somewhat resembles a watch Glass. It consists of dense connective tissue and is quite sturdy (easily withstanding resistances such as Water pressure while swimming); therefore, one can swim and dive quite safely with open eyes, thus having The ability to orient oneself in water using vision. The cornea contains A large number of nerve endings and is devoid of Blood Vessels. Its Nutrition is carried out through the diffusion of necessary substances from the fluid of the anterior chamber of THE EYE AND from the Vessels of the sclera adjacent to the edge of the cornea.

The vascular tunic lies behind the fibrous tunic of the eyeball. It contains a significant number of blood vessels and pigment Cells, which give it a dark color. The vascular tunic is conventionally divided into three parts: the posterior part—the choroid proper, the middle part—the ciliary body, and the anterior part—the iris.

The choroid proper covers the largest part (approximately 2/3) of the posterior surface of the eyeball. It has a brown color and is tightly connected to the sclera at the site where the optic nerve enters the eyeball and at the junction of the cornea and the sclera.

The ciliary body is the middle, thickened part of the vascular tunic, arranged like a ring in the region where the sclera transitions into the cornea. The ciliary body has about 70 ciliary processes running in a radial direction, from which ciliary zonules (suspensory ligaments) extend and weave into the lens capsule.

Within the thickness of the ciliary body lies the ciliary Muscle. It consists of bundles of smooth muscle fibers arranged in three directions: circular, radial, and meridional. Meridional fibers make up the main bulk of the ciliary muscle. Upon contraction, this muscle relaxes the zonule, and through it the lens capsule, which, owing to its elastic properties, thereby becomes more convex—a process necessary when viewing objects at close range. Upon relaxation of the muscle, the ciliary body returns to its initial position, the ciliary zonules become taut, and the lens flattens. In old age, the elasticity of the zonule and the resilience of the lens decrease, leading to visual impairment.

The iris, i.e., the anterior part of the vascular tunic, has the appearance of a frontally positioned circular disk with an opening in the center—the pupil. It is constructed of muscle fibers running in circular and radial directions. The circular fibers form the sphincter pupillae muscle, while the radial fibers form the dilator pupillae muscle. The iris Functions as an optical Diaphragm located inside the eyeball.

Fig. 135. Structure OF THE retina:

I, II, III — first, second, and third Neurons of the retina; 1 — pigmented layer; 2 — layer of rods and cones; 3 — outer limiting membrane; 4 — outer nuclear layer; 5 — outer plexiform layer; 6 — inner nuclear layer; 7 — inner plexiform layer; 8 — ganglion cells; 9 — fibers of the optic n.; 10 — inner limiting membrane (after S.V. Kravkov)

The iris has anterior and posterior surfaces. The anterior surface is clearly visible through the cornea. It contains pigment, upon The Nature and amount of which the eye color depends: the more pigment there is, the darker the eye color.

The sensitive (inner) tunic of the eyeball is the retina, which develops as an outgrowth of the Diencephalon and, in its origin, structure, and function, forms a single whole with the optic nerve. Corresponding to the three PARTS OF THE vascular tunic, the adjacent retina is subdivided into the optic, ciliary, and iridial parts. The optic part is distinguished by the greatest complexity of structure, with up to a dozen layers discernible under a Microscope (Fig. 135). One of these layers includes rod-like and cone-like visual cells (rods and cones). Rods perceive light stimuli, while cones provide the ability to distinguish colors and their shades. The rods of the retina contain the so-called visual purple, or rhodopsin, which is produced by the Cells of the pigmented layer. In the light, visual purple decomposes, and in the dark, it regenerates, giving the entire retina a pinkish hue.

The outer layer of the retina, facing the vascular tunic of the eyeball, contains pigment and represents the retinal pigment epithelium, which is connected to the vascular tunic much more firmly than to the inner layers of the retina itself facing the cavity of the eyeball. Two regions can be distinguished in the optic part of the retina that differ in their Structure and Functional features: the optic disc and the macula. The optic disc is the site where the nerve enters the eyeball. It is about 1.7 mm in diameter and is located medial to the path of the optical axis of the eyeball. The macula (so named because of its yellowish color) is the site of clearest vision. Its diameter is approximately 1 mm. In the middle of the macula is the fovea centralis—the site of highest retinal sensitivity to light stimuli. In contrast, the optic disc, lacking both rods and cones, does not perceive light stimuli and constitutes the physiological blind spot of the retina.

The remaining two parts of the retina, the ciliary and iridial parts, are relatively simple in structure. The iridial part consists of the pigment epithelium mentioned earlier, while the ciliary part consists of two layers of epithelial cells (the outer layer being the pigment epithelium).

The eyeball has the following transparent (refracting) media: the cornea (see p. 335), the fluid of the anterior and posterior Chambers of the eyeball, the crystalline lens, and the vitreous body. Light rays, entering the eye, are refracted and form an inverted and diminished image on the retina.

The anterior chamber of the eyeball is the space between the posterior surface of the cornea, the anterior surface of the iris, and partly the anterior surface of the lens. The slit between the posterior surface of the iris and the anterior surface of the ciliary zonule, as well as partly the anterior surface of the lens, is called the posterior chamber of the eyeball. Both chambers are filled with a transparent fluid produced by blood vessels located in large numbers within the ciliary processes. The fluid of the anterior chamber, together with the cornea of the eye, forms a biconvex lens of about 30 diopters, thus constituting the refractive medium for passing light rays.

The most important light-refracting medium is the crystalline lens. It is built of fibers that are hexagonal in shape and run meridionally. The lens is enclosed in a transparent capsule. At the margin of the lens, it is attached to the ciliary zonule, which consists of fibers extending to the ciliary body. In appearance, the lens is compared to a biconvex lens. The anterior surface of the lens has a lesser convexity than the posterior surface. Its anteroposterior dimension is 3.7 mm. When contraction of the ciliary muscle reduces the tension of the transparent lens capsule, the lens, due to its elastic properties, becomes more convex, and its anteroposterior dimension can reach 4.4 mm. When viewing distant objects, the lens flattens, and when viewing close objects, it becomes thicker. The adjustment of the eye for clearest vision at near and far distances is called accommodation. In animals living in water, the lens is spherical in shape, and its light-refracting properties are higher than those of terrestrial animals. Human beings, however, do not see the outlines of objects clearly enough in water. This is due to the fact that the light-refracting Properties of the transparent media of the human eye are very close to the light-refracting property of water. When rays pass directly from water into the eye, their refraction is insignificant, and the point of intersection lies no longer on the retina, as usual, but behind it.

The entire cavity of the eyeball behind the lens and ciliary zonule is occupied by the vitreous body, which abuts the retina. Anteriorly, it has a depression corresponding in shape to the posterior surface of the lens. The vitreous body is a transparent gelatinous substance enclosed in a transparent membrane and consisting of fine connective tissue fibers, Proteins, and hyaluronic acid.

Accessory structures of the eye. The Accessory Organs of the eye include the Muscles, eyelids, conjunctiva, and Lacrimal Apparatus.

The eyeball is set into motion by 6 muscles: 4 rectus and 2 oblique muscles. The following muscles are distinguished: superior, inferior, medial, and lateral rectus, and superior and inferior oblique (Figs. 120, 136). All these muscles are constructed of Striated Muscle tissue. They originate from the common tendinous ring, which is located deep within the orbit and surrounds the optic nerve. The only exception is the shortest inferior oblique muscle, which originates directly from the periosteum of the inferior wall of the orbit and runs to the eyeball. The rectus Muscles of the eyeball run anteriorly and insert in the region of its equator, slightly anterior to it, attaching to the fibrous tunic of the eyeball. The superior oblique muscle runs along the superior-medial margin of the orbit and its tendon loops through a fibrous pulley attached to the Frontal bone. From this pulley, the tendon runs at an acute angle laterally and attaches to the fibrous tunic of the eyeball superiorly and somewhat lateral to its median plane.

The function of the extraocular muscles is that the oblique muscles rotate the eyeball around the anteroposterior axis, the medial and lateral rectus muscles around the vertical axis, and the superior and inferior rectus muscles around the transverse axis. Thus, the eyeball is capable of rotating around three mutually perpendicular axes. In practice, thanks to the combined action of individual muscles, it can rotate around any axis passing through its center. When viewing distant objects, the optical axes of the eyes become more parallel and intersect when extended at a more acute angle than when viewing close objects.

Fig. 136. The lacrimal apparatus:

1 — levator palpebrae superioris m.; 2 — eyeball; 3 — trochlea; 4 — lacrimal lake; 5 — lacrimal sac; 6 — nasolacrimal duct; 7 — Nasal cavity wall; 8 — lower eyelid; 9 — tarsal glands; 10 — inferior oblique m.; 11 — adipose tissue; 12 — inferior orbital wall; 13 — inferior rectus m.; 14 — lacrimal gland ductules; 15 — lateral rectus m.; 16 — superior rectus m.; 17 — lacrimal gland; 18 — superior oblique m. (after Kahn)

The entire eyeball, together with its muscles, lies within the orbital cavity and is surrounded by adipose tissue. The walls of the orbit are lined with periosteum. The Adipose tissue is separated from the eyeball by a connective tissue layer known as the sheath of the eyeball (fascia bulbi). Between this sheath and the fibrous layer of the eyeball wall lies a slit-like space resembling the cavity of a ball-and-socket joint. Unlike joint cavities, however, it contains fine strands connecting the Fascia of the eyeball to its wall. As the muscles approach the eyeball, their tendons pass through this fascia.

The eyelids are structures that protect the eyeball from the front. The upper and lower eyelids are distinguished. The upper eyelid is larger than the lower one and considerably more mobile due to the action of the levator palpebrae superioris muscle, which attaches to its tarsus. Eyelashes grow along the margins of the eyelids. Between the free margins of the upper and lower eyelids lies the palpebral fissure. Its lateral angle is sharp, whereas the medial angle is rounded, forming the so-called lacrimal lake. Within this angle lies a small, reddish elevation known as the lacrimal caruncle, which contains adipose tissue and Sebaceous Glands. The framework of each eyelid is formed by the tarsus (tarsal plate). The eyelids are equipped with tarsal glands, as well as sebaceous glands whose secretion lubricates the eyelid margins and eyelashes. Directly beneath the Skin of the eyelids lies a muscle that forms part of the orbicularis oculi muscle (see p. 196). It acts as an antagonist to the levator palpebrae superioris muscle.

The conjunctiva is a mucous membrane that lines the inner surface of the eyelids and a portion of the eyeball. The transition zone where the conjunctiva passes from the eyelids to the eyeball is termed the fornix. The superior and inferior conjunctival fornices are distinguished.

The lacrimal apparatus (see Fig. 136) comprises the lacrimal gland and the lacrimal duct system. The lacrimal gland is located in the superior lateral angle of the orbit. It is classified as a tubulo-alveolar gland and has 5 to 12 excretory ductules that open into the upper conjunctival fornix within its lateral region. The lacrimal gland produces a secretion that moistens the eyeball during blinking.

Tears flow through the lacrimal passages toward the medial angle of the eye. When the eyelids are closed, a triangular slit forms between them along the line of closure, known as the lacrimal rivus, through which tears drain into the lacrimal lake and from there into the lacrimal canaliculi. The superior and inferior lacrimal canaliculi run medially and converge to form an expansion called the lacrimal sac, which is surrounded by fibrous tissue and attached to the Lacrimal bone. Attached to the wall of the lacrimal sac is the lacrimal part of the orbicularis oculi muscle (see p. 196); upon contraction, it can dilate the lacrimal sac, thereby facilitating the suction of accumulated tears into the lacrimal canaliculi. The lacrimal sac continues downward as the nasolacrimal duct, which runs within the bony nasolacrimal canal and opens into the nasal cavity beneath the Inferior nasal concha.

The Blood supply to the retina and the optic nerve is provided by the central retinal artery, which enters the eyeball within the substance of the optic nerve and is a branch of the ophthalmic artery (a branch of the Internal Carotid Artery). The central retinal vein runs alongside the central retinal artery.

At the equator are four vortex Veins that empty into the ophthalmic veins, which in turn drain into the cavernous sinus.

Innervation of the eyeball (aside from the optic nerve) is supplied by branches belonging to the Trigeminal nerve system and Branches of the associated ciliary ganglion. The innervation of the smooth muscles of the eyeball and the extrinsic muscles composed of striated muscle tissue has already been discussed (see p. 362).

Pathway of visual information. Light rays, after passing through the transparent light-refracting media of the eyeball, strike the retina, where they are perceived by its rods and cones. Visual information travels to bipolar cells, which transmit impulses to the ganglion cells of the retina (see Fig. 135); these are larger and feature well-defined tigroid substance (Nissl bodies) in their Cytoplasm. The neurites of these cells form bundles of fibers that constitute the optic nerve—the conductor of the visual analyzer. Emerging from the orbit, the optic nerve passes through the optic canal into the cranial cavity, where, at the Base of the Brain in the region of the sella turcica, it forms an incomplete decussation and continues as the optic tract. Fibres of the optic tract run to the thalamus (lateral geniculate body), where the third neuron of the pathway is located, and subsequently to the central part of the analyzer—the visual cortex of the cerebrum, located in the occipital lobe along the margins of the calcarine sulcus. Some fibers extend to the lateral geniculate bodies and the superior colliculi of the corpora quadrigemina. Thanks to the connections of the latter with the Cranial Nerves and the Autonomic Nervous system, automatic regulation of pupil size and accommodation of the eyes for viewing an object are made possible, while connections with the anterior horns of the Spinal Cord facilitate the transmission of impulses to striated skeletal muscles, ensuring appropriate movements in response to visual information.