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.
- 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.
- Infraorbital margin is formed by the zygomatic bone laterally and maxilla medially; it is continuous with anterior lacrimal crest medially.
- Medial orbital margin is ill-defined. It is formed by the frontal bone above and lacrimal crest of the frontal process of maxilla below.
- Lateral orbital margin is formed by zygomatic process of the frontal bone above and frontal process of the zygomatic bone below.
- [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.
- Infraorbital and zygomatic branches of the maxillary nerve (and the accompanying vessels)
- Orbital branches from the pterygopalatine ganglion
- Connection between the inferior ophthalmic vein and the pterygoid venous plexus.
- [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).
- Eyeball
- Muscles
- Fascia bulbi
- Nerves
(a) Optic
(b) Oculomotor (c) Trochlear (d) Abducens (e) Ophthalmic
(f) Ciliary ganglion - Ophthalmic artery
- Ophthalmic veins
- Lacrimal gland
- 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.
- Lateral check ligament: Extends from the fascial sleeve of lateral rectus for attachment to the lateral wall of the orbit on Whitnall’s tubercle.
- 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.
- 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.
- [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?
- Orbital septa [these are extensions into the upper and lower eyelids]
- Lacrimal fascia [this fascia covers the lateral surface of the lacrimal sac]
- It forms a fibrous pulley, the trochlea to hold the superior oblique muscle.
- 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.]
