The Orbit and The Eyeball Flashcards
Make sure you check out our Histology flashcards for more on the eye, especially what hasn't been covered here.
Name the bones that form the various walls of the bony orbit.
Medial wall (thinnest): Formed by:
1. Frontal process of maxilla.
2. Lacrimal bone.
3. Orbital plate of ethmoid.
Lateral wall (strongest): Formed by:
1. Orbital surface of the zygomatic bone in front.
2. Orbital surface of greater wing of sphenoid behind.
Floor: Formed by:
1. Orbital surface of the body of maxilla.
2. Orbital surface of the zygomatic bone, anterolaterally.
3. Orbital process of the palatine bone, posteromedially.
Roof: Formed by:
1. Orbital plate of the frontal bone in front.
2. Orbital surface of lesser wing of the sphenoid behind.
[Diagram: Bony parts of the orbit]
List bony features present in each of the walls of the orbit.
Medial wall:
1. Lacrimal fossa: communicates with the nasal cavity through nasolacrimal canal. The lacrimal fossa and nasolacrimal canal lodge lacrimal sac and nasolacrimal duct, respectively.
2. Anterior and posterior ethmoidal foramina, lie at the junction between medial wall and the roof of the orbit.
3. Locate these structures here: [Diagram].
Lateral wall:
1. Zygomatic foramen. Sometimes there are two small foramina, for zygomaticofacial and zygomaticotemporal nerves.
2. Whitnall’s tubercle, a small bony tubercle just behind the lateral orbital margin and slightly below the frontozygomatic suture.
Floor:
1. Infraorbital groove and canal.
2. Small rough impression in anteromedial angle for origin of inferior oblique muscle.
Roof:
1. Fossa for lacrimal gland in the anterolateral part. [Note that the lacrimal sac and lacrimal gland are not the same structure. The lacrimal fossa lodges the lacrimal sac. Here’s a diagram to clear things up.]
2. Trochlear notch or spine at the anteromedial angle.
3. Optic canal at the extreme posterior part of the roof.
4. [Diagram]
Apex: Lies at the posterior end of the orbit and is formed by sphenoid. More precisely it is formed by the centre of the bony bridge between optic canal and superior orbital fissure. [Diagram]
Base: Open and quadrangular, its boundaries form the orbital margins.
Further notes:
The primary function of the infraorbital groove is to act as the passage for the infraorbital artery, infraorbital vein and infraorbital nerve. The infraorbital nerve is a branch of the maxillary division of the trigeminal nerve.
The infraorbital gorove is an important surgical landmark for local anesthesia of the infraorbital nerve.
State the orbital margins.
(1) Supraorbital margin is formed by the frontal bone and presents a notch—supraorbital notch or foramen at the junction of its lateral two-third and medial one-third.
(2) Infraorbital margin is formed by the zygomatic bone laterally and maxilla medially; it is continuous with anterior lacrimal crest medially.
(3) Medial orbital margin is ill-defined. It is formed by the frontal bone above and lacrimal crest of the frontal process of maxilla below.
(4) Lateral orbital margin is formed by zygomatic process of the frontal bone above and frontal process of the zygomatic bone below.
(5) [Diagram]
Further notes:
The orbital margins provide a fair bony protection to the eye except at the lateral margin. For this reason protective eye guards are designed to compensate for it, in squash and handball players and still permitting good peripheral vision.
The inferior orbital fissure connects the orbit with the pterygopalatine and infratemporal fossae. State the structures transmitted by the fissure.
(1). Infraorbital and zygomatic branches of the maxillary nerve (and the accompanying vessels)
(2) Orbital branches from the pterygopalatine ganglion
(3) Connection between the inferior ophthalmic vein and the pterygoid venous plexus.
(4) [Diagram]
State the relations of the orbit.
Superior: Anterior cranial fossa and frontal air sinus
Lateral: Temporal fossa in front and middle cranial fossa behind
Inferior: Maxillary air sinus
Medial: Ethmoidal air sinuses
[Diagram 1: sinuses] [Diagram 2: temporal fossa]
Briefly discuss the orbital fascia or periorbita.
It is the periosteum of the bony orbit, which lines the bony boundaries of the orbit and forms a funnel-shaped fascial sheath that encloses the orbital contents. It is loosely attached to the bones, hence can be easily stripped off especially from roof and medial wall of the orbit.
At the optic canal and superior orbital fissure, it becomes continuous with the periosteum lining the interior of the skull (endocranium). At the infraorbital fissure and orbital margins, it becomes continuous with the periosteum covering the external surface of the skull (pericranium).
[Diagram: periorbita]
List the contents of the bony orbit (8).
(1) Eyeball
(2) Muscles
(3) Fascia bulbi
(4) Nerves: optic, oculomotor, trochlear, abducens, ophthalmic, ciliary ganglion
(5) Ophthalmic artery
(6) Ophthalmic veins
(7) Lacrimal gland
(8) Orbital fat
[4-minute video: Dissection of the Orbit]
All these structures lie within the orbital periosteum that lines the bony walls of the orbit.
[Diagram: Contents of the orbit]
NOTE: Infraorbital nerve and vessels and zygomatic nerve lie outside the orbital periosteum, hence are not included in the contents of the orbit.
(a) What is the fascia bulbi/fascial sheath of the eyeball? State its extents.
(b) State two functions of fascia bulbi.
(c) What structures pierce the fascia bulbi?
(a) 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.
(b) The Tenon’s capsule forms a socket for the eyeball to facilitate free ocular movements. It separates the eyeball from orbital fat.
(c) Structures that pierce fascia bulbi:
☛ tendons of four recti and two oblique muscles of the eyeball
☛ ciliary nerves and vessels around the entrance of the optic nerve
Note: At the posterior pole of the eyeball the fascia bulbi becomes continuous with sheath of the optic nerve. The optic nerve hence does not pierce fascia bulbi!
[Image: Tenon’s capsule]
[Diagram 1: Tenon’s capsule]
[Diagram 2: Tenon’s capsule]
The fascia bulbi provides a tubular sleeve around each muscle that pierces it. From them, ligaments emerge. List these ligaments, stating their extents.
(1) Lateral check ligament: Extends from the fascial sleeve of lateral rectus for attachment to the lateral wall of the orbit on Whitnall’s tubercle.
(2) Medial check ligament: Extends from the fascial sleeve of medial rectus for attachment to the medial wall of the orbit on posterior lacrimal crest of the lacrimal bone.
(3) Suspensory ligament of the eye (or suspensory ligament of Lockwood): The hammock-like support for the eyeball formed by the fascial sleeve of inferior rectus thickening on its underside, blending with the sleeve of inferior oblique as well as with the medial and lateral check ligaments. It is expanded in the centre and narrows at its extremities.
(4) [Diagram 1] [Diagram 2] [Diagram 3]
State the clinical correlations related to the Tenon’s capsule and suspensory ligament of the eye.
◾ If suspensory ligament of the eye remains intact when the floor of the orbit is fractured or the maxilla is removed surgically, the eyeball does not sag.
◾ During enucleation of the eye, if Tenon’s fascia is not damaged, and an artificial eye is planted subsequently within the socket of Tenon’s capsule it is able to move.
Further notes:
Enucleation is the removal of the eye that leaves the eye muscles and remaining orbital contents intact. This type of ocular surgery is indicated for a number of ocular tumors, in eyes that have sustained severe trauma, and in eyes that are otherwise blind and painful.
What are four modifications of the periorbita/orbital fascia?
(1) Orbital septa [these are extensions into the upper and lower eyelids]
(2) Lacrimal fascia [this fascia covers the lateral surface of the lacrimal sac]
(3) It forms a fibrous pulley, the trochlea to hold the superior oblique muscle.
(4) In the posterior part of the orbit, it thickens around the optic canal and the central part of the superior orbital fissure. This is the point of origin of the four rectus muscles and is the common tendinous ring.
Name the contents of the superior orbital fissure that do not pass through the tendinous ring (of Zinn).
trochlear nerve, lacrimal nerve, frontal nerve, superior ophthalmic vein
[Diagram: Tendinous ring of Zinn]: Extraocular muscles attach onto the tendinous ring. Appreciate the structures passing through the ring, and structures that don’t pass through.
The extraocular muscles are classified into 2 groups: voluntary and involuntary.
List the voluntary extraocular muscles.
There are seven voluntary muscles in the orbit. Of these, six muscles move the eyeball and one muscle moves the upper eyelid.
1. Four recti muscles
(a) Superior rectus, (b) Inferior rectus, (c) Medial rectus, and (d) Lateral rectus.
2. Two oblique muscles (a) Superior oblique, (b) Inferior oblique.
3. One levator palpebrae superioris.
[Diagram 1: Voluntary extraocular muscles of the eye.]
[Diagram 2: Voluntary extraocular muscles of the eye.]
For this card, click on Diagram 1 and Diagram 2 to view the position of the extraocular voluntary muscles. In the Answer section, you’ll see a brief description of origin and insertion of the recti muscles. Do not stress with attachments. Nevertheless, I’ve put them there with linked images for your understanding. 😎
Origin: All the recti arise from the corresponding margins of the common tendinous ring. The lateral rectus arises by two heads.
{Note: The common tendinous ring of Zinn encloses the optic canal and middle part of the superior orbital fissure. It is attached medially to apex of the orbit and laterally to a small tubercle (tubercle of Zinn) on the lower border of superior orbital fissure.}
Insertion: All the recti are inserted into sclera a little posterior to the limbus (corneoscleral junction) in front of the equator of the eyeball. Average distance from limbus is:
Medial rectus: 5 mm, Inferior rectus: 6 mm, Lateral rectus: 7 mm, Superior rectus: 8 mm
Name the involuntary extraorbital muscles.
- superior tarsal/Muller’s muscle
- inferior tarsal
- orbitalis
For understanding (rate it a 5): Are the actions of the superior and inferior recti really straight?
No. The ocular muscles and optic nerve come from the apex of the orbit near the back of the medial wall and pass forwards and laterally to be attached to the eyeball. The actions of superior and inferior recti are, therefore, not straight, despite their name. The superior and inferior oblique muscles, therefore, have to act in concert with two recti in order to produce direct upward and downward movements of the eyeball.
[Diagram]
State the axis of the following movements of the eyeball:
(a) elevation and depression
(b) adduction and abduction
(c) rotation (torsion)
(a) Elevation and depression: around the transverse axis (x-axis of Fick) passing through the equator.
(b) Adduction and abduction: around the vertical axis (z-axis of Fick) passing through the equator.
(c) Rotation (torsion): around the anteroposterior axis (y-axis of Fick) extending from anterior pole to posterior pole of the eyeball.
[Diagram: movements of the eyeball around the particular axes]
Note:
✓ Click on this image to appreciate the axes of the globe, globe here being our eyeballs.
✓ When 12 o’clock position of the cornea rotates medially, it is called intorsion and when it rotates laterally, it called extorsion.
✓ When you tilt your head to one direction, your eyes torque in the opposite direction, to help maintain binocular vision.
State the actions of each of the extraocular voluntary muscles.
Superior rectus: elevation, adduction, and intorsion.
Medial rectus: adduction.
Inferior rectus: depression, adduction, and extorsion.
Lateral rectus: abduction.
Superior oblique: depression, abduction, and intorsion .
Inferior oblique: elevation, abduction, and extorsion.
Levator palpebrae superioris muscle: elevation of the upper eyelid to open the eye.
[Diagram 1]: voluntary extraocular muscles of the eye
[Diagram 2]: voluntary extraocular muscles of the eye
[Video]: actions of the extraocular muscles
State the two types of associated movements of the eyeball.
- Conjugate movements: when both the eyes move in the same direction with visual axes being parallel.
- Disconjugate movements: when the axes of both eyes converge or diverge.
State the innervation of levator palpebrae superioris muscle.
- Striped (skeletal muscle) part is supplied by the upper division of oculomotor nerve.
- Unstriped (smooth muscle) part is supplied by the post-ganglionic sympathetic fibres from the superior cervical ganglion.
Further notes:
The levator palpebrae superioris muscle, which is responsible for elevating the upper eyelid, receives its motor innervation from the oculomotor nerve (CN III). However, it also has a component called the superior tarsal muscle (or Müller’s muscle) that is innervated by postganglionic sympathetic axons from the superior cervical ganglion. This sympathetic innervation is crucial for maintaining eyelid elevation and contributes to the fight-or-flight response, causing the eyelid to widen during moments of alertness.
Comment on paralysis of levator palpebrae superioris when:
(a) the upper division of CN III is injured
(b) the postganglionic sympathetic fibres from the superior cervical ganglion are injured
(a) Complete ptosis due to involvement of CN III.
(b) Partial ptosis due to involvement of cervical sympathetic chain (as seen in Horner’s syndrome).
Note:
Ptosis: drooping of the upper eyelid
Outline the visual pathway.
- Retina: Light enters the eye and is detected by photoreceptors in the retina.
- Optic Nerve (CN II): The optic nerve carries the visual information from the retina.
- Optic Chiasm: At the optic chiasm, nerve fibers from the nasal half of each retina cross to the opposite side.
- Optic Tract: The optic tracts carry visual information from the chiasm to the brain.
- Lateral Geniculate Nucleus (LGN): The LGN of the thalamus processes the information.
- Optic Radiations: Fibers from the LGN form the optic radiations.
- Visual Cortex: The optic radiations project to the primary visual cortex in the occipital lobe, where visual perception occurs.
Further details:
1. At the retina: rods and cones ⇒ rod and cone cells ⇒ bipolar cells ⇒ ganglion cells
2. Optic nerve: formed by axons of ganglion cells
3. Optic nerves join at the optic chiasma: temporal fibres enter the ipsilateral optic tract, whereas nasal fibres cross to enter the contralateral optic tract.
4. Optic tract: contains temporal fibres of the same side and nasal fibres of the opposite side
5. Lateral geniculate body: axons form the optic radiation
6. Optic radiation: has 2 loops; they pass through the retrolenticular part of the internal capsule
7. Optic radiation terminates in Primary Visual Cortex (area 17): lips/banks of calcarine sulcus
8. Visual Association Cortex (areas 18 and 19)
Discuss the loops/divisions of the optic radiation.
- Upper division/Baum’s loop: runs in the parietal lobe and comprises of upper retinal fibres. It terminates above the calcarine sulcus in the cuneus.
- Lower division/Meyer’s loop: runs in the temporal lobe and contains lower retinal fibres. It terminates below the calcarine sulcus in the lingual gyrus.
Discuss the representation of the retina in the Primary Visual Area.
(a) Upper retinal fibres terminate above the calcarine sulcus; temporal fibres of the same eye and nasal fibres of the opposite eye.
(b) Lower retinal fibres terminate below the calcarine sulcus (lingual gyrus); temporal fibres of the same eye and nasal fibres of the opposite eye.
(c) The macular fibres terminate in the posterior 1/3 of the visual area.
Outline the effects of the following lesions of the visual pathway:
(a) optic nerve
(b) central lesion of optic chiasma
(c) peripheral lesion of optic chiasma
(d) optic tract, lateral geniculate body
(e) Baum’s loop
(f) Meyer’s loop
(a) blindness of the ipsilateral eye
(b) bitemporal hemianopia [usually caused by pituitary adenoma (benign tumor)]
(c) binasal hemianopia [usually caused by aneurysm of the internal carotid artery]
(d) contralateral homonymous hemianopia
(e) contralateral inferior quadrantanopia
(f) contralateral superior quadrantanopia
Further notes (Important for understanding the quadrantanopias):
Damage to Baum’s tract, which carries the upper retinal fibers, causes contralateral inferior quadrantanopia because of the way the visual system is organized. Here’s a simplified explanation:
(a) Upper retinal fibres: These fibers correspond to the lower half of the visual field because the visual system inverts images as they are projected onto the retina.
(b) Baum’s Tract: This tract is part of the superior optic radiation and carries information from the upper retina.
(c) Visual Processing: The fibers terminate in the upper bank of the calcarine sulcus, which processes the lower quadrant of the visual field.
(d) Lesion effect: a lesion in Baum’s tract interrupts the pathway for the upper retinal fibres, resulting in a loss of vision in the lower visual field of the opposite side.
The same reasoning can be applied to lesions of Meyer’s tract.
[Diagram 1] [Diagram 2]
Outline the effects of the following lesions of the visual pathway:
(a) Visual area; cuneus
(b) Visual area; lingual gyrus
(c) Visual area; total
(a) contralateral homonymous lower quadrantanopia
(b) contralateral homonymous upper quadrantanopia
(c) contralateral homonymous hemianopia with macular sparing
State the extra-geniculate projections of the optic tract and state the role of each.
Majority of the fibres of the optic tract project to the lateral geniculate body. However, some go to the:
(a) superior colliculus which mediates visual reflex
(b) pretectal nucleus which mediates the pupillary light reflex [reflex constriction due to increased light intensity]
(c) hypothalamus: which mediates pupillary dilation [occurs in response to sudden decrease in light intensity], and also to suprachiasmatic nucleus for circadian rhythms.
(Remember that the posterior hypothalamic nucleus is responsible for sympathetic functions and pupillary dilation is a sympathetic function.)
(d) pulvinar: visual association
Outline the pathway of the pupillary light reflex.
retina ⇒ optic nerve ⇒ optic chiasma ⇒ optic tracts ⇒ pretectal nuclei via superior brachium ⇒ Edinger-Westphal nucleus of both sides ⇒ oculomotor nerves ⇒ ciliary ganglion ⇒ short ciliary nerves ⇒ constriction of the sphincter pupillae muscle
[Diagram: Pupillary Light Reflex Pathway]
What are three accommodative changes that take place in the eye when one focuses from a distant object to a near object?
- Constriction of pupils: due to action of sphincter pupillae to restrict the light waves to the thickest central part of the lens.
- Thickening of the lenses: due to contraction of the ciliary muscles, to increase the refractive index of the lenses.
- Convergence of the eyeballs: due to the contraction of medial recti, to bring about the convergence of the ocular axes
- [These three reactions together constitute the accommodation or near reflex. Here’s an animation.]
anatomical basis of Argyll-Robertson pupils
Damage to the dorsal aspect of rostral midbrain causing loss of constriction of pupils via pupillary light reflex but not the accommodation reflex.
Outline the pathway of the accommodation reflex.
retina ⇒ optic nerve ⇒ optic chiasma ⇒ optic tracts ⇒ lateral geniculate body ⇒ optic radiation ⇒ visual area ⇒ Frontal Eye Field (area 8) ⇒ superior colliculus ⇒ projects to:
(a) Somatic oculomotor nucleus of the 2 sides ⇒ medial recti ⇒ causing convergence.
(b) Edinger-Westphal nucleus of the 2 sides ⇒ sphincter pupillae muscle for pupillary constriction, and ciliary muscle for thickening of the lens.
Convergence of the medial recti muscles may also influence another neural pathway leading to pupillary constriction and thickening of the lens. Outline that pathway.
Medial recti contraction ⇒ proprioception from medial recti ⇒ nucleus of Perlia (midbrain) ⇒ both Edinger-Westphal nuclei ⇒ oculomotor nerve ⇒ ciliary ganglion ⇒ short ciliary nerves ⇒ sphincter pupillae and ciliaris
Outline the course of the ophthalmic division of the trigeminal nerve.
After arising from the trigeminal ganglion (aka. Gasserian ganglion), it runs forwards in the lateral wall of the cavernous sinus below the oculomotor and trochlear nerves and above the maxillary nerve. It receives a sympathetic branch from the plexus around the internal carotid artery. It leaves the cavernous sinus then divides into three branches; lacrimal, frontal and nasocilliary, all of which enter the orbit via the superior orbital fissure.
Outline the branches of the ophthalmic nerve.
(a) Tentorial nerve [an intracranial meningeal branch]
(b) Lacrimal nerve [terminal portion also receives post-synaptic parasympathetic fibres from the zygomatic nerve and conveys them to lacrimal gland]
(c) Frontal nerve [it branches into the supraorbital and supratrochlear nerves]
(d) Nasociliary nerve [it is the sensory root of the ciliary ganglion. Branches: posterior ethmoidal nerve, anterior ethmoidal nerve, long ciliary nerves, infratrochlear nerve, communicating branch to ciliary ganglion.]
[Diagram]: Branches of ophthalmic nerve
[Cadaveric image 1]
[Cadaveric image 2]
Further notes:
Zygomatic nerve is a branch of CN V2. It enters the orbit through the inferior orbital fissure.
State the distribution of the following branches of ophthalmic nerve:
(a) Frontal nerve
(b) Nasociliary nerve
(a) Frontal nerve:
Supratrochlear: Skin of lower forehead
Supraorbital: Skin of scalp up to vertex, frontal sinus
(b) Nasociliary nerve:
Anterior ethmoidal: Anterior ethmoidal air cells, middle ethmoidal air cells, anterosuperior quadrant of lateral wall of nose, anterosuperior part of septum of nose
Posterior ethmoidal: Posterior ethmoidal air cells, sphenoidal air sinus
Infratrochlear: Conjunctiva over medial aspect, skin of root of nose
External nasal branch of anterior ethmoidal: Skin of external aspect of nose
Outline the pathway for afferent sensory fibres of the ophthalmic nerve from the receptors until the trigeminal nuclei.
(a) Territory of supply of the ophthalmic nerve: ✔forehead and scalp, ✔upper eyelid and its conjunctiva, ✔cornea of the eye, ✔the dorsum/bridge of the nose
(b) Preganglionic fibres from the ciliary ganglion and fibres from the peripheral branches merge into the ophthalmic branch. The ophthalmic branch gives off a recurrent tentorial branch which supplies the tentorium cerebelli and supratentorial falx cerebri.
(c) The ophtalmic nerve then synapses on the trigeminal ganglion and the fibres then continue to the pontine nucleus (chief sensory) and then the spinal nucleus.
[Diagram]: cutaneous territories of the three divisions of the trigeminal nerve
[5-minute video]: the Ciliary Ganglion
The corneal reflex, also known as the blink reflex, is an involuntary blinking of the eyelids elicited by stimulation of the cornea (such as by touching or by a foreign body), or bright light, though could result from any peripheral stimulus. Outline the pathway of the corneal reflex.
stimulation of the cornea ⇒ nasociliary branch of ophthalmic nerve (afferent limb) ⇒ Gasserian/trigeminal ganglion ⇒ main sensory nucleus of trigeminal ⇒ motor nucleus of facial nerve ⇒ both of the facial nerves (temporal and zygomatic branches) (efferent limbs) ⇒ rapid and involuntary closure of both eyelids
[Diagram illustrating this pathway]
State the components of the lacrimal apparatus.
Describe the pathway of secretomotor (parasympathetic) innervation to the lacrimal gland.
Preganglionic fibres arise from the lacrimatory nucleus in the pons and pass successively through nervus intermedius, geniculate ganglion, greater petrosal nerve and nerve of pterygoid canal (vidian nerve) to reach the pterygopalatine ganglion where they relay.
The postganglionic fibres arise from the neurons of the pterygopalatine ganglion and pass successively through the maxillary nerve, zygomatic nerve, zygomaticotemporal nerve and lacrimal nerve to reach the lacrimal gland.
[Diagram: Parasympathetic (secretomotor) and Sympathetic (vasomotor) Innervation to the Lacrimal Gland.]
Further notes:
~ Vasomotor actions pertain to the regulation of blood vessel diameter. In the context of the lacrimal gland, vasomotor control influences the blood flow within the gland’s blood vessels. Changes in blood vessel diameter can impact the gland’s overall function.
~ Secretomotor function involves stimulating glands to secrete specific substances. The secretomotor innervation of the lacrimal gland comes from the lacrimal nerve, a branch of the ophthalmic division of the trigeminal nerve. These secretomotor fibers activate the gland, leading to the secretion of aqueous tears.
~ The vidian nerve, also known as the nerve of the pterygoid canal is formed by the confluence of two nerves: the greater superficial petrosal nerve and the deep petrosal nerve. The greater superficial petrosal nerve arises from the geniculate ganglion of the nervus intermedius division of the facial nerve. It carries parasympathetic fibers from the superior salivary nucleus. The deep petrosal nerve arises from the sympathetic plexus on the internal carotid artery. It carries sympathetic fibers.
Outline the pathway of sympathetic (vasomotor) supply of the lacrimal gland.
T1 spinal segment ⇒ superior cervical sympathetic ganglion (gives origin to postanglionic sympathetic fibres) ⇒ sympathetic plexus around internal carotid artery ⇒ deep petrosal nerve ⇒ pterygopalatine ganglion ⇒ zygomatic nerve ⇒ zygomaticotemporal nerve ⇒ lacrimal nerve to lacrimal gland
Further notes:
Vasomotor actions pertain to the regulation of blood vessel diameter. In the context of the lacrimal gland, vasomotor control influences the blood flow within the gland’s blood vessels. Changes in blood vessel diameter can impact the gland’s overall function.
Secretomotor function involves stimulating glands to secrete specific substances. The secretomotor innervation of the lacrimal gland comes from the lacrimal nerve, a branch of the ophthalmic division of the trigeminal nerve. These secretomotor fibers activate the gland, leading to the secretion of aqueous tears.
The deep petrosal nerve is a post-ganglionic branch of the (sympathetic) internal carotid (nervous) plexus (which is in turn derived from the superior cervical ganglion, a part of the cervical sympathetic trunk) that enters the cranial cavity through the carotid canal, then passes perpendicular to the carotid canal in the cartilaginous substance which fills the foramen lacerum to unite with the (parasympathetic) greater petrosal nerve to form the nerve of pterygoid canal (Vidian nerve).
Describe the general sensory supply to the lacrimal gland.
It is by sensory fibres of the lacrimal nerve, a branch of the ophthalmic division of the trigeminal nerve.
Name the tunics of the eyeball.
(i) Outer fibrous coat (tunica fibrosa) consisting of sclera and cornea.
(ii) Middle vascular coat consisting of choroid, ciliary body, and iris.
(iii) Inner nervous coat consisting of the retina.
Discuss the sclera (its structural features)
~ The sclera is the posterior five-sixth of the outer coat.
~ It consists of dense fibrous tissue. It is opaque and a small portion of it is seen as the white of the eye in the palpebral fissure.
~ The sclera is continuous anteriorly with the cornea.
~ The junction between the sclera and cornea is termed corneoscleral junction (limbus).
~ Just behind the corneoscleral junction, within the sclera is a circularly running canal called sinus venosus sclerae/scleral venous sinus (canal of Schlemm).
~ Posterior to the canal is a triangular projection—the scleral spur—which points forwards and inwards and provides attachment to the ciliary muscle. It is thinnest at the equator and thickest at the back but weakest at the site of emergence of optic nerve.
[Diagram: Meridional section of the eyeball showing ciliary region and the iridocorneal angle.]
[Diagram: Horizontal section of the eyeball showing its internal structure including three coats and visual axis.]
State the functions of the sclera.
- Helps to maintain the shape of the eyeball.
- Protects internal structures.
- Provides attachment to muscles that move the eyeball.
State the structures piercing the sclera.
- Optic nerve pierces the sclera, a little inferomedial to the posterior pole of the eyeball. The perforating fibres of nerve make the area sieve-like (hence called lamina cribrosa).
- Posterior ciliary vessels and nerves around the optic nerve.
- Anterior ciliary arteries pierce the sclera close to corneoscleral junction.
- Four choroidal veins (also called venae vorticosae) pierce the sclera, just behind the equator.
- [Diagram]
The cornea is the anterior one-sixth of the outer coat. It bulges forwards from the sclera at the limbus. It is transparent and more convex than sclera. State its features.
- It is avascular and nourished by permeation of nutrients from loops of capillaries at the limbus, aqueous humour, and lacrimal fluid.
- It not only permits the light to enter the eye but also refracts the entering light.
- It is highly sensitive and supplied by the ophthalmic division of trigeminal nerve. [nasociliary nerve to be specific]
- The nerves of cornea form the afferent limb of the corneal reflex (closure of the eyelids on stimulation of the cornea).
The cornea consists of five layers. State these layers from outside inwards.
- Corneal epithelium: stratified squamous non-keratinised epithelium.
- Bowman’s membrane (anterior limiting membrane): this thick homogenous layer consists of collagen fibres crossing at random, a condensation of the intercellular substance, and no cells, contributing greatly to the stability and strength of the cornea.
- Corneal stroma (substantia propria): made up of about 200–250 lamellae of fine collagen fibres, which cross each other at right angles to form corneal spaces. Flattened fibroblasts are located between the lamellae. The transparency of cornea is due to precise lattice arrangement of its lamellae embedded in the ground substance.
- Descemet’s membrane: thick homogenous structure composed of fine collagenous filaments organised in 3-D network.
- Corneal endothelium: simple squamous epithelium.
- [Diagram: layers of the cornea]
- [Transmission electron micrograph: collagen fibres of the corneal stroma crossing at right angles]
Note:
~ The corneal endothelium and epithelium are responsible for maintaining the transparency of the cornea. Both layers are capable of transporting sodium ions toward their apical surfaces. Chloride ions and water follow passively, maintaining the corneal stroma in a relatively dehydrated state. This state, along with the regular orientation of the very thin collagen fibrils of the stroma, accounts for the transparency of the cornea.
State the various clinical correlations of the cornea.
- The periphery of the cornea frequently displays a whitish ring in older persons, owing to fatty degeneration. This whitish ring is termed arcus senilis.
- Plastic lens: The central part of the cornea receives oxygen from the outside air. Therefore, the soft plastic contact lenses worn for long periods must be permeable to the air so that oxygen can reach the cornea.
- Corneal opacity: The injury to cornea may cause opacity that may interfere with vision. The most common injuries of the eye are the cuts or tears of the cornea caused by foreign bodies.
- Corneal graft: The normal lack of vascularity and of lymph vessels accounts for the great success of corneal grafts. The cornea is successfully grafted from one person to the other.
- The corneal reflex is elicited clinically by gentle touching of the cornea with wisp of cotton wool. As the cornea is touched both the eyes are closed.
The middle vascular coat/tunica vascularis/the uvea/uveal tract consists of the choroid, ciliary body and iris. What is the choroid? What arteries and veins are found in the choroid?
~ The choroid is the posterior part of the vascular coat of the eyeball.
~ It is brown, thin, and highly vascular membrane lining the inner surface of the sclera.
~ Anteriorly, it is connected to the iris by the ciliary body and posteriorly, it is pierced by the optic nerve.
~ Arteries: They are derived from short ciliary arteries, which pierce the sclera around the optic nerve. [The short ciliary arteries are branches of the ophthalmic artery, which is a branch of the internal carotid artery.]
~ Veins: They are arranged in the form of whorls, which converge to form the 4 or 5 venae vorticosae, which pierce the sclera just behind the equator to open into the ophthalmic veins.
N.B.
The inner surface of the choroid is firmly attached to the retina and nourishes the rods and cones of the retina by diffusion.
State the four layers of the choroid, from outside inwards.
- Suprachoroid lamina (lamina fusca): It consists of loose network of elastic and collagen fibres and is traversed by long posterior ciliary vessels and nerves.
- Vascular lamina: It consists of loose areolar tissue and pigment cells. It contains branches of short posterior ciliary arteries and veins, which converge in whorls to form 4 or 5 venae vorticosae, which pierce the sclera and drain into ophthalmic veins.
- Capillary lamina (capillary layer of choroid): It consists of fine network of capillaries, which nourish photoreceptors of the retina (rods and cones) by diffusion.
- Basal lamina (membrane of Bruch): It is a thin transparent membrane, which is firmly attached to the pigment cell layer of the retina.
- [Diagram: layers of the choroid]
Further notes:
~ Its characteristic black color is from the abundant melanocytes within it.
~ The inner layer of the choroid is richer in small vessels than the outer layer, and is called the choriocapillary layer.
~ The last three layers form the choroid proper, which is separated from sclera by suprachoroid lamina.
~ In some animals such as cat, tiger, lion, etc. the specialized cells of choroid form a reflecting media called tapetum, which produces greenish glare in the eyes of these animals in the night.
The ciliary body is the thickening in the vascular tunic. It is continuous with the ____(a)____ posteriorly and the ____(b)____ anteriorly. It is situated posterior to the limbus in front of the ora serrata of the ____(c)____.
The ciliary bodies suspend the lens via suspensory ligaments.
(a) choroid
(b) iris
(c) retina
The ciliary body is triangular in cross section, thick in front and thin behind. What are the components of the ciliary body?
- Ciliary ring is an outer fibrous ring, which is continuous with the choroid.
- Ciliary processes are a group of 60–90 folds on the inner aspect of the ciliary body. They are arranged radially between the ciliary ring and the iris. The grooves between the processes provide attachment to the fibres of suspensory ligament of the lens.
- The ciliary muscle is a small unstriped (smooth) muscle mass consisting of mainly two types of fibres: outer radial fibres and inner circular fibres.
[Diagram]: Parts of the Ciliary Body
[Diagram]: the eye showing the anterior and posterior chambers
Must note:
~ The ciliary processes are a complex of capillaries and cuboidal epithelium, which secretes aqueous humour. The ciliary processes may be compared with choroidal plexus of the brain ventricles involved in the secretion of CSF.
State the functions of the ciliary body.
- Its main function is to focus the lens for near vision. The ciliary muscle as a whole acts as a sphincter, therefore, when its muscle fibres, both radial and circular contract, the choroid is pulled towards the lens reducing the tension on the suspensory ligaments. This allows the lens to assume a more spherical form because of its own elastic nature. Now lens can cause more refraction needed for accommodation.
[Diagram 1] [Diagram 2] - It secretes aqueous humour in the posterior chamber.
The iris is a contractile diaphragm between the cornea and the lens. An opening in its centre is called the ____(I)____. The iris is attached at its periphery to the middle of the anterior surface of the ciliary body. Peripheral to this attachment the ciliary body and narrow rim of sclera form the ____(II)____.
(I) pupil
(II) iridocorneal angle
List the layers of the iris from outer inwards.
- An anterior mesothelial lining/Anterior limiting layer
- A connective tissue stroma containing pigment cells and blood vessels.
- A layer of smooth muscle, which consists of two parts.
(a) Constrictor pupillae—an inner (near the margin of the pupil) part made of circular fibres.
(b) Dilator pupillae—a peripheral part made up of radial fibres.
Nerve supply: Constrictor pupillae is supplied by the parasympathetic fibres and dilator pupillae by the sympathetic fibres.
Actions: The constrictor and dilator pupillae constricts and dilates the pupil, respectively. - Posterior pigment epithelium, which is continuous with the ciliary part of the retina.
- [Diagram: layers of the iris]
The retina is present between the choroid and the hyaloid membrane of the vitreous humour. The retina diminishes in thickness from behind forwards. Anteriorly, it presents an irregular edge called _____________.
ora serrata
[Diagram]: note the hyaloid membrane of the vitreous humour and the ora serratae
State the blood supply of the retina.
The deeper part of the retina, i.e., up to the bipolar neurons is supplied by the central artery of the retina (a branch of the ophthalmic artery), while the superficial part of the retina up to the rods and cones is nourished by diffusion from the capillaries of the choroid.
State the venous drainage of the retina.
It is by central vein of the retina, which drains into the cavernous sinus.
An ophthalmoscope will show the central artery of retina enter the eye through the optic disc and divides into an upper division and a lower division to supply the upper and lower halves of retina. What will injury of the superior division of the central artery of retina lead to?
lower hemianopia with a sharp horizontal separation of upper and lower halves
Further notes:
The optic disc is the round spot on the retina formed by the passage of the axons of the retinal ganglion cells, which transfer signals from the photoreceptors of the eye to the optic nerve. [Diagram]
State the arterial supply of the eyeball.
- Central artery of the retina
- Long and short posterior ciliary arteries
- Anterior ciliary arteries
In the region of ciliary body, a major arterial circle is formed by the anastomosis between long posterior ciliary artery and anterior ciliary artery. From major arterial circle the vessel passes centripetally and anastomoses close to the pupillary margin to form the minor arterial circle.
[Diagram: arterial supply of the eyeball]
[Diagram: anterior ciliary arteries]
Further notes:
The anterior ciliary arteries are seven arteries in each eye-socket that arise from muscular branches of the ophthalmic artery and supply the conjunctiva, sclera, rectus muscles, and the ciliary body.
The interior of the eyeball is divided into two compartments by the lens: a small anterior compartment in front of the lens and a large posterior compartment behind the lens. The anterior compartment is divided into two chambers, i.e?
Anterior chamber (between cornea and iris)
Posterior chamber (between iris and lens)
These 2 chambers communicate with each other through pupil.
[Diagram]
The two chambers are separated by aqueous humour. State the roles of the aqueous humour.
- maintains the intraocular pressure
- nourishes the cornea and the lens which are otherwise avascular [The aqueous humour is rich in ascorbic acid, glucose, and amino acids.]
Describe the circulation of aqueous humour.
The aqueous humour is secreted in the posterior chamber by the ciliary processes. From here it passes into the anterior chamber through the pupil. Here it passes through the spaces in the iridocorneal angle, located between the fibres of ligamentum pectinatum and then enter into the canal of Schlemm, from where it is drained by the anterior ciliary veins.
The posterior compartment is behind the lens and much larger than the anterior compartment (comprising four fifths of the eyeball). It is surrounded almost completely by the retina and is filled with a colorless, transparent, jelly-like substance called vitreous humour/vitreous body.
a) The vitreous humour is enclosed in a delicate hyaloid membrane. Anteriorly, this membrane forms the ________________ in which rests the lens.
b) State the functions of the vitreous humour.
a) hyaloid fossa
b) Functions of the vitreous humour:
~ maintains intraocular pressure and therefore maintains the shape of the eyeball
~ holds the lens and the retina in place
Further notes:
The hyaloid fossa is a depression on the anterior surface of the vitreous body within the eye. It specifically lines the crystalline lens.
The hyaloid canal extends from optic disc to the lens. The canal marks the site of the ________________ in the fetus. It is a continuation of central artery of the retina which disappears 6 weeks before birth.
hyaloid artery
[Diagram]
(a) What is retinal detachment?
(b) State the anatomical basis for retinal detachment?
(a) This refers to separation of two layers of the retina (i.e., pigment and neural layers).
[Diagram: retinal detachment]
[Image 1: retinal detachment] [Image 2: retinal detachment]
(b) loss of adherent junctions*
What causes Horner’s syndrome? What is the commonest cause for Horner’s syndrome? What are the characteristic features of this syndrome?
~ Horner’s syndrome is caused by any lesion that leads to a loss of sympathetic function in the head.
~ The commonest cause for Horner’s syndrome is a tumor eroding the cervicothoracic ganglion/stellate ganglion, which is typically an apical lung tumor.
~ Characteristic features include:
■ pupillary constriction due to paralysis of the dilator pupillae muscle (i.e. miosis. Note. Mydriasis is the opposite; excessive pupillary dilation),
■ partial ptosis (drooping of the upper eyelid) due to paralysis of the superior tarsal muscle, and
■ absence of sweating on the ipsilateral side of the face and the neck due to absence of innervation of the sweat glands.
~ Secondary changes may also include:
■ ipsilateral vasodilation due to loss of the normal sympathetic control of the subcutaneous blood vessels, and
■ enophthalmos (sinking of the eye)—believed to result from paralysis of the orbitalis muscle, although this is an uncommon feature of Horner’s syndrome.
NB: The orbitalis muscle spans the inferior orbital fissure and helps maintain the forward position of orbital contents.
Briefly discuss Marcus Gunn pupil (description, causes, symptoms).
Description:
✓ [aka. Relative Afferent Pupil Defect (RAPD)] A Marcus Gunn pupil presents a disturbance to the consensual light reflex. When light is shone into the affected eye, there is reduced afferent input, and the pupils constrict fully. If light is then immediately shone into the unaffected eye, both pupils constrict normally. This abnormal response can be observed during the swinging flashlight test.
✓ Unilateral optic nerve dysfunction or retinal dysfunction
[Diagram]
Causes:
1. Disease of retina:
✓ retinal detachment
✓ retinal ischemia
2. Optic nerve disease:
✓ optic neuritis
✓ inflammation from multiple sclerosis (remember that the myelin sheath is destroyed in multiple sclerosis)
3. Severe glaucoma
4. Trauma
Symptoms:
Retinal detachment:
✓ floaters, ✓ flashes of light, ✓ shadow in vision field
Optic neuritis:
✓ pain with eye movement, ✓ loss of vision
Glaucoma:
✓ headaches, ✓ nausea or vomiting, ✓ eye redness, ✓ blurred vision
Further notes:
What is multiple sclerosis? Multiple sclerosis is a potentially disabling disease that affects the central nervous system, including the brain and spinal cord. It is an autoimmune disease where the immune system mistakenly attacks the protective myelin sheath covering the nerve fibres. This results in disruption of communication between the brain and the rest of the body.
Compare Adie’s pupil/tonic pupil and Argyll-Robertson pupil.
✓ Adie’s pupil, also called tonic pupil, was first reported by the British neurologist William John Adie in 1931. It manifests as unilateral or bilateral pupil dilation, direct and indirect loss or weakening of light reflexes, abnormal adjustment reflex, and pupil contraction disorder.
✓ Argyll Robertson pupil is sometimes confused with tonic or Adie’s tonic pupil. Adie’s tonic pupil is also characterized by light–near dissociation. However, while AR pupils are small and misshapen, Adie’s tonic pupils remain dilated, regardless of light levels
[Cartoon illustration: Argyll-Robertson pupil vs Adie pupil]
Further notes:
Light-near dissociation occurs when the pupillary light reaction is impaired, while the near reaction (accomodative response) remains intact. It can manifest unilaterally or bilaterally.
Briefly discuss glaucoma.
✓ If the drainage of aqueous humour is blocked, there occurs an abnormal increase in the intraocular pressure—a condition called glaucoma. As a result, there is severe pain in the eye due to pressure on the highly sensitive cornea.
✓ The glaucoma may cause variety of visual problems, viz. blindness due to compression of retina and its blood supply.
✓ The intraocular pressure which normally is about 15 mmHg can be measured on anesthetized cornea (tonometry).
Briefly discuss presbyopia.
Presbyopia (short vision): The lens plays an important role in accommodation. The lens absorbs much of the ultraviolet light and becomes increasingly yellowish with age. It also becomes harder with age. As a result of which the power of accommodation is lessened in old age producing a clinical condition called presbyopia.
Briefly discuss cataracts.
The opacity of the lens is termed cataract. With increasing age and in certain disease states, the lens becomes opaque. The increasing opacity leads to increasing visual impairment. This can be treated surgically by excision of opaque lens and replacement with an artificial lens. [Image]
Discuss the functions of the eye.
The main function of the eye is focusing of light waves and stimulation of photoreceptors of the retina. This requires five basic processes, viz.
1. Transmission of light waves through transparent media of the eyeball.
2. Refraction (bending) of light waves through different refractive media of different densities.
3. Accommodation of the lens to focus the light waves.
4. Regulation of amount of light entering the eye through pupil by iris diaphragm.
5. Convergence of eyeballs.
List the refractive media of the eye that form the refractive apparatus of the eye.
- Cornea (very refractive but not adjustable).
- Aqueous humour.
- Lens (refractive and adjustable).
- Vitreous humour/vitreous body.
Note: Most of the refraction by eye takes place at the anterior surface of the cornea (not in the lens as generally thought).
State the function of the iris.
It regulates the amount of light entering the eye.
What is the main factor contributing to the transparency of the cornea?
The collagen fibrils in the cornea run parallel to its surface. Due to their regular arrangement, they allow light to pass through without significant scattering. The fibrils are arranged in an orthogonal orientation to the direction of light.
