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43 Cards in this Set
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Anatomy of scalp |
The scalp consists of skin (normally hair bearing) and subcutaneous tissue that cover the neurocranium from the superior nuchal lines on the occipital bone to the supra-orbital margins of the frontal bone Laterally, the scalp extends over the temporal fascia to the zygomatic arches. The scalp is composed of five layers, the first three of which are connected intimately and move as a unit (e.g., when wrinkling the forehead and moving the scalp). Each letter in the word scalp serves as a memory key for one of its five layers |
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SKin of scalp |
Skin: Thick
containing many sweat and sebaceous glands hair follicles.
It has anabundant arterial supply and good venous and lymphatic drainage. |
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Connective tissue |
forms the thick, dense, richly vascularized subcutaneous layer that is well supplied with cutaneous nerves. |
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APONEUROSIS |
Aponeurosis (epicranial aponeurosis): the broad, strong, tendinous sheet that covers the calvaria and serves as the attachment for muscle bellies converging from the forehead and occiput (the occipitofrontalis muscle) (Fig. 7.15B) and from the temporal bones on each side (the temporoparietalis and superior auricular muscles). Collectively, these structures constitute the musculoaponeurotic epicranius. The frontal belly of the occipitofrontalis pulls the scalp anteriorly, wrinkles the forehead, and elevates the eyebrows; the occipital belly of the occipitofrontalis pulls the scalp posteriorly, smoothing the skin of the forehead. The superior auricular muscle (actually a specialized posterior part of the temporoparietalis) elevates the auricle of the external ear. All parts of the epicranius are innervated by the facial nerve. |
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Loose Aerolar tissue |
Loose areolar tissue: a sponge-like layer including potential spaces that may distend with fluid as a result of injury or infection. This layer allows free movement of the scalp proper (the first three layers—skin, connective tissue, and epicranial aponeurosis) over the underlying calvaria. |
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Pericranium |
Pericranium: a dense layer of connective tissue that forms the external periosteum of the neurocranium. It is firmly attached but can be stripped fairly easily from the crania of living persons, except where the pericranium is continuous with the fibrous tissue in the cranial sutures. |
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Muscles of Mouth lips and cheeks |
1. Levator labii superioris alaeque nasi. 2. Levator labii superioris. 3. Levator anguli oris. 4. Zygomaticus minor. 5. Zygomaticus major. 6. Depressor labii inferioris. 7. Depressor anguli oris. 8. Risorius. 9. Buccinator.
At rest, the lips are in gentle contact and the teeth are close together.
At rest, the lips are in gentle contact and the teeth are close together. The orbicularis oris, the first of the series of sphincters associated with the alimentary system (digestive tract), encircles the mouth within the lips, controlling entry and exit through the oral fissure (L. rima oris, the opening between the lips). The orbicularis oris is important during articulation (speech). At rest, the lips are in gentle contact and the teeth are close together. The orbicularis oris, the first of the series of sphincters associated with the alimentary system (digestive tract), encircles the mouth within the lips, controlling entry and exit through the oral fissure (L. rima oris, the opening between the lips). The orbicularis oris is important during articulation (speech). At rest, the lips are in gentle contact and the teeth are close together. The orbicularis oris, the first of the series of sphincters associated with the alimentary system (digestive tract), encircles the mouth within the lips, controlling entry and exit through the oral fissure (L. rima oris, the opening between the lips). The orbicularis oris is important during articulation (speech). At rest, the lips are in gentle contact and the teeth are close together. The orbicularis oris, the first of the series of sphincters associated with the alimentary system (digestive tract), encircles the mouth within the lips, controlling entry and exit through the oral fissure (L. rima oris, the opening between the lips). The orbicularis oris is important during articulation (speech). At rest, the lips are in gentle contact and the teeth are close together. The orbicularis oris, the first of the series of sphincters associated with the alimentary system (digestive tract), encircles the mouth within the lips, controlling entry and exit through the oral fissure (L. rima oris, the opening between the lips). The orbicularis oris is important during articulation (speech). At rest, the lips are in gentle contact and the teeth are close together. The orbicularis oris, the first of the series of sphincters associated with the alimentary system (digestive tract), encircles the mouth within the lips, controlling entry and exit through the oral fissure (L. rima oris, the opening between the lips). The orbicularis oris is important during articulation (speech). The orbicularis oris, the first of the series of sphincters associated with the alimentary system (digestive tract), encircles the mouth within the lips, controlling entry and exit through the oral fissure (L. rima oris, the opening between the lips).
The orbicularis oris is important during articulation (speech).
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Buccinator muscle |
BUCCINATOR MUSCLE The buccinator (L. trumpeter) is a thin, flat, rectangular muscle that attaches laterally to the alveolar processes of the maxillae and mandible, opposite the molar teeth, and to the pterygomandibular raphe, a tendinous thickening of the buccopharyngeal fascia separating and giving origin to the superior pharyngeal constrictor posteriorly. It occupies a deeper, more medially placed plane than the other facial muscles, passing deep to the mandible so that it is more closely related to the buccal mucosa than to the skin of the face. The buccinator, active in smiling, also keeps the cheek taut, thereby preventing it from folding and being injured during chewing. Anteriorly, the fibers of the buccinator mingle medially with those of the orbicularis oris, and the tonus of the two muscles compresses the cheeks and lips against the teeth and gums. The tonic contraction of the buccinator and especially of the orbicularis oris, provides a gentle but continual resistance to the tendency of the teeth to tilt in an outward direction. In the presence of a short upper lip, or retractors that remove this force, crooked or protrusive (“buck”) teeth develop. The orbicularis oris (from the labial aspect) and buccinator (from the buccal aspect) work with the tongue (from the lingual aspect) to keep food between the occlusal surfaces of the teeth during mastication (chewing) and to prevent food from accumulating in the oral vestibule. |
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Muscles of the orbital opening |
These include: 1. Orbicularis oculi. 2. Corrugator supercilii. These include: 1. Orbicularis oculi. 2. Corrugator supercilii. 3. Frontalis. 4. Levator palpebrae superioris. These include: 1. Orbicularis oculi. 2. Corrugator supercilii. 3. Frontalis. 4. Levator palpebrae superioris. These include: 1. Orbicularis oculi. 2. Corrugator supercilii. 3. Frontalis. 4. Levator palpebrae superioris. These include: 1. Orbicularis oculi. 2. Corrugator supercilii. 3. Frontalis. 4. Levator palpebrae superioris. These include: 1. Orbicularis oculi. 2. Corrugator supercilii. 3. Frontalis. 4. Levator palpebrae superioris. 3. Frontalis. 4. Levator palpebrae superioris. |
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Orbit of the eye |
The orbitsare bilateral bony cavities in the facial skeleton that resemble hollow quadrangular pyramids with their bases directed anterolaterally and their apices, posteromedially The medial walls of the two orbits, separated bythe ethmoidal sinuses and the upper parts of the nasal cavity,are nearly parallel, whereas their lateral walls are approxi-mately at a right (90°) angle. Consequently, the axes of the orbits (orbital axes) divergeat approximately 45°. The optical axes(axes of gaze, the directionor line of sight) for the two eyeballs, are parallel, how-ever, and in the anatomical position run directly anteriorly(“looking straight ahead”), the eyeballs being in the primary position. |
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Tenons capsule |
The fascia bulbi (Tenon’s capsule) is a loose membranous sheath that envelops the eyeball and extends from optic nerve to the sclerocorneal junction.
It is separated from the sclera by the episcleral space.
The Tenon’s capsule forms a socket for the eyeball to facilitate free ocular movements. The fascia bulbi is pierced by: (a) tendons of four recti and two oblique muscles of the eyeball, The fascia bulbi is pierced by: (a) tendons of four recti and two oblique muscles of the eyeball, (b) ciliary nerves and vessels around the entrance of the optic nerve. The fascia bulbi is pierced by: (a) tendons of four recti and two oblique muscles of the eyeball, (b) ciliary nerves and vessels around the entrance of the optic nerve. The fascia bulbi is pierced by: (a) tendons of four recti and two oblique muscles of the eyeball, (b) ciliary nerves and vessels around the entrance of the optic nerve. The fascia bulbi is pierced by: (a) tendons of four recti and two oblique muscles of the eyeball, (b) ciliary nerves and vessels around the entrance of the optic nerve. The fascia bulbi is pierced by: (a) tendons of four recti and two oblique muscles of the eyeball, (b) ciliary nerves and vessels around the entrance of the optic nerve. The fascia bulbi is pierced by: (a) tendons of four recti and two oblique muscles of the eyeball, (b) ciliary nerves and vessels around the entrance of the optic nerve. The fascia bulbi is pierced by: (a) tendons of four recti and two oblique muscles of the eyeball, (b) ciliary nerves and vessels around the entrance of the optic nerve. The fascia bulbi is pierced by: (a) tendons of four recti and two oblique muscles of the eyeball, (b) ciliary nerves and vessels around the entrance of the optic nerve. The fascia bulbi is pierced by: (a) tendons of four recti and two oblique muscles of the eyeball, (b) ciliary nerves and vessels around the entrance of the optic nerve. The fascia bulbi is pierced by: (a) tendons of four recti and two oblique muscles of the eyeball, (b) ciliary nerves and vessels around the entrance of the optic nerve. The fascia bulbi is pierced by: (a) tendons of four recti and two oblique muscles of the eyeball, (b) ciliary nerves and vessels around the entrance of the optic nerve. The fascia bulbi is pierced by: (a) tendons of four recti and two oblique muscles of the eyeball, (b) ciliary nerves and vessels around the entrance of the optic nerve. The fascia bulbi is pierced by: (a) tendons of four recti and two oblique muscles of the eyeball, (b) ciliary nerves and vessels around the entrance of the optic nerve. The fascia bulbi is pierced by: (a) tendons of four recti and two oblique muscles of the eyeball, (b) ciliary nerves and vessels around the entrance of the optic nerve. The fascia bulbi is pierced by: (a) tendons of four recti and two oblique muscles of the eyeball, (b) ciliary nerves and vessels around the entrance of the optic nerve. The fascia bulbi is pierced by: (a) tendons of four recti and two oblique muscles of the eyeball, (b) ciliary nerves and vessels around the entrance of the optic nerve. The fascia bulbi is pierced by: (a) tendons of four recti and two oblique muscles of the eyeball, (b) ciliary nerves and vessels around the entrance of the optic nerve. The fascia bulbi is pierced by: (a) tendons of four recti and two oblique muscles of the eyeball, (b) ciliary nerves and vessels around the entrance of the optic nerve. The fascia bulbi is pierced by: (a) tendons of four recti and two oblique muscles of the eyeball, (b) ciliary nerves and vessels around the entrance of the optic nerve. The fascia bulbi is pierced by: (a) tendons of four recti and two oblique muscles of the eyeball, (b) ciliary nerves and vessels around the entrance of the optic nerve. The fascia bulbi is pierced by: (a) tendons of four recti and two oblique muscles of the eyeball, (b) ciliary nerves and vessels around the entrance of the optic nerve. The fascia bulbi is pierced by: (a) tendons of four recti and two oblique muscles of the eyeball, (b) ciliary nerves and vessels around the entrance of the optic nerve. The fascia bulbi is pierced by: (a) tendons of four recti and two oblique muscles of the eyeball, (b) ciliary nerves and vessels around the entrance of the optic nerve. The fascia bulbi is pierced by: (a) tendons of four recti and two oblique muscles of the eyeball, (b) ciliary nerves and vessels around the entrance of the optic nerve. The fascia bulbi is pierced by: (a) tendons of four recti and two oblique muscles of the eyeball, (b) ciliary nerves and vessels around the entrance of the optic nerve. The fascia bulbi is pierced by: (a) tendons of four recti and two oblique muscles of the eyeball, (b) ciliary nerves and vessels around the entrance of the optic nerve. The fascia bulbi is pierced by: (a) tendons of four recti and two oblique muscles of the eyeball, (b) ciliary nerves and vessels around the entrance of the optic nerve. The fascia bulbi is pierced by: (a) tendons of four recti and two oblique muscles of the eyeball, (b) ciliary nerves and vessels around the entrance of the optic nerve. The fascia bulbi is pierced by: (a) tendons of four recti and two oblique muscles of the eyeball, (b) ciliary nerves and vessels around the entrance of the optic nerve. The fascia bulbi is pierced by: (a) tendons of four recti and two oblique muscles of the eyeball, (b) ciliary nerves and vessels around the entrance of the optic nerve. The fascia bulbi is pierced by: (a) tendons of four recti and two oblique muscles of the eyeball, (b) ciliary nerves and vessels around the entrance of the optic nerve. (b) ciliary nerves and vessels around the entrance of the optic nerve. |
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Roof of the orbit |
Roof •The superior wall (roof) is approximately horizontal and is formed mainly by the orbital part of the frontal bone, which separates the orbital cavity from the anterior cranial fossa. Near the apex of the orbit, the superior wall is formed by the lesser wing of the sphenoid. Anterolaterally, a shallow depression in the orbital part of the frontal bone, called the fossa for the lacrimal gland (lacrimal fossa), accommodates the lacrimal gland. |
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Medial wall |
Medial wall •The medial walls of the contralateral orbits are essentially parallel and are formed primarily by the orbital plate of the ethmoid bone, along with contributions from the 1. frontal process of the maxilla, 2.lacrimal,3. sphenoid bones. Features of medial wall Anteriorly, the medial wall is indented by the lacrimal groove and fossa for the lacrimal sac; the trochlea (pulley) for the tendon of one of the extraocular muscles is located superiorly. Much of the bone forming the medial wall is paper thin; the ethmoid bone is highly pneumatized with ethmoidal cells, often visible through the bone of a dried cranium. |
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Inferior wall of orbit |
•The inferior wall (orbital floor) is formed mainly by the maxilla and partly by the zygomatic and palatine bones. The thin inferior wall is shared by the orbit and maxillary sinus. It slants inferiorly from the apex to the inferior orbital margin. The inferior wall is demarcated from the lateral wall of the orbit by the inferior orbital fissure, a gap between the orbital surfaces of the maxilla and the sphenoid. |
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Lateral wall |
Lateral wall •The lateral wall is formed by the frontal process of the zygomatic bone and the greater wing of the sphenoid. This is the strongest and thickest wall, which is important because it is most exposed and vulnerable to direct trauma. Its posterior part separates the orbit from the temporal and middle cranial fossae. The lateral walls of the contralateral orbits are nearly perpendicular to each other. •The apex of the orbit is at the optic canal in the lesser wing of the sphenoid just medial to the superior orbital fissure. |
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Base |
The baseof the orbit is outlined by the orbital margin, which surrounds the orbital opening. The bone forming the orbital margin is reinforced to afford protection to the orbital contents and provides attachment for the orbital septum, a fibrous membrane that extends into the eyelids. |
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Apex of the orbit |
The apex of the orbit is at the optic canal in the lesser wing of the sphenoid just medial to the superior orbital fissure. |
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What is periorbitia? |
The bones forming the orbit are lined with periorbita, the periosteum of orbit. The periorbita is continuous: • At the optic canal and superior orbital fissure with the periosteal layer of the dura mater. •Over the orbital margins and through the inferior orbital fissure with the periosteum covering the external surface of the cranium (pericranium). •With the orbital septa at the orbital margins . •With the fascial sheaths of the extraocular muscles. •With the orbital fascia that forms the fascial sheath of the eyeball. |
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Eyelids |
Eyelids The eyelids are moveable folds that cover the eyeball anteriorly when closed, thereby protecting it from injury and excessive light.
They also keep the cornea moist by spreading the lacrimal fluid.
Conjunctiva
The eyelids are covered externally by thin skin and internally by transparent mucous membrane, the palpebral conjunctiva
This part of the conjunctiva is reflected onto the eyeball, where it is continuous with the bulbar conjunctiva.
This part of the conjunctiva is thin and transparent and attaches loosely to the anterior surface of the eyeball.
The bulbar conjunctiva, loose and wrinkled over the sclera (where it contains small, visible blood vessels), is adherent to the periphery of the cornea
Conjunctival fornices
The lines of reflection of the palpebral conjunctiva onto the eyeball form deep recesses, the superior and inferior conjunctival fornices
Conjuntival sac The conjunctival sac is the space bound by the palpebral and bulbar conjunctivae; it is a closed space when the eyelids are closed, but opens via an anterior aperture, the palpebral fissure (L. rima palpebrae, the gap between the eyelids), when the eye is open (eyelids are parted) .
The conjunctival sac is a specialized form of mucosal “bursa” that enables the eyelids to move freely over the surface of the eyeball as they open and close.
TARSI (dense band of connective tissue)
The superior (upper) and inferior (lower) eyelids are strengthened by dense bands of connective tissue, the superior and inferior tarsi (sing. = tarsus) which form the “skeleton” of the eyelids
Fibers of the palpebral portion of the orbicularis oculi (the sphincter of the palpebral fissure) are in the connective tissue superficial to these tarsi and deep to the skin of the eyelids
Tarsal glands Embedded in the tarsi are tarsal glands, the lipid secretion of which lubricates the edges of the eyelids and prevents them from sticking together when they close.
The lipid secretion also forms a barrier that lacrimal fluid does not cross when produced in normal amounts. When production is excessive, it spills over the barrier onto the cheeks as tears.
The eyelashes (L. cilia) are in the margins of the lids.
The large sebaceous glands associated with the eyelashes are ciliary glands.
Commisures The junctions of the superior and inferior eyelids make up the medial and lateral palpebral commissures, defining the medial and lateral angles of the eye (G. kanthos, corner of eye), or canthi
Palpebral ligaments Between the nose and the medial angle of the eye is the medial palpebral ligament, which connects the tarsi to the medial margin of the orbit (Fig. 7.47A).
The orbicularis oculi originates and inserts onto this ligament.
A similar lateral palpebral ligament attaches the tarsi to the lateral margin of the orbit but does not provide for direct muscle attachment Orbital septum The orbital septum is a fibrous membrane that spans from the tarsi to the margins of the orbit, where it becomes continuous with the periosteum (Figs. 7.45A and 7.47A). It keeps the orbital fat contained and, owing to its continuity with the periorbita, can limit the spread of infection to and from the orbit. The septum constitutes in large part the posterior fascia of the orbicularis oculi muscle. |
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Lacrimal apparatus |
The lacrimal apparatus (Figs. 7.46A and 7.47B) consists of the: •
Lacrimal gland: secretes lacrimal fluid, a watery physiological saline containing the bacteriocidal enzyme lysozyme.
The fluid moistens and lubricates the surfaces of the conjunctiva and cornea
provides some nutrients and dissolved oxygen to the cornea;
when produced in excess, it constitutes tears.
• Excretory ducts of lacrimal gland: convey lacrimal fluid from the lacrimal glands to the conjunctival sac (Fig. 7.46A).
• Lacrimal canaliculi (L. small canals): commence at a lacrimal punctum (opening) on the lacrimal papilla near the medial angle of the eye and drain lacrimal fluid from the lacrimal lake (L. lacus lacrimalis; a triangular space at the medial angle of the eye where the tears collect) to the lacrimal sac (the dilated superior part of the nasolacrimal duct) (Figs. 7.46A and 7.47B).
• Nasolacrimal duct:conveys the lacrimal fluid to the inferior nasal meatus (part of the nasal cavity inferior to the inferior nasal concha.
The lacrimal gland, almond shaped and approximately 2 cm long, lies in the fossa for the lacrimal gland in the superolateral part of each orbit (Figs. 7.44B, 7.46A, and 7.47B).
The gland is divided into superior (orbital) and inferior (palpebral) parts by the lateral expansion of the tendon of the levator palpebrae superioris (Fig. 7.46A).
Accessory lacrimal glands are also present, sometimes in the middle part of the eyelid or along the superior or inferior fornices of the conjunctival sac.
They are more numerous in the superior eyelid than in the inferior eyelid.
Production of lacrimal fluid is stimulated by parasympathetic impulses from CN VII. It is secreted through 8–12 excretory ducts, which open into the lateral part of the superior conjunctival fornix of the conjunctival sac.
The fluid flows inferiorly within the sac under the influence of gravity. When the cornea becomes dry, the eye blinks.
The eyelids come together in a lateral to medial sequence pushing a film of fluid medially over the cornea, somewhat like windshield wipers.
In this way, lacrimal fluid, containing foreign material such as dust is pushed toward the medial angle of the eye, accumulating in the lacrimal lake from which it drains by capillary action through the lacrimal puncta and lacrimal canaliculi to the lacrimal sac (Figs. 7.46A and 7.47B).
From this sac, the tears pass to the inferior nasal meatus of the nasal cavity through the nasolacrimal duct.
They drain posteriorly across the floor of the nasal cavity to the nasopharynx and are eventually swallowed.
In addition to cleansing particles and irritants from the conjunctival sac, lacrimal fluid provides the cornea with nutrients and oxygen.
The nerve supply of the lacrimal gland is both sympathetic and parasympathetic (Fig. 7.48). The presynaptic parasympathetic secretomotor fibers are conveyed from the facial nerve by the greater petrosal nerve and then by the nerve of the pterygoid canal to the pterygopalatine ganglion, where they synapse with the cell body of the postsynaptic fiber.
Vasoconstrictive, postsynaptic sympathetic fibers, brought from the superior cervical ganglion by the internal carotid plexus and deep petrosal nerve, join the parasympathetic f ibers to form the nerve of the pterygoid canal and traverse the pterygopalatine ganglion.
The zygomatic nerve (from the maxillary nerve) brings both types of fibers to the lacrimal branch of the ophthalmic nerve, by which they enter the gland (see Chapter 9). |
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Eye Ball |
1. The eyeball (L. oculus; Gk. ophthalmos) or globe of the eye is an organ of sight and closely resembles a camera in its structure. 2. It has light-sensitive retina and is provided with a lens system (cornea, lens, and refractive media) for focusing images and device for controlling the amount of light admitted (the iris diaphragm). 3. like a camera, its inside is black to prevent reflection of light 4. The eyeball is a highly durable structure for its wall enclosing the refractory media, 1. The eyeball (L. oculus; Gk. ophthalmos) or globe of the eye is an organ of sight and closely resembles a camera in its structure. 2. It has light-sensitive retina and is provided with a lens system (cornea, lens, and refractive media) for focusing images and device for controlling the amount of light admitted (the iris diaphragm). 3. like a camera, its inside is black to prevent reflection of light4. The eyeball is a highly durable structure for its wall enclosing the refractory media, 5. made up of three coats, and the fluid filled within it distributes hydraulic pressure uniformly to maintain its shape. 6. Location The eyeball occupies the anterior one-third of the orbital cavity and is embedded in the fat.7. It is enclosed in the thin fibrous sheath (Tenon’s fascia), which separates the eyeball from the fat. 8. The optic nerve emerges from it, a little medial to its posterior pole.9. Shape and Size It is almost spherical in shape and has a diameter of about 24 mm. 1. The eyeball (L. oculus; Gk. ophthalmos) or globe of the eye is an organ of sight and closely resembles a camera in its structure. 2. It has light-sensitive retina and is provided with a lens system (cornea, lens, and refractive media) for focusing images and device for controlling the amount of light admitted (the iris diaphragm). 3. like a camera, its inside is black to prevent reflection of light4. The eyeball is a highly durable structure for its wall enclosing the refractory media, 5. made up of three coats, and the fluid filled within it distributes hydraulic pressure uniformly to maintain its shape. 6. Location The eyeball occupies the anterior one-third of the orbital cavity and is embedded in the fat.7. It is enclosed in the thin fibrous sheath (Tenon’s fascia), which separates the eyeball from the fat. 8. The optic nerve emerges from it, a little medial to its posterior pole.9. Shape and Size It is almost spherical in shape and has a diameter of about 24 mm. 1. The eyeball (L. oculus; Gk. ophthalmos) or globe of the eye is an organ of sight and closely resembles a camera in its structure. 2. It has light-sensitive retina and is provided with a lens system (cornea, lens, and refractive media) for focusing images and device for controlling the amount of light admitted (the iris diaphragm). 3. like a camera, its inside is black to prevent reflection of light4. The eyeball is a highly durable structure for its wall enclosing the refractory media, 5. made up of three coats, and the fluid filled within it distributes hydraulic pressure uniformly to maintain its shape. 6. Location The eyeball occupies the anterior one-third of the orbital cavity and is embedded in the fat.7. It is enclosed in the thin fibrous sheath (Tenon’s fascia), which separates the eyeball from the fat. 8. The optic nerve emerges from it, a little medial to its posterior pole.9. Shape and Size It is almost spherical in shape and has a diameter of about 24 mm. 1. The eyeball (L. oculus; Gk. ophthalmos) or globe of the eye is an organ of sight and closely resembles a camera in its structure. 2. It has light-sensitive retina and is provided with a lens system (cornea, lens, and refractive media) for focusing images and device for controlling the amount of light admitted (the iris diaphragm). 3. like a camera, its inside is black to prevent reflection of light4. The eyeball is a highly durable structure for its wall enclosing the refractory media, 5. made up of three coats, and the fluid filled within it distributes hydraulic pressure uniformly to maintain its shape. 6. Location The eyeball occupies the anterior one-third of the orbital cavity and is embedded in the fat.7. It is enclosed in the thin fibrous sheath (Tenon’s fascia), which separates the eyeball from the fat. 8. The optic nerve emerges from it, a little medial to its posterior pole.9. Shape and Size It is almost spherical in shape and has a diameter of about 24 mm. 1. The eyeball (L. oculus; Gk. ophthalmos) or globe of the eye is an organ of sight and closely resembles a camera in its structure. 2. It has light-sensitive retina and is provided with a lens system (cornea, lens, and refractive media) for focusing images and device for controlling the amount of light admitted (the iris diaphragm). 3. like a camera, its inside is black to prevent reflection of light4. The eyeball is a highly durable structure for its wall enclosing the refractory media, 5. made up of three coats, and the fluid filled within it distributes hydraulic pressure uniformly to maintain its shape. 6. Location The eyeball occupies the anterior one-third of the orbital cavity and is embedded in the fat.7. It is enclosed in the thin fibrous sheath (Tenon’s fascia), which separates the eyeball from the fat. 8. The optic nerve emerges from it, a little medial to its posterior pole.9. Shape and Size It is almost spherical in shape and has a diameter of about 24 mm. 1. The eyeball (L. oculus; Gk. ophthalmos) or globe of the eye is an organ of sight and closely resembles a camera in its structure. 2. It has light-sensitive retina and is provided with a lens system (cornea, lens, and refractive media) for focusing images and device for controlling the amount of light admitted (the iris diaphragm). 3. like a camera, its inside is black to prevent reflection of light4. The eyeball is a highly durable structure for its wall enclosing the refractory media, 5. made up of three coats, and the fluid filled within it distributes hydraulic pressure uniformly to maintain its shape. 6. Location The eyeball occupies the anterior one-third of the orbital cavity and is embedded in the fat.7. It is enclosed in the thin fibrous sheath (Tenon’s fascia), which separates the eyeball from the fat. 8. The optic nerve emerges from it, a little medial to its posterior pole.9. Shape and Size It is almost spherical in shape and has a diameter of about 24 mm. 1. The eyeball (L. oculus; Gk. ophthalmos) or globe of the eye is an organ of sight and closely resembles a camera in its structure. 2. It has light-sensitive retina and is provided with a lens system (cornea, lens, and refractive media) for focusing images and device for controlling the amount of light admitted (the iris diaphragm). 3. like a camera, its inside is black to prevent reflection of light4. The eyeball is a highly durable structure for its wall enclosing the refractory media, 5. made up of three coats, and the fluid filled within it distributes hydraulic pressure uniformly to maintain its shape. 6. Location The eyeball occupies the anterior one-third of the orbital cavity and is embedded in the fat.7. It is enclosed in the thin fibrous sheath (Tenon’s fascia), which separates the eyeball from the fat. 8. The optic nerve emerges from it, a little medial to its posterior pole.9. Shape and Size It is almost spherical in shape and has a diameter of about 24 mm. 1. The eyeball (L. oculus; Gk. ophthalmos) or globe of the eye is an organ of sight and closely resembles a camera in its structure. 2. It has light-sensitive retina and is provided with a lens system (cornea, lens, and refractive media) for focusing images and device for controlling the amount of light admitted (the iris diaphragm). 3. like a camera, its inside is black to prevent reflection of light4. The eyeball is a highly durable structure for its wall enclosing the refractory media, 5. made up of three coats, and the fluid filled within it distributes hydraulic pressure uniformly to maintain its shape. 6. Location The eyeball occupies the anterior one-third of the orbital cavity and is embedded in the fat.7. It is enclosed in the thin fibrous sheath (Tenon’s fascia), which separates the eyeball from the fat. 8. The optic nerve emerges from it, a little medial to its posterior pole.9. Shape and Size It is almost spherical in shape and has a diameter of about 24 mm. 1. The eyeball (L. oculus; Gk. ophthalmos) or globe of the eye is an organ of sight and closely resembles a camera in its structure. 2. It has light-sensitive retina and is provided with a lens system (cornea, lens, and refractive media) for focusing images and device for controlling the amount of light admitted (the iris diaphragm). 3. like a camera, its inside is black to prevent reflection of light4. The eyeball is a highly durable structure for its wall enclosing the refractory media, 5. made up of three coats, and the fluid filled within it distributes hydraulic pressure uniformly to maintain its shape. 6. Location The eyeball occupies the anterior one-third of the orbital cavity and is embedded in the fat.7. It is enclosed in the thin fibrous sheath (Tenon’s fascia), which separates the eyeball from the fat. 8. The optic nerve emerges from it, a little medial to its posterior pole.9. Shape and Size It is almost spherical in shape and has a diameter of about 24 mm. 1. The eyeball (L. oculus; Gk. ophthalmos) or globe of the eye is an organ of sight and closely resembles a camera in its structure. 2. It has light-sensitive retina and is provided with a lens system (cornea, lens, and refractive media) for focusing images and device for controlling the amount of light admitted (the iris diaphragm). 3. like a camera, its inside is black to prevent reflection of light4. The eyeball is a highly durable structure for its wall enclosing the refractory media, 5. made up of three coats, and the fluid filled within it distributes hydraulic pressure uniformly to maintain its shape. 6. Location The eyeball occupies the anterior one-third of the orbital cavity and is embedded in the fat.7. It is enclosed in the thin fibrous sheath (Tenon’s fascia), which separates the eyeball from the fat. 8. The optic nerve emerges from it, a little medial to its posterior pole.9. Shape and Size It is almost spherical in shape and has a diameter of about 24 mm. 1. The eyeball (L. oculus; Gk. ophthalmos) or globe of the eye is an organ of sight and closely resembles a camera in its structure. 2. It has light-sensitive retina and is provided with a lens system (cornea, lens, and refractive media) for focusing images and device for controlling the amount of light admitted (the iris diaphragm). 3. like a camera, its inside is black to prevent reflection of light4. The eyeball is a highly durable structure for its wall enclosing the refractory media, 5. made up of three coats, and the fluid filled within it distributes hydraulic pressure uniformly to maintain its shape. 6. Location The eyeball occupies the anterior one-third of the orbital cavity and is embedded in the fat.7. It is enclosed in the thin fibrous sheath (Tenon’s fascia), which separates the eyeball from the fat. 8. The optic nerve emerges from it, a little medial to its posterior pole.9. Shape and Size It is almost spherical in shape and has a diameter of about 24 mm. 1. The eyeball (L. oculus; Gk. ophthalmos) or globe of the eye is an organ of sight and closely resembles a camera in its structure. 2. It has light-sensitive retina and is provided with a lens system (cornea, lens, and refractive media) for focusing images and device for controlling the amount of light admitted (the iris diaphragm). 3. like a camera, its inside is black to prevent reflection of light4. The eyeball is a highly durable structure for its wall enclosing the refractory media, 5. made up of three coats, and the fluid filled within it distributes hydraulic pressure uniformly to maintain its shape. 6. Location The eyeball occupies the anterior one-third of the orbital cavity and is embedded in the fat.7. It is enclosed in the thin fibrous sheath (Tenon’s fascia), which separates the eyeball from the fat. 8. The optic nerve emerges from it, a little medial to its posterior pole.9. Shape and Size It is almost spherical in shape and has a diameter of about 24 mm. 1. The eyeball (L. oculus; Gk. ophthalmos) or globe of the eye is an organ of sight and closely resembles a camera in its structure. 2. It has light-sensitive retina and is provided with a lens system (cornea, lens, and refractive media) for focusing images and device for controlling the amount of light admitted (the iris diaphragm). 3. like a camera, its inside is black to prevent reflection of light4. The eyeball is a highly durable structure for its wall enclosing the refractory media, 5. made up of three coats, and the fluid filled within it distributes hydraulic pressure uniformly to maintain its shape. 6. Location The eyeball occupies the anterior one-third of the orbital cavity and is embedded in the fat.7. It is enclosed in the thin fibrous sheath (Tenon’s fascia), which separates the eyeball from the fat. 8. The optic nerve emerges from it, a little medial to its posterior pole.9. Shape and Size It is almost spherical in shape and has a diameter of about 24 mm. 1. The eyeball (L. oculus; Gk. ophthalmos) or globe of the eye is an organ of sight and closely resembles a camera in its structure. 2. It has light-sensitive retina and is provided with a lens system (cornea, lens, and refractive media) for focusing images and device for controlling the amount of light admitted (the iris diaphragm). 3. like a camera, its inside is black to prevent reflection of light4. The eyeball is a highly durable structure for its wall enclosing the refractory media, 5. made up of three coats, and the fluid filled within it distributes hydraulic pressure uniformly to maintain its shape. 6. Location The eyeball occupies the anterior one-third of the orbital cavity and is embedded in the fat.7. It is enclosed in the thin fibrous sheath (Tenon’s fascia), which separates the eyeball from the fat. 8. The optic nerve emerges from it, a little medial to its posterior pole.9. Shape and Size It is almost spherical in shape and has a diameter of about 24 mm. 1. The eyeball (L. oculus; Gk. ophthalmos) or globe of the eye is an organ of sight and closely resembles a camera in its structure. 2. It has light-sensitive retina and is provided with a lens system (cornea, lens, and refractive media) for focusing images and device for controlling the amount of light admitted (the iris diaphragm). 3. like a camera, its inside is black to prevent reflection of light4. The eyeball is a highly durable structure for its wall enclosing the refractory media, 5. made up of three coats, and the fluid filled within it distributes hydraulic pressure uniformly to maintain its shape. 6. Location The eyeball occupies the anterior one-third of the orbital cavity and is embedded in the fat.7. It is enclosed in the thin fibrous sheath (Tenon’s fascia), which separates the eyeball from the fat. 8. The optic nerve emerges from it, a little medial to its posterior pole.9. Shape and Size It is almost spherical in shape and has a diameter of about 24 mm. 1. The eyeball (L. oculus; Gk. ophthalmos) or globe of the eye is an organ of sight and closely resembles a camera in its structure. 2. It has light-sensitive retina and is provided with a lens system (cornea, lens, and refractive media) for focusing images and device for controlling the amount of light admitted (the iris diaphragm). 3. like a camera, its inside is black to prevent reflection of light4. The eyeball is a highly durable structure for its wall enclosing the refractory media, 5. made up of three coats, and the fluid filled within it distributes hydraulic pressure uniformly to maintain its shape. 6. Location The eyeball occupies the anterior one-third of the orbital cavity and is embedded in the fat.7. It is enclosed in the thin fibrous sheath (Tenon’s fascia), which separates the eyeball from the fat. 8. The optic nerve emerges from it, a little medial to its posterior pole.9. Shape and Size It is almost spherical in shape and has a diameter of about 24 mm. 1. The eyeball (L. oculus; Gk. ophthalmos) or globe of the eye is an organ of sight and closely resembles a camera in its structure. 2. It has light-sensitive retina and is provided with a lens system (cornea, lens, and refractive media) for focusing images and device for controlling the amount of light admitted (the iris diaphragm). 3. like a camera, its inside is black to prevent reflection of light4. The eyeball is a highly durable structure for its wall enclosing the refractory media, 5. made up of three coats, and the fluid filled within it distributes hydraulic pressure uniformly to maintain its shape. 6. Location The eyeball occupies the anterior one-third of the orbital cavity and is embedded in the fat.7. It is enclosed in the thin fibrous sheath (Tenon’s fascia), which separates the eyeball from the fat. 8. The optic nerve emerges from it, a little medial to its posterior pole.9. Shape and Size It is almost spherical in shape and has a diameter of about 24 mm. 1. The eyeball (L. oculus; Gk. ophthalmos) or globe of the eye is an organ of sight and closely resembles a camera in its structure. 2. It has light-sensitive retina and is provided with a lens system (cornea, lens, and refractive media) for focusing images and device for controlling the amount of light admitted (the iris diaphragm). 3. like a camera, its inside is black to prevent reflection of light4. The eyeball is a highly durable structure for its wall enclosing the refractory media, 5. made up of three coats, and the fluid filled within it distributes hydraulic pressure uniformly to maintain its shape. 6. Location The eyeball occupies the anterior one-third of the orbital cavity and is embedded in the fat.7. It is enclosed in the thin fibrous sheath (Tenon’s fascia), which separates the eyeball from the fat. 8. The optic nerve emerges from it, a little medial to its posterior pole.9. Shape and Size It is almost spherical in shape and has a diameter of about 24 mm. 1. The eyeball (L. oculus; Gk. ophthalmos) or globe of the eye is an organ of sight and closely resembles a camera in its structure. 2. It has light-sensitive retina and is provided with a lens system (cornea, lens, and refractive media) for focusing images and device for controlling the amount of light admitted (the iris diaphragm). 3. like a camera, its inside is black to prevent reflection of light4. The eyeball is a highly durable structure for its wall enclosing the refractory media, 5. made up of three coats, and the fluid filled within it distributes hydraulic pressure uniformly to maintain its shape. 6. Location The eyeball occupies the anterior one-third of the orbital cavity and is embedded in the fat.7. It is enclosed in the thin fibrous sheath (Tenon’s fascia), which separates the eyeball from the fat. 8. The optic nerve emerges from it, a little medial to its posterior pole.9. Shape and Size It is almost spherical in shape and has a diameter of about 24 mm. 1. The eyeball (L. oculus; Gk. ophthalmos) or globe of the eye is an organ of sight and closely resembles a camera in its structure. 2. It has light-sensitive retina and is provided with a lens system (cornea, lens, and refractive media) for focusing images and device for controlling the amount of light admitted (the iris diaphragm). 3. like a camera, its inside is black to prevent reflection of light4. The eyeball is a highly durable structure for its wall enclosing the refractory media, 5. made up of three coats, and the fluid filled within it distributes hydraulic pressure uniformly to maintain its shape. 6. Location The eyeball occupies the anterior one-third of the orbital cavity and is embedded in the fat.7. It is enclosed in the thin fibrous sheath (Tenon’s fascia), which separates the eyeball from the fat. 8. The optic nerve emerges from it, a little medial to its posterior pole.9. Shape and Size It is almost spherical in shape and has a diameter of about 24 mm. 1. The eyeball (L. oculus; Gk. ophthalmos) or globe of the eye is an organ of sight and closely resembles a camera in its structure. 2. It has light-sensitive retina and is provided with a lens system (cornea, lens, and refractive media) for focusing images and device for controlling the amount of light admitted (the iris diaphragm). 3. like a camera, its inside is black to prevent reflection of light4. The eyeball is a highly durable structure for its wall enclosing the refractory media, 5. made up of three coats, and the fluid filled within it distributes hydraulic pressure uniformly to maintain its shape. 6. Location The eyeball occupies the anterior one-third of the orbital cavity and is embedded in the fat.7. It is enclosed in the thin fibrous sheath (Tenon’s fascia), which separates the eyeball from the fat. 8. The optic nerve emerges from it, a little medial to its posterior pole.9. Shape and Size It is almost spherical in shape and has a diameter of about 24 mm. 1. The eyeball (L. oculus; Gk. ophthalmos) or globe of the eye is an organ of sight and closely resembles a camera in its structure. 2. It has light-sensitive retina and is provided with a lens system (cornea, lens, and refractive media) for focusing images and device for controlling the amount of light admitted (the iris diaphragm). 3. like a camera, its inside is black to prevent reflection of light4. The eyeball is a highly durable structure for its wall enclosing the refractory media, 5. made up of three coats, and the fluid filled within it distributes hydraulic pressure uniformly to maintain its shape. 6. Location The eyeball occupies the anterior one-third of the orbital cavity and is embedded in the fat.7. It is enclosed in the thin fibrous sheath (Tenon’s fascia), which separates the eyeball from the fat. 8. The optic nerve emerges from it, a little medial to its posterior pole.9. Shape and Size It is almost spherical in shape and has a diameter of about 24 mm. 1. The eyeball (L. oculus; Gk. ophthalmos) or globe of the eye is an organ of sight and closely resembles a camera in its structure. 2. It has light-sensitive retina and is provided with a lens system (cornea, lens, and refractive media) for focusing images and device for controlling the amount of light admitted (the iris diaphragm). 3. like a camera, its inside is black to prevent reflection of light4. The eyeball is a highly durable structure for its wall enclosing the refractory media, 5. made up of three coats, and the fluid filled within it distributes hydraulic pressure uniformly to maintain its shape. 6. Location The eyeball occupies the anterior one-third of the orbital cavity and is embedded in the fat.7. It is enclosed in the thin fibrous sheath (Tenon’s fascia), which separates the eyeball from the fat. 8. The optic nerve emerges from it, a little medial to its posterior pole.9. Shape and Size It is almost spherical in shape and has a diameter of about 24 mm. 1. The eyeball (L. oculus; Gk. ophthalmos) or globe of the eye is an organ of sight and closely resembles a camera in its structure. 2. It has light-sensitive retina and is provided with a lens system (cornea, lens, and refractive media) for focusing images and device for controlling the amount of light admitted (the iris diaphragm). 3. like a camera, its inside is black to prevent reflection of light4. The eyeball is a highly durable structure for its wall enclosing the refractory media, 5. made up of three coats, and the fluid filled within it distributes hydraulic pressure uniformly to maintain its shape. 6. Location The eyeball occupies the anterior one-third of the orbital cavity and is embedded in the fat.7. It is enclosed in the thin fibrous sheath (Tenon’s fascia), which separates the eyeball from the fat. 8. The optic nerve emerges from it, a little medial to its posterior pole.9. Shape and Size It is almost spherical in shape and has a diameter of about 24 mm. 1. The eyeball (L. oculus; Gk. ophthalmos) or globe of the eye is an organ of sight and closely resembles a camera in its structure. 2. It has light-sensitive retina and is provided with a lens system (cornea, lens, and refractive media) for focusing images and device for controlling the amount of light admitted (the iris diaphragm). 3. like a camera, its inside is black to prevent reflection of light4. The eyeball is a highly durable structure for its wall enclosing the refractory media, 5. made up of three coats, and the fluid filled within it distributes hydraulic pressure uniformly to maintain its shape. 6. Location The eyeball occupies the anterior one-third of the orbital cavity and is embedded in the fat.7. It is enclosed in the thin fibrous sheath (Tenon’s fascia), which separates the eyeball from the fat. 8. The optic nerve emerges from it, a little medial to its posterior pole.9. Shape and Size It is almost spherical in shape and has a diameter of about 24 mm. 1. The eyeball (L. oculus; Gk. ophthalmos) or globe of the eye is an organ of sight and closely resembles a camera in its structure. 2. It has light-sensitive retina and is provided with a lens system (cornea, lens, and refractive media) for focusing images and device for controlling the amount of light admitted (the iris diaphragm). 3. like a camera, its inside is black to prevent reflection of light4. The eyeball is a highly durable structure for its wall enclosing the refractory media, 5. made up of three coats, and the fluid filled within it distributes hydraulic pressure uniformly to maintain its shape. 6. Location The eyeball occupies the anterior one-third of the orbital cavity and is embedded in the fat.7. It is enclosed in the thin fibrous sheath (Tenon’s fascia), which separates the eyeball from the fat. 8. The optic nerve emerges from it, a little medial to its posterior pole.9. Shape and Size It is almost spherical in shape and has a diameter of about 24 mm. 1. The eyeball (L. oculus; Gk. ophthalmos) or globe of the eye is an organ of sight and closely resembles a camera in its structure. 2. It has light-sensitive retina and is provided with a lens system (cornea, lens, and refractive media) for focusing images and device for controlling the amount of light admitted (the iris diaphragm). 3. like a camera, its inside is black to prevent reflection of light4. The eyeball is a highly durable structure for its wall enclosing the refractory media, 5. made up of three coats, and the fluid filled within it distributes hydraulic pressure uniformly to maintain its shape. 6. Location The eyeball occupies the anterior one-third of the orbital cavity and is embedded in the fat.7. It is enclosed in the thin fibrous sheath (Tenon’s fascia), which separates the eyeball from the fat. 8. The optic nerve emerges from it, a little medial to its posterior pole.9. Shape and Size It is almost spherical in shape and has a diameter of about 24 mm. 1. The eyeball (L. oculus; Gk. ophthalmos) or globe of the eye is an organ of sight and closely resembles a camera in its structure. 2. It has light-sensitive retina and is provided with a lens system (cornea, lens, and refractive media) for focusing images and device for controlling the amount of light admitted (the iris diaphragm). 3. like a camera, its inside is black to prevent reflection of light4. The eyeball is a highly durable structure for its wall enclosing the refractory media, 5. made up of three coats, and the fluid filled within it distributes hydraulic pressure uniformly to maintain its shape. 6. Location The eyeball occupies the anterior one-third of the orbital cavity and is embedded in the fat.7. It is enclosed in the thin fibrous sheath (Tenon’s fascia), which separates the eyeball from the fat. 8. The optic nerve emerges from it, a little medial to its posterior pole.9. Shape and Size It is almost spherical in shape and has a diameter of about 24 mm. 1. The eyeball (L. oculus; Gk. ophthalmos) or globe of the eye is an organ of sight and closely resembles a camera in its structure. 2. It has light-sensitive retina and is provided with a lens system (cornea, lens, and refractive media) for focusing images and device for controlling the amount of light admitted (the iris diaphragm). 3. like a camera, its inside is black to prevent reflection of light4. The eyeball is a highly durable structure for its wall enclosing the refractory media, 5. made up of three coats, and the fluid filled within it distributes hydraulic pressure uniformly to maintain its shape. 6. Location The eyeball occupies the anterior one-third of the orbital cavity and is embedded in the fat.7. It is enclosed in the thin fibrous sheath (Tenon’s fascia), which separates the eyeball from the fat. 8. The optic nerve emerges from it, a little medial to its posterior pole.9. Shape and Size It is almost spherical in shape and has a diameter of about 24 mm. 1. The eyeball (L. oculus; Gk. ophthalmos) or globe of the eye is an organ of sight and closely resembles a camera in its structure. 2. It has light-sensitive retina and is provided with a lens system (cornea, lens, and refractive media) for focusing images and device for controlling the amount of light admitted (the iris diaphragm). 3. like a camera, its inside is black to prevent reflection of light4. The eyeball is a highly durable structure for its wall enclosing the refractory media, 5. made up of three coats, and the fluid filled within it distributes hydraulic pressure uniformly to maintain its shape. 6. Location The eyeball occupies the anterior one-third of the orbital cavity and is embedded in the fat.7. It is enclosed in the thin fibrous sheath (Tenon’s fascia), which separates the eyeball from the fat. 8. The optic nerve emerges from it, a little medial to its posterior pole.9. Shape and Size It is almost spherical in shape and has a diameter of about 24 mm. 1. The eyeball (L. oculus; Gk. ophthalmos) or globe of the eye is an organ of sight and closely resembles a camera in its structure. 2. It has light-sensitive retina and is provided with a lens system (cornea, lens, and refractive media) for focusing images and device for controlling the amount of light admitted (the iris diaphragm). 3. like a camera, its inside is black to prevent reflection of light4. The eyeball is a highly durable structure for its wall enclosing the refractory media, 5. made up of three coats, and the fluid filled within it distributes hydraulic pressure uniformly to maintain its shape. 6. Location The eyeball occupies the anterior one-third of the orbital cavity and is embedded in the fat.7. It is enclosed in the thin fibrous sheath (Tenon’s fascia), which separates the eyeball from the fat. 8. The optic nerve emerges from it, a little medial to its posterior pole.9. Shape and Size It is almost spherical in shape and has a diameter of about 24 mm. 5. made up of three coats, and the fluid filled within it distributes hydraulic pressure uniformly to maintain its shape. 6. Location The eyeball occupies the anterior one-third of the orbital cavity and is embedded in the fat. 7. It is enclosed in the thin fibrous sheath (Tenon’s fascia), which separates the eyeball from the fat. 8. The optic nerve emerges from it, a little medial to its posterior pole. 9. . Shape and Size It is almost spherical in shape and has a diameter of about 24 mm. |
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Fibrous layer of eyeball |
The fibrous layer of the eyeball is the external fibrous skeleton of the eyeball, providing shape and resistance.
The sclera is the tough opaque part of the fibrous layer (coat) of the eyeball, covering the posterior five sixths of the eyeball and providing attachment for both the extrinsic (extraocular) and the intrinsic muscles of the eye.
The anterior part of the sclera is visible through the transparent bulbar conjunctiva as “the white of the eye”
The cornea is the transparent part of the fibrous layer covering the anterior one sixth of the eyeball
The convexity of the cornea is greater than that of the sclera, and so it appears to protrude from the eyeball when viewed laterally.
The two parts of the fibrous coat differ primarily in terms of the regularity of the arrangement of the collagen fibers of which they are composed and the degree of hydration of each.
While the sclera is relatively avascular, the cornea is completely avascular, receiving its nourishment from capillary beds around its periphery and fluids on its external and internal surfaces (lacrimal fluid and aqueous humor, respectively).
Lacrimal fluid also provides oxygen absorbed from the air.
The cornea is highly sensitive to touch, its innervation is provided by the ophthalmic nerve (CN V1). Even very small foreign bodies (e.g., dust particles) elicit blinking, flow of tears, and sometimes severe pain.
Its nourishment is derived from the capillary beds at its periphery, the aqueous humor, and lacrimal fluid.
The latter also provides oxygen absorbed from air.
Drying of the corneal surface may cause ulceration.
Corneal limbus The corneal limbus is the angle formed by the intersecting curvatures of sclera and cornea at the corneoscleral junction The junction is a 1-mm-wide, gray, translucent circle that includes numerous capillary loops involved in nourishing the avascular cornea. |
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Vascular layer of eyeball |
The vascular layer of the eyeball (also called the uvea or uveal tract) consists of the choroid, ciliary body, and iris (Fig. 7.49B).
The choroid, a dark reddish brown layer between the sclera and the retina, forms the largest part of the vascular layer of the eyeball and lines most of the sclera (Fig. 7.50A).
Within this pigmented and dense vascular bed, larger vessels are located externally (near the sclera). The finest vessels (the capillary lamina of the choroid, or choriocapillaris, an extensive capillary bed) are innermost, adjacent to the avascular light-sensitive layer of the retina, which it supplies with oxygen and nutrients.
Engorged with blood in life (it has the highest perfusion rate per gram of tissue of all vascular beds of the body),
this layer is responsible for the “red eye” reflection that occurs in flash photography.
The choroid attaches firmly to the pigment layer of the retina, but it can easily be stripped from the sclera.
The choroid is continuous anteriorly with the ciliary body.
The ciliary body, is a ring-like thickening of the layer posterior to the corneoscleral junction, which is muscular as well as vascular (Figs. 7.49B and 7.50).
It connects the choroid with the circumference of the iris.
The ciliary body provides attachment for the lens.
The contraction and relaxation of the circularly arranged smooth muscle of the ciliary body controls the thickness, and therefore the focus, of the lens.
Folds on the internal surface of the ciliary body, the ciliary processes, secrete aqueous humor.
Aqueous humor fills the anterior segment of the eyeball,
the interior of the eyeball anterior to the lens, suspensory ligament, and ciliary body.
(Fig. 7.50B).
The iris, which literally lies on the anterior surface of the lens, is a thin contractile diaphragm with a central aperture, the pupil, for transmitting light (Figs. 7.49B, 7.50, and 7.51A).
When a person is awake, the size of the pupil varies continually to regulate the amount of light entering the eye (Fig. 7.51B).
Two involuntary muscles control the size of the pupil: the parasympathetically stimulated, circularly arranged sphincter pupillae decreases its diameter (constrict or contracts the pupil, pupillary miosis),
the sympathetically stimulated, radially arranged dilator pupillae increases its diameter (dilates the pupil).
The nature of the pupillary responses is paradoxical: sympathetic responses usually occur immediately, yet it may take up to 20 min for the pupil to dilate in response to low lighting, as in a darkened theater.
Parasympathetic responses are typically slower than sympathetic responses, yet parasympathetically stimulated papillary constriction is normally instantaneous.
Abnormal sustained pupillary dilation (mydriasis) may occur in certain diseases or as a result of trauma or the use of certain drugs. |
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Inner layer of eyeball |
The inner layer of the eyeball is the retina
It is the sensory neural layer of the eyeball.
Grossly, the retina consists of two functional parts with distinct locations: an optic part and a non-visual retina.
The optic part of the retina is sensitive to visual light rays and has two layers: a neural layer and pigmented layer. The neural layer is light receptive. The pigmented layer consists of a single layer of cells that reinforces the light-absorbing property of the choroid in reducing the scattering of light in the eyeball.
The non-visual retina is an anterior continuation of the pigmented layer and a layer of supporting cells.
The non-visual retina extends over the ciliary body (ciliary part of the retina) and the posterior surface of the iris (iridial part of the retina), to the pupillary margin.
Clinically, the internal aspect of the posterior part of the eyeball, where light entering the eyeball is focused, is referred to as the fundus of the eyeball (ocular fundus).
The retina of the fundus includes a distinctive circular area called the optic disc (optic papilla), where the sensory fibers and vessels conveyed by the optic nerve (CN II) enter the eyeball (Figs. 7.49C, 7.50A and 7.52).
Because it contains no photoreceptors, the optic disc is insensitive to light.
Consequently, this part of the retina is commonly called the blind spot.
Just lateral to the optic disc is the macula of the retina or macula lutea (L. yellow spot).
The yellow color of the macula is apparent only when the retina is examined with red-free light.
The macula lutea is a small oval area of the retina with special photoreceptor cones that is specialized for acuity of vision.
It is not normally observed with an ophthalmoscope (a device for viewing the interior of the eyeball through the pupil).
At the center of the macula lutea is a depression, the fovea centralis (L., central pit), the area of most acute vision.
The fovea is approximately 1.5 mm in diameter; its center, the foveola, does not have the capillary network visible elsewhere deep to the retina.
The optic part of the retina terminates anteriorly along the ora serrata (L. serrated edge), the irregular posterior border of the ciliary body
Except for the cones and rods of the neural layer, the retina is supplied by the central artery of the retina, a branch of the ophthalmic artery.
The cones and rods of the outer neural layer receive nutrients from the capillary lamina of the choroid, or choriocapillaris
It has the finest vessels of the inner surface of the choroid, against which the retina is pressed.
A corresponding system of retinal veins unites to form the central vein of the retina. |
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External Ear |
The external ear consists of (a) pinna or auricle and (b) external auditory meatus, concerned with collection and transmission of sound waves to the tympanic membrane, respectively. AURICLE/PINNA The auricle is trumpet-like undulating projection on the side of the head The entire pinna except its lobule is made up of a single piece of crumpled yellow elastic cartilage covered with skin (Fig. 18.2B). The lobule of pinna is made of fibrofatty tissue covered with skin. The auricular cartilage is continuous with the cartilage of the external auditory meatus. Features The auricle presents two surfaces: lateral and medial. The lateral surface of auricle (Fig. 18.2A) displays following elevations and depressions: 1. Concha, a large depression that leads into the external auditory meatus. It is guarded in front by a triangular flap of cartilage, the tragus. 2. Helix forms a prominent peripheral rim of the pinna. It consists of two limbs—anterior and posterior. An anterior limb ends as crus of helix, which divides the concha into smaller upper and larger lower parts. The posterior limb ends below as flabby ear lobe and its upper end sometimes presents a small elevation called Darwin’s tubercle. It is probably erroneously thought to represent the vestige of the pointed part of the quadruped ear. 3. Antihelix is another prominent ridge present in front and parallel to the posterior part of helix, partly encircling the concha. Its upper end divides into two crura enclosing a triangular depression called triangular fossa. The narrow gutter between the helix and antihelix is called scaphoid fossa. 4. Tragus is a small triangular flap in front of concha. 5. Antitragus is a small elevation opposite to tragus from which it is separated by an intertragic notch. 6. Cymba conchae is a small area of concha above the crus of helix. Clinically it is important as it corresponds to the suprameatal triangle (McEwen’s triangle). 7. Lobule of the ear hangs below the antitragus as a large skin covered flap of fibrofatty tissue. • There is no cartilage between tragus and crus and the gap between the two is called incisura terminalis. • The thick hair on pinna particularly on tragus in male represents Y-linked inheritance. • The pinna collects and directs the sound waves to the external auditory meatus. For surgery of external auditory meatus, the incision is made in the region of incisura terminalis as it will not cut through the cartilage. The medial/cranial surface of (pinna) presents the following features: 2 eminances 1. Eminentia concha, which corresponds to the depression of the concha. 2. Eminentia triangularis, which corresponds to the triangular fossa between the crura of the antihelix. |
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Muscles of ear |
The extrinsic muscles pass from scalp or skull to the auricle. They are as follows: 1. Auricularis anterior. 2. Auricularis superior. 3. Auricularis posterior. The anterior and superior muscles arise from epicranial aponeurosis and are inserted into the upper part of the helix and upper part of the cranial surface of the auricle, respectively. The auricularis posterior arises from the mastoid process and gets inserted into eminentia concha Theintrinsic muscles are small muscular slips, which pass between the cartilaginous parts of the auricle. Actions The extrinsic muscles may play a role in positioning of the auricle to catch the sound, while intrinsic muscles may change the shape of the auricle. Such movements are rarely seen in human beings. However in animals they modify the shape of the pinna. |
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Vasculature and innervation |
SkinThe skin covering the auricle is closely adherent to the underlying cartilage and fibrofatty tissue. Sometimes coarse hair projects out of the tragus, antitragus, intertragic notch, and helix in elderly males. The hairy pinna is an expression of Y-linked genes. Arterial Supply 1. The cranial surface and posterior part of the lateral surface is supplied by the posterior auricular branch of the external carotid artery. 2. The anterior part of the lateral surface is supplied by the superficial temporal artery.N.B. Few branches of occipital artery supply the upper part of the cranial surface. Venous Drainage The veins accompany the arteries and drain into superficial temporal and external jugular veins. Lymphatic Drainage The lymph from auricle drains into: 1. Preauricular (parotid) lymph nodes 2. Mastoid lymph nodes. 3. Upper group of deep cervical lymph nodes. Nerve SupplyMotor supply: All the extrinsic and intrinsic muscles of the auricle are supplied by the facial nerve. The auricularis anterior and auricularis superior are supplied by the temporal branch of the facial nerve, while auricularis posterior is supplied by the posterior auricular branch of the facial nerve. Sensory supply (Fig. 18.3) 1. Lateral (facial) surface (Fig. 18.3A) (a) Lower one-third, by great auricular nerve (C2, C3). (b) Upper two-third, by auriculotemporal nerve [(a branch of mandibular division of the trigeminal nerve (CNV)]. (c) Concha, by auricular branch of the vagus (Alderman’s nerve) nerve (CNX). 2. Medial (cranial) surface (Fig. 18.3B)(a) Lower one-third, by great auricular nerve (C2, C3). (b) Upper two-third, by lesser occipital nerve (C2). 7(c) Eminentia conchae, by auricular branch of the vagus. |
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External Auditory meatus |
The external auditory meatus (syn. external acoustic meatus) extends from the bottom of the concha to the tympanic membrane and measures about 24 mm along its posterior wall. Note that it is not a straight tube but it has a typical S-shaped course. Its outer part is directed upwards, backwards, and medially (UBM), whereas its inner part is directed downwards, forwards, and medially (DFM). Therefore, to examine the tympanic membrane the pinna has to be pulled upwards, backwards, and laterally, to bring the two parts in alignment. |
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Tympanic membrane |
The tympanic membrane (or ear drum) is a thin (0.1 mm thick) semitransparent membrane, which forms the partition between external acoustic meatus and middle ear. It is oval, measuring 9–10 mm in length, and 8–9 mm in width. It is placed obliquely making an angle of about 55° with the floor of the external acoustic meatus. The tympanic membrane faces downwards, forwards, and laterally as though to catch the sounds reflected from the ground. Consequently the anterior wall and the floor of external auditory meatus are longer than the posterior wall and the roof. Structure The tympanic membrane is made of three layers (Fig. 18.4); from lateral to medial these are as follows: 1. Outercuticular layer of stratified squamous epithelium, which is continuous with the skin lining the external auditory, meatus. 2. Middle fibrous layer, which encloses the handle of the malleus. It contains outer radiating and inner circular fibres. 3. Innermucosal layer is lined by low columnar epithelium, which is continuous with the mucous lining of the middle, ear. Parts The tympanic membrane is divided into two parts: pars tensa and pars flaccida Pars tensa forms most of the tympanic membrane. Its periphery is thickened to form a fibrocartilaginous rim called annulus tympanicus, which fits into the tympanic sulcus. The fibrocartilaginous rim presents a notch above. From the margins of the notch the anterior and posterior malleolar folds in mucous membrane of tympanic cavity pass to gain attachment to the lateral process of the malleus. The handle of the malleus is firmly attached to the inner surface of the pars tensa. This part is rendered tough by the inward pull of the tensor tympani muscle, attached to the root of handle of the malleus and radial fibres. Pars flaccida (Shrapnell’s membrane) is a small triangular area above the lateral process of malleus between anterior and posterior malleolar folds (now called malleal folds). This part is thin and lax because intermediate fibrous layer here is replaced by loose areolar tissue. It appears slightly pinkish. Surfaces Lateral surface of the tympanic membrane is concave towards the meatus and directed downwards, forwards, and laterally. Medial surface is convex and bulges towards the middle ear. The point of maximum convexity is called umbo (Fig. 18.5). When the tympanic membrane is illuminated for inspection, the concavity of the membrane produces a ‘cone of light’ radiating from the umbo over the anteroinferior quadrant. This surface receives the attachment of malleus up to the center of the membrane. Here the handle of the malleus is crossed medially by chorda tympani nerve, which runs forwards between the fibrous and mucosal layer at the junction of pars flaccida and pars tensa. |
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Middle ear |
The middle ear (syn. tympanum, tympanic cavity) is a narrow slit-like air-filled cavity within the petrous part of the temporal bone. The middle ear is sandwiched between the external and internal ear. It contains three auditory ossicles, which transmit sound vibrations from tympanic membrane in its lateral wall to the internal ear via its medial wall. The tympanic cavity is really the intermediate portion of a blind diverticulum from the respiratory mucous membrane of the nasopharynx. From front to back, the diverticulum consists of pharyngotympanic tube, tympanic cavity, and mastoid antrum. In a section through long axis of petromastoid bone, the outline of tympanic cavity together with mastoid antrum and pharyngotympanic tube resembles a pistol, the nozzle being represented by the tube, the body by tympanic cavity, and handle by mastoid antrum (Fig. 18.7). Size and Shape The middle ear is shaped like a cube, compressed from side to side. In coronal section, it resembles a biconcave disc, like a red blood cell (Fig. 18.8). Measurements Vertical diameter: 15 mm. Anteroposterior diameter: 15 mm. Transverse diameters (a) At roof: (b) In the center: (c) At the floor: Communication 6 mm. 2 mm. 4 mm. The middle ear communicates: Anteriorly with nasopharynx through pharyngotympanic tube. Posteriorly with mastoid antrum and mastoid air cells through aditus to antrum called aditus ad antrum. |
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Contents of middle ear |
Inside the mucous lining Air Outside the mucous lining: 1. Three small bones called ear ossicles: malleus, incus, and stapes. 2. Two muscles: tensor tympani and stapedius. 3. Two nerves: chorda tympani and tympanic plexus. 4. Vessels supplying and draining the middle ear. 5. Ligaments of the ear ossicles. N.B. The mucous membrane lining of the middle ear invests all the structures within it and forms several folds, which project into the cavity giving it a honey-comb appearance. Thus strictly speaking, the middle ear contains only air. |
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Subdivisions of Middle Ear |
The tympanic cavity extends much beyond the limits of tympanic membrane, which forms its lateral boundary. It is divided into three parts, viz. 1. Epitympanum (attic), a part above the tympanic membrane containing head of malleus, body, and short process of incus. 2. Mesotympanum, a part opposite to tympanic membrane containing handle of malleus, long process of incus, and stapes. It is the narrowest part of the middle ear. 3. Hypotympanum, a part below the tympanic membrane. |
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Walls of tympanic cavity |
Roof: It is formed by a thin plate of bone called tegmen tympani. It separates the tympanic cavity from the middle cranial fossa. The tegmen tympani also extend posteriorly to form the roof of aditus ad antrum. 1. Aditus ad antrum, an opening in the upper part through which tympanic cavity communicates with the mastoid antrum. Floor: The floor is also formed by a thin plate of bone, which separates the tympanic cavity from the jugular bulb. Sometimes it is congenitally deficient and the jugular bulb then projects into the middle ear, being separated from cavity only by mucosa. The tympanic branch of glossopharyngeal nerve pierces the floor between the jugular fossa and lower opening of the carotid canal and enters the tympanic cavity to take part in the formation of tympanic plexus. Anterior wall: It is formed by a thin plate of bone. In the lower part it separates the cavity from internal carotid artery. The upper part of anterior wall presents two openings or canals, the upper one for the tensor tympani muscle and the lower one for the auditory tube. The bony partition between the two canals extends backwards along the medial wall in the tympanic cavity as a curved lamina called processus cochleariformis. Posterior wall: The posterior wall separates the tympanic cavity from mastoid antrum and mastoid air cells, and presents the following features: 1. Aditus ad antrum, an opening in the upper part through which tympanic cavity communicates with the mastoid antrum. 2. Fossa incudis, a small depression close to the aditus, lodging the short process of the incus. 3. Pyramid, a hollow conical bony projection below the aditus containing stapedius muscle whose tendon appears through its summit, passes forwards to be attached to the neck of the stapes. 4. Vertical part of facial canal runs in the posterior wall just behind the pyramid and descends up to the stylomastoid foramen. 5. Posterior canaliculus for chorda tympani, a small aperture for emergence of this nerve. Medial wall: It separates the tympanic cavity from the internal ear; thus it is actually formed by the bony lateral wall of the internal ear. The medial wall presents the following features: 1. Promontory, a rounded prominence in the centre produced by first (basal) turn of the cochlea. The tympanic branch of the glossopharyngeal nerve ramifies on it to form tympanic plexus. 2. Oval window (fenestra vestibuli), a reniform aperture located above and behind the promontory. It is closed by the base of stapes and annular ligament 3. Round window (fenestra cochleae), a small round opening below and behind the promontory which in life, is closed by fibrous secondary tympanic membrane. The secondary tympanic membrane separates the middle ear from the scala tympani. 4. Sinus tympani, a depression behind the promontory between fenestra vestibuli and fenestra cochleae, which indicates the position of ampulla of the posterior semicircular canal. 5. Prominence of oblique part of the facial canal that extends backwards and downwards above the oval window until it joins the vertical part of the facial canal in the posterior wall of the tympanic cavity. Sometimes the bony covering of the facial nerve may be absent, thus exposing the nerve for injuries and infection. 6. Prominence of lateral semicircular canal of the internal ear, which is seen as a small ridge high up in the angle between the medial and posterior walls. Lateral wall: Most of the lateral wall is formed by tympanic membrane, which separates the tympanic cavity from the external auditory meatus (Fig. 18.10). The chorda tympani nerve, a branch of facial nerve passes across the tympanic membrane lying lateral to the long process of the incus and medial to the handle of the malleus. It enters the tympanic cavity through the posterior canaliculus in the posterior wall and leaves through the anterior canaliculus medial to the petrotympanic fissure. |
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Otitis media |
Infection of the middle ear (otitis media): It is common especially in infants and children. Infective agents reach the middle ear from the upper respiratory tract through pharyngotympanic tube. The long-standing infection leads to chronic suppurative otitis media (CSOM), which clinically presents as ear discharge and perforation of tympanic membrane. The spread of infection from the middle ear may give rise to the following clinical conditions:– Acute mastoiditis and mastoid abscess, when infection spreads into mastoid antrum and mastoid air cells through aditus ad antrum in the posterior wall.– Meningitis and temporal lobe abscess may occur, if infection spreads upwards through the thin roof (tegmen tympani).– Lower motor neuron type of facial palsy, when infection erodes the papery thin bony wall of facial canal.– Transverse and sigmoid sinus thrombosis, when infection spreads through the floor.– Labyrinthitis, when infection spreads deep into medial wall. The labyrinthitis causes vomiting and vertigo. Cerebellar abscess, when infection spreads too far posteromedially. |
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Ear ossicles |
The three ear ossicles (malleus, incus, and stapes) within the middle ear are connected to one another by synovial joints and form a bony chain that extend across the tympanic cavity from the tympanic membrane to the oval window (Fig. 18.11). They conduct sound vibrations from tympanic membrane to the oval window and subsequently to the inner ear fluid. Malleus It resembles a hammer and, therefore, known as malleus. It has head, neck, handle (manubrium), a lateral process, and an anterior process. The head and neck lie in the epitympanum, whereas the handle is embedded in the fibrous layer of tympanic membrane. The lateral process forms a knob-like projection on the outer surface of the tympanic membrane and provides attachment to the anterior and posterior malleolar folds. The head of malleus articulates with the body of the incus forming the incudomalleolar joint (saddle type of synovial joint). Incus It resembles an anvil or a premolar tooth in shape. It consists of a relatively large body and two slender processes: a short process and a long process. The body and short process lie in the attic, whereas its long process hangs vertically behind and parallel with the handle of the malleus. Its bulbous tip (lentiform nodule) is directed medially to articulate with the head of the stapes, forming the incudostapedial joint (ball and socket type of synovial joint). Stapes It resembles a stirrup. It consists of head, neck, anterior and posterior crura, and footplate. The footplate closes the oval window and is attached to its margin by annular ligament. The features of ear ossicles are summarized in Table 18.1. |
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Hyperaccusia |
Hyperacusis: Both, tensor tympani and stapedius contract reflexly and simultaneously to dampen very loud sounds, thus preventing noise trauma to the internal ear. The paralysis of stapedius results in hyperacusis (an abnormally increased power of hearing) where even whisper appears as noise. |
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Otoscelrosis |
Otosclerosis: Abnormal ossification of annular ligament, which anchors the footplate of stapes to the oval window is called otosclerosis. This impedes the movements of stapes and causes deafness. The otosclerosis is the most common cause of conductive deafness in adults. |
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Mastoid Antrum |
It is a large air-containing space in the upper part of the mastoid process. It communicates anteriorly with tympanic cavity through aditus ad antrum. Its roof is formed by tegmen antri, which is the backward continuation of tegmen tympani. It separates the antrum from middle cranial fossa. The lateral wall of the antrum is formed by a plate of bone, which is on an average 1.5 cm thick in the adult. (It is only 2 mm thick in a newborn.) It is marked on the surface of mastoid by suprameatal triangle (McEwen’s triangle). The floor of antrum receives the openings of mastoid air cells. Its posterior wall is related to sigmoid sinus whereas its medial wall presents bulging of the lateral semicircular canal. Development Mastoid antrum develops as a backward extension of tympanic cavity and assumes the full adult size at birth |
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Pharyngotympanic tube |
The pharyngotympanic tube (auditory tube) is an osseocartilaginous tube, which connects the nasopharynx with tympanic cavity (middle ear). It is directed downwards, forwards, and medially from the tympanic cavity to the nasopharynx. The auditory tube maintains equilibrium of air pressure on either side of the tympanic membrane for its proper vibration. |
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Internal Ear |
The internal ear consists of a closed system of fluid filled intercommunicating membranous sacs and ducts called membranous labyrinth. The fluid filled in the membranous labyrinth is called endolymph. The membranous labyrinth lies within the complex intercommunicating bony cavities and canals (bony labyrinth) in the petrous part of the temporal bone. The space between the membranous and bony labyrinth is filled with fluid called perilymph. The sensory receptors within the membranous labyrinth are responsible for hearing and balancing (equilibrium). |
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Discuss Inner layer of eyeball |
The retina is the innermost coat of the eyeball (Fig. 19.22). It consists of two layers, viz. 1. An outer pigment layer. 2. An inner sensory layer. The space between the two layers contains a gummy substance that glues the two layers. The retina is present between the choroid and the hyaloid membrane of the vitreous. The retina diminishes in thickness from behind forwards. Anteriorly, it presents an irregular edge called ora serrata. Structure 1. The outer layer of the retina is insensitive to light and made up of pigmented cuboidal epithelium. 2. The inner sensory layer of the retina is sensitive to light and is made up of photoreceptors cells called rods and cones; as well as numerous relay neurons, viz. bipolar neurons and ganglion cells. The very thin non-cellular continuation of the retina in front of the ora serrata covers the ciliary body and iris. Thus the photosensitive part of the retina lines the inner surface of the eyeball posterior to the ciliary body. The nerve fibres arising from its ganglion cells covers inner surface and collect in the inferomedial region, where they pierce the outer two coats of eyeball and emerge as the optic nerve. N.B. Histologically, the retina is composed of following 10 layers: 1.Outer pigmented layer Layer of 2.rods and cones (photoreceptor cells) 3.External limiting membrane 4.Outer nuclear layer (Cell bodies of rods and cones) 5.Outer plexiform layer 6.Inner nuclear layer (Cell bodies of bipolar neurons) 7.Inner plexiform layer 8.Ganglion cell layer 9.Nerve fibre layer 10.Internal limiting membrane |
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Write a note on ophthalmic artery |
It is branch of the internal carotid artery. Origin It arises from internal carotid artery as it emerges from the roof of the cavernous sinus medial to anterior clinoid process close to the optic canal.
Course The artery enters the orbit through optic canal inferolateral to the optic nerve, both lying in a common dural sheath The artery pierces the dura mater, ascends over the lateral side of the optic nerve to cross it superiorly from lateral to medial side along with the nasociliary nerve. It then runs forwards along the medial wall of the orbit tortuously to allow the movements of the eyeball.
Termination Near the medial angle of the eye it terminates by dividing into two branches: supratrochlear and dorsal nasal. Branches Branches The branches of the ophthalmic artery are as follows: 1. Central artery of the retina (first and most important) arises from ophthalmic artery (still in dural sheath) below the optic nerve, runs forwards in the dural sheath and pierces the optic nerve inferomedially about 1.25 cm behind the eyeball. The central artery reaches the optic disc through the central part of the nerve. It supplies the optic nerve and inner 6/7 layers of the retina. Clinical correlation The central artery of retina is an example of a typical end artery. Its damage produces sudden total blindness on the side of the lesion. 2. Lacrimal artery arises from ophthalmic artery just before it crosses the optic nerve. It passes forwards along the upper border of lateral rectus and supplies the lacrimal gland, eyelids, and conjunctiva. The lacrimal artery gives off the following branches: (a) Glandular branches to lacrimal gland. (b) Two lateral palpebral arteries—one to each eyelid. (c) Two zygomatic branches: zygomaticofacial and zygomaticotemporal. (d) Recurrent meningeal branch runs backwards to enter the middle cranial fossa through the superior orbital fissure. (e) Muscular branches. 3. Posterior ciliary arteries consist of two sets: long and short, both of which pierce the sclera of the eyeball around the optic nerve and chiefly supply the choroid and sclera. Long ciliary arteries are usually two and short ciliary arteries are usually seven in number. (Remember that anterior ciliary arteries arise from muscular arteries.) 4. Supraorbital artery accompanies the supraorbital nerve. It passes through supraorbital notch to enter the scalp and divides into medial and lateral branches. 5. Posterior ethmoidal artery enters the posterior ethmoidal foramen in the medial wall of the orbit and supplies the ethmoidal air sinuses, nasal cavity, and dura mater. 6. Anterior ethmoidal artery enters the anterior ethmoid foramen in the medial wall of the orbit and supplies the ethmoidal air sinuses, medial and lateral wall of nasal cavity, and dura mater. 7. Dorsal (external) nasal artery supplies the lower part of the dorsum of nose. 8. Supratrochlear artery accompanies the supratrochlear nerve to supply the forehead. 9. Medial palpebral branches, one to each eyelid, anastomose with the corresponding lateral palpebral branches of the lacrimal artery. N.B. Branches of ophthalmic artery accompany all the branches of nasociliary frontal and lacrimal nerves (derived from ophthalmic nerve), and within the orbit supply all the extraocular muscles, lacrimal gland, and the eyeball. |
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Optic Nerve |
Optic nerve is the 2nd cranial nerve. It is purely sensory and responsible for vision; hence, it is also called the nerve of sight.
Unique Features
The optic nerve is not a true peripheral (cranial) nerve. It is actually a tract of brain for it develops as an outgrowth of diencephalon during embryonic life.
Hence, it presents the following unique features:
1. It consists of second-order sensory neurons.
2. Its fibres are myelinated by oligodendrocytes.
3. It is surrounded by meninges.
4. Its fibres cannot regenerate if cut/damaged.
Functional Component
Special somatic afferent fibres:
They carry sense of sight from the visual field of the corresponding eye.
Course and Relations
The fibres of optic nerve arise from ganglion cells (second- order neurons) in the neural layer of the retina of the eyeball, converge toward the optic disc at the posterior pole of the eyeball, pierce the outer layer of retina, choroid, and sclera to leave the eyeball.
Immediately after emerging from the eyeball, the fibres unite to form the optic nerve, which passes posteromedially through the posterior half of the orbit and enters the middle cranial fossa through the optic canal.
In the middle cranial fossa, optic nerves of two sides unite to form the optic chiasma.
The midregion of optic chiasma is composed of crossed fibres from the medial/nasal halves of the retina of both eyes, while the lateral region is made up of fibres from the lateral/temporal half of the retina of the ipsilateral eye.
Diverging from the chiasma are the optic tracts.
Most of the fibres of the optic tract relay in the lateral geniculate body.
The third-order neurons arise in the lateral geniculate body,
run in the retrolenticular part of the internal capsule, and form optic radiations.
The fibres of optic radiation terminate in and around the calcarine sulcus of the occipital lobe (visual cortex).
Some of the fibres from the lateral geniculate body reach the pretectal area of the midbrain and form a part of the pathway for light reflex.
Thus, visual pathway consists of the following components in craniocaudal order (Fig. 22.2):
Retina → optic nerve → optic tract → lateral geniculate body → optic radiation → visual cortex.
The optic nerve is 4 cm in length. It is divided into three parts: (a) intraorbital part, (b) canalicular part, and (c) intracranial part.
It is enclosed by three meninges of the brain |
