Organ of Vision PDF

Organ of vision 1. Structure of eyeball 2. Visual accessory organs 3. Eyeball development  External anatomy Eyelids or palpebrae An eyelid is a thin fold of skin that covers and protects the human eye. The levator palpebrae superioris muscle retracts the eyelid to "open" th...

Organ of vision 1. Structure of eyeball 2. Visual accessory organs 3. Eyeball development  External anatomy Eyelids or palpebrae An eyelid is a thin fold of skin that covers and protects the human eye. The levator palpebrae superioris muscle retracts the eyelid to "open" the eye. This can be either voluntarily or involuntarily. The human eyelid features a row of eyelashes along the eyelid margin, which serve to heighten the protection of the eye from dust and foreign debris, as well as from perspiration. "Palpebral" (and "blepharal") means relating to the eyelids. Its key function is to regularly spread the tears and other secretions on the eye surface to keep it moist, since the cornea must be continuously moist. They keep the eyes from drying out when asleep. Moreover, the blink reflex protects the eye from foreign bodies  Features of eyelids The space between the two eyelids is the palpebral fissure. The two lids are fused with each other to form the medial and lateral angles or canthi of the eye. At the inner canthus, there is a small triangular space, the lacus lacrimalis. Within it, there is an elevated lacrimal caruncule, made up of modified skin and akin glands. Lateral to the caruncle, the bulbar conjunctiva is pinched up to form a vertical fold called the plica semilunaris. The free edge of eyelids is broad and has two lips. The outer lip presents two or more rows of eyelashes or cilia, except in the boundary of the lacus lacrimalis. At the point where eyelashes cease, there is a lacrimal papilla on the summit of which there is the lacrimal punctum. Each eyelids made up of: 1. The skin is thin and loose. Structu re of 2. The superficial fascia contains the palpebral part of the orbicularis oculi. Deep to the muscle is loose areolar tissue which is continuous with loose areolar tissue of the eyelids scalp. 3. The palpebral fascia of the two lids forms the orbital septum. Its thickenings form tarsal plates or tarsi in the lids and the palpebral ligaments at the angles. Tarsi are thin plates of condensed fibrous tissue located near the lid margins. They give stiffness to the lids (Muller’s muscles). Tarsal glands or meibomian glands are embedded in the posterior surface of the tarsi; their ducts open in a row behind the cilia. 4. The conjunctiva lines the posterior surface of the tarsus.  Glands of eyelids 1. Large sebaceous glands also called as Zeis’s glands at the lid margin associated with cilia. 2. Modified sweat glands or Moll’s glands at the lid margin closely associated with Zeis’s glands and cilia. 3. Sebaceous or tarsal glands are also known as meibomian glands.  Lacrimal apparatus It is made up of : 1. Lacrimal gland and its ducts 2. Conjunctival sac 3. Lacrimal puncta and lacrimal canaliculi 4. Lacrimal sac 5. Nasolacrimal duct.  Lacrimal gland It is a serous gland situated chiefly in the lacrimal fossa on the anterolateral part of the roof of the bony orbit and partly on the upper eyelid. Small accessory lacrimal glands are found in the conjunctival fornices. These are also called as Krause’s gland. The gland is “J” shaped, being indented by the tendon of the levator palpebrae superioris muscle. It has: 1. An orbital part which is larger and deeper 2. A palpebral part smaller and superficial, lying within the eyelid. About 10 of its ducts pierce the conjunctiva of the upper lid and open into the conjunctival sac near the superior fornix.  Conjunctival sac The conjunctiva is a mucous membrane that lines the inner surface of the eyelids and folds back to cover the anterior surface of the eyeball, except for its central portion (cornea). The potential space between the palpebral and bulbar parts is the conjunctival sac. The palpebral conjunctiva is thick, highly vascular. The bulbar conjunctiva covers the sclera. It is thin, transparent, and loosely attached to the eyeball. Over the cornea, it is represented by the anterior epithelium of the cornea.  Lacrimal puncta and canaliculi, lacrimal sac and nasolacrimal duct Each lacrimal canaliculi (the superior and inferior canaliculi) begins at the lacrimal punctum, collect tears, which flow into lacrimal sac, which lies in a deep groove of the lacrimal bone, and then into the lacrimal duct, which empties into the nasal cavity. A fold of mucous membrane called the valve of Hasner forms an imperfect valve at the lower end of the duct. Secretion of the lacrimal gland moistens and lubricates the surface of the eye and the lining of the lids. Tears also contain an enzyme (lysozyme) that is an antibacterial agent, reducing the risk of eye infections. Organ of vision The eye, lacrimal gland, and associated extrinsic muscle are housed within the pear-shaped orbital cavity of the skull. This orbit, which is lined with the periosteum of various bones, also contains fat, blood vessels, nerves, and connective tissues. Orbit  Structure of the eye The eye is a hollow, spherical structure about 2.5 cm in diameter. Its wall has three distinct layers – a fibrous outer tunic, a vascular middle tunic, and a nervous inner tunic. The spaces within the eye are filled with fluids that support its wall and internal parts, and help maintain its shape.  Outer tunic (fibrous tunic) Sclera Cornea The sclera is forms the The cornea is transparent. It posterior 5/6 of the eyeball. replaces the sclera over the It is composed of dense fibrous tissue which is anterior one-sixth of the maintains the shape of the eyeball. The cornea is more eyeball. The sclera protects convex than sclera. It is the eye and is an attachment for the extrinsic separated from the iris by a muscles. In the back of the space called the anterior eye, the optic nerve chamber of the eye. The pierces the sclera. Here the sclera shows numerous cornea is avascular and is perforations for passage of nourished by lymph which fibres of the optic nerve, circulates in the numerous this region is called the lamina cribrosa corneal spaces and by the (crib=sieve). lacrimal fluid. The outer surface of the sclera is white and smooth, it is covered by Tenon’s capsule. Its anterior part is covered by conjunctiva.  Outer tunic (fibrous tunic) Sclera Cornea The cornea consists of : The inner surface is brown and 1. Corneal epithelium (stratified grooved for the ciliary nerves squamous nonkeratinized and vessels. It is separated type) from the choroid by the 2. Bowman’s membrane or perichoroidal space which anterior elastic lamina. contains a delicate cellular tissue, termed the 3. The substantia propria. suprachoroidal lamina. 4. Descemet’s membrane or The sclera is continuous posterior elastic lamina. anteriorly with the cornea at 5. Simple squamous the sclerocorneal junction mesothelium. or limbus. The deep part of the limbus contains a circular canal (sinus venosus sclera or the canal of Schlemm). The sclera is almost avascular. However, the loose connective tissue between the conjunctiva and sclera called the episclera is vascular. Middle coat Choroid Ciliary body Iris  Choroid Choroid is a thin pigmented layer which separates the posterior part of the sclera from the retina. Anteriorly, it ends at the ora serrata by merging with the ciliary body. Posteriorly, it is perforated by the optic nerve to which it is firmly attached. Its outer surface is separated from the sclera by the suprachoroidal lamina which is traversed by the ciliary vessels and nerves. Its attachment to the sclera is loose, so that it can be easily stripped. The inner surface is firmly united to the retina  Ciliary body Ciliary body is a thickened part of middle coat, lying just posteriorly to the corneal limbus. It is continuous anteriorly with the iris and posteriorly with the choroid. It suspends the lens and helps it in accommodation for near vision. The ciliary body is triangular in cross- section. It is thick in front and thin behind. The scleral surface of this body contains the ciliary muscle. The posterior part of the vitreous surface is smooth and black (pars plana). The anterior part is ridged anteriorly (pars plicata) to form about 70 ciliary processes. The central ends of the processes are free and rounded. Ciliary zonule is thickened vitreous membrane fitted to the posterior surfaces of ciliary processes. The cilliary muscle is a ring unstripped muscle which are longitudinal, radial and circular. The contraction of ciliary muscle relaxes the suspensory ligament so that the lens becomes more convex. Ciliary body  Iris The iris is a thin diaphragm with an opening in the centre, called the pupil. Iris composed mostly of connective tissue, smooth muscle fibres, and blood vessels. From the outside, the iris is the colored portion of the eye. The iris extends forward from the periphery of the ciliary body and lies between the cornea and lens, and divides the anterior segment of the eye into anterior and posterior chamber, both containing aqueous humour. Its peripheral margin is attached to the middle of the anterior surface of the ciliary body and is separated from the cornea by the iridocorneal angle. The central free margin forming the boundary of the pupil. The iris contains a well-developed ring of muscle called the sphincter pupillae which lies near the margin of pupil. Its nerve supply (parasympathetic) is similar to that of the ciliary muscle. The dilatator pupilae is placed near the posterior surface of the iris. It is supplied by sympathetic nerve.  Inner coat (Retina) This is the thin, delicate inner layer of the eyeball. It is continuous posteriorly with the optic nerve. The outer surface of the retina (formed by pigment cells)is attached to the choroid, while the inner surface is in contact with the hyaloid membrane (of the vitreous). Inferomedial to the posterior pole there is a circular area known as the optic disc. Here, nerve fibers from the retina leave the eye and join the optic nerve. Because the optic disc region has no receptor cells, it is commonly known as the blind spot of the eye. The retina diminishes in thickness from behind forwards and is divided into optic, ciliary, and iridial parts. The optic part of the retina contains nervous tissue and is sensitive to light. It extends from the optic disc to the posterior end of the ciliary body. The anterior margin of the optic part of the retina forms a wavy line called the orra serrata. The ciliary and iridal parts of the retina that covers the ciliary body and iris are non-nervous insensitive parts.  Inner coat (Retina) At the posterior pole of the eye 3mm lateral to the optic disc, there is another depression called the macula lutea. It is avascular and yellow in colour. The center of the macula is depressed to form the fovea centralis. This is the thinnest part of the retina. It contains cones only. Fovea centralis is in the region of the retina that produces the sharpest vision. The retina is composed of ten layers: 1. The outer pigmented 2. Layer of cones and rods 3. External limiting membrane 4. Outer nuclear layer 5. Outer plexiform layer 6. Inner nuclear layer (bipolar cells) 7. Inner plexiform layer 8. Ganglion cell layer 9. Nerve fiber layer 10. The internal limiting membrane  Lens The lens is a transparent biconvex structure which is placed between anterior and posterior segments of the eye. The lens is enclosed in a transparent, structureless elastic capsule. Its elastic nature keeps it under constant tension. As a result, the lens can assume a globular shape. The suspensory ligaments attached to the margin of the capsule are also under tension. They pull outward, flattening the capsule and the lens inside. If the tension on the suspensory ligaments relaxes, the elastic capsule rebounds, and the lens surface becomes more convex. The ciliary muscle accomplish this. When the fibers contract, the choroid coat is pulled forward and the ciliary body shortens. This relaxes the suspensory ligaments; the lens thickens in respons and is now focused for viewing closer objects than before. To allow focus on more distant objects, the ciliary muscle relax, tension on the suspensory ligaments increases, and the lens becomes thinner and less convex again. This ability of the lens to adjust shape to facilitate focusing is called accommodation.  Aqueous humor This is a clear fluid is secreted into the posterior chamber from the capillaries in the cilliary processes. The fluid circulates from this chamber through the pupil into the anterior chamber. From the anterior chamber, it is drained into the anterior ciliary veins through the spaces of the iridocorneal angle (located between the fibres of the ligamentum pectinatum) and the canal of Schlemm. Aqueous humor aids in maintaining the shape of the front of the eye. It is also rich in ascorbic acid, glucose and amino acids, and nourishes the avascular tissues of the cornea and lens. Aqueous humor  Vitreous body It is a colourless, jelly-like transparent mass which fills the posterior segment of the eyeball. It is enclosed in a delicate homogeneous hyaloid membrane. Behind it is attached to the optic disc, and in front to the ora serrata; in between it is free and lies in contact with the retina. The anterior surface of the vitreous body is indented by the lens and ciliary processes. As a person ages, tiny, dense clumps of gel or deposits of crystal-like substances form in the vitreous humor. When these clumps cast shadows on the retina, the person sees small, moving specks in the field of vision. Such specks, known as floaters, are most apparent when looking at a plain background, such as the sky or a wall.  Extrinsic muscle 1. Four recti muscle: a) Superior recti b) Inferior recti c) Medial recti d) Lateral recti 2. Two obliques muscle: a) Superior oblique b) Inferior oblique The four recti arise from a common annular tendon or tendinous ring of Zinn. The ring is attached to the middle Origin part of superior orbital fissure. The lateral rectus has an additional small tendinous head which arises from the orbital surface of the greater wing of the sphenoid bone lateral to the tendinous ring. The superior oblique arises from the undersurface of lesser wing of the sphenoid, superomedial to the optic canal. The inferior oblique arises from the orbital surface of the maxilla, lateral to the lacrimal groove. The recti are inserted into the sclera, a little posterior to the limbus. Insertion The tendon of the superior oblique passes through a fibrocartilaginous pulley attached to the trochlear fossa of the frontal bone. The tendon then passes laterally, downwards and backward below the superior rectus. It is inserted into the sclera behind the equator of the eyeball, between the superior rectus and the lateral rectus. The inferior oblique passes lateraly, upwards and backwards below the inferior rectus and then deep to the lateral rectus. The inferior oblique is inserted close to the superior oblique a little below and posterior to the latter. Blood supply  Second cranial nerve Optic nerve, constituted by the axons of ganglion cells in retina, project from the nasal hemiretina to the contralateral lateral geniculate body and from the temporal hemiretina to the ipsilateral geniculate body. Optic nerve axons are covered by a membrane continuous with the dura and leave the orbit by passing through the optic canal. Optic chiasma contains decussating fibers from the two nasal hemiretinas and non-crossing fibers from the two temporal hemiretinas. Optic tract contains fibres from the ipsilateral temporal hemiretina and the contralateral nasal hemiretina. It projects to the ipsilateral lateral geniculate body, pretectal nuclei, and superior colliculus. Lesions of visual pathway Total anopsia-one side complete cut of n. opticus Bitemporal hemianopia-Chiasma opticum sagittal defect (Nasal retina of both eyes) Contralateral homonymus hemianopia-Visual pathways of behind of optic chiasma  Eyeball development Development of the eye involves a series of inductive interactions between neighboring tissues in the embryonic head. These are the neuroectoderm of the forebrain (which forms the sensory retina and accessory pigmented structures), the surface ectoderm (which forms the lens and the anterior corneal epithelium) and the intervening neural crest and their mesenchyme (which contributes to the fibrous coats of the eye and to tissues of the anterior segment of the eye) and the primary mesenchyme. Neuroectoderm of the forebrain evaginates to form the optic vesicle, which in turn invaginates to form the optic cup and optic stalk. The inner layer of the optic cup is made up neuroepithelium (neural retina), while the outer layer is composed of retinal pigment epithelium. The middle portion of the optic cup develops into the ciliary body and iris Optic cup forms: retina, epithelium of iris and ciliary body and iris muscle. Optic stalk forms the optic nerve, optic chiasma and optic tract.  Eyeball development The optic vesicle contact the surface ectoderm and induce the formation of lens placode, which eventually separates from the ectoderm to form the lens vesicle (and eye lens) at the open end of the optic cup. Surface ectoderm also forms the anterior epithelium of cornea. Through a groove at the bottom of the optic vesicle known as choroid fissure the hyaloid blood vessels enter the eye. Hyaloid artery and vein form the central artery and vein of the retina. The extracellular mesenchyme forms the sclera, the corneal endothelium and stroma, blood vessels, muscles and vitreous. The preotic myotomes, each supplied by its own nerve, form the extrinsic muscle of the eye. Conjunctiva is derived from surface ectoderm and so are the skin of eye and glands like lacrimal gland. Connective tissue and bony structure of the orbit are derived from neural crest cells. Questions 1. Retina consists of the following number of layers: a) Eight b) Ten c) Nine d) Eleven Questions 2. These can be found at the fovea centralis a) Cones b) Rods c) None of the above d) All receptors Questions 3. Smooth muscle, attaches to the lens a) Ciliary body b) Choroid c) Sclera d) Retina Questions 4. The innermost tunic of the eye, containing the photoreceptors a) Ciliary body b) Retina c) Sclera d) Cornea Questions 5. The white portion of the eye is the: a) Retina b) Sclera c) Cornea d) Iris