Lecture 13 - The Orbit: Complete Simple Explanation

1. ORBIT / BONY ORBIT

• The eyeball
• The optic nerve
• The extra-ocular (eye-moving) muscles
• The lacrimal apparatus (tear system)
• Fat, connective tissue (fascia)
• Nerves and blood vessels supplying all of these

Shape

• The apex of the pyramid = the optic foramen (the hole for the optic nerve)
• The base (orbital rim) is formed by:
  • Superiorly: the frontal bone
  • Medially: the frontal process of the maxilla
  • Inferiorly: the zygomatic process of the maxilla + the zygomatic bone
  • Laterally: the zygomatic bone, the frontal process of the zygomatic bone, and the zygomatic process of the frontal bone

Seven Bones

1. Maxilla
2. Zygomatic bone
3. Frontal bone
4. Ethmoid bone
5. Lacrimal bone
6. Sphenoid bone
7. Palatine bone

Roof (Superior Wall)

• Made mostly of the orbital part of the frontal bone, with a small contribution from the sphenoid bone.
• This thin plate of bone separates the orbit from the brain in the anterior cranial fossa.
• Unique features of the roof:
  • Anteromedially: a small pit called the trochlear fovea, where a pulley for the superior oblique muscle attaches. Part of the frontal sinus may also intrude here.
  • Anterolaterally: a depression called the lacrimal fossa, which holds the orbital part of the lacrimal (tear) gland.
• Posteriorly, the lesser wing of the sphenoid completes the roof.

Medial Wall

• Made of four bones: maxilla, lacrimal, ethmoid, and sphenoid.
• The largest contributor is the orbital plate of the ethmoid bone. This part contains collections of ethmoidal air cells (clearly visible in a dry skull).
• At the junction of the roof and medial wall (near the frontoethmoidal suture): the anterior and posterior ethmoidal foramina. These are where the anterior and posterior ethmoidal nerves and vessels leave the orbit into the ethmoid bone.
• In front of the ethmoid is the small lacrimal bone, and in front of that is the frontal process of the maxilla.
• These two bones (lacrimal bone + frontal process of maxilla) form the lacrimal groove, which holds the lacrimal sac. The groove is bordered by:
  • Posterior lacrimal crest (part of lacrimal bone)
  • Anterior lacrimal crest (part of maxilla)
• Behind the ethmoid, a small part of the sphenoid bone completes the medial wall, forming part of the medial wall of the optic canal.

Floor (Inferior Wall)

• The floor is also the roof of the maxillary sinus.
• Made primarily of the orbital surface of the maxilla, with small contributions from the zygomatic bone and the palatine bone.
• Starting posteriorly and continuing along the lateral boundary of the floor is the inferior orbital fissure.
• Beyond the front end of the inferior orbital fissure, the zygomatic bone completes the floor.
• Posteriorly, the orbital process of the palatine bone makes a tiny contribution near where the maxilla, ethmoid, and sphenoid meet.

Lateral Wall

• Made of two bones:
  • Anteriorly: the zygomatic bone
  • Posteriorly: the greater wing of the sphenoid bone
• The superior orbital fissure lies between the greater wing and the lesser wing of the sphenoid (the lesser wing forms part of the roof).

2. EYELIDS

Layers (front to back)

1. Skin
2. Subcutaneous tissue
3. Voluntary muscle (orbicularis oculi)
4. Orbital septum
5. Tarsus
6. Conjunctiva

Skin and Subcutaneous Tissue

• The skin of the eyelids is thin and not particularly tough.
• Only a thin, loosely arranged layer of connective tissue separates the skin from the voluntary muscle underneath.
• Because this connective tissue is so loose, fluid and blood accumulate easily after an injury (this is why black eyes swell so much).

3. ORBITAL SEPTUM

• It is an extension of the periosteum (the membrane lining the bones) from the margin of the orbit into both the upper and lower eyelids.
• It extends downward into the upper eyelid and upward into the lower eyelid.
• It is continuous with the periosteum both outside and inside the orbit.
• In the upper eyelid, the orbital septum attaches to the tendon of the levator palpebrae superioris muscle.
• In the lower eyelid, the orbital septum attaches directly to the tarsus.

4. CONJUNCTIVA

• Covers the posterior (inner) surface of each eyelid completely.
• Reflects (folds back) onto the outer surface of the eyeball (the sclera, which is the white of the eye).
• Attaches to the eyeball at the junction between the sclera and the cornea.

• Superior conjunctival fornix (upper recess)
• Inferior conjunctival fornix (lower recess)

5. ORBICULARIS OCULI

Orbital Part

• Surrounds the orbit (the bony ring around the eye).
• The larger, outer ring of muscle.

Palpebral Part

• Located within the eyelids themselves (the thin inner part).
• Anchored medially by the medial palpebral ligament, which attaches to the anterior lacrimal crest.
• Laterally, it blends with fibers from the muscle in the lower eyelid at the lateral palpebral ligament.

Lacrimal Part (Third Part)

• A third part can be identified: fibers on the medial border that pass deeply and attach to the posterior lacrimal crest.
• This is the lacrimal part of the orbicularis oculi.
• It may be involved in the drainage of tears - when it contracts during blinking, it may dilate the lacrimal sac and draw tears into it through the canaliculi from the conjunctival sac.

6. TARSUS AND LEVATOR PALPEBRAE SUPERIORIS

Tarsus (Tarsal Plates)

• Superior tarsus - larger, in the upper eyelid.
• Inferior tarsus - smaller, in the lower eyelid.

• Medially: to the anterior lacrimal crest of the maxilla by the medial palpebral ligament.
• Laterally: to the orbital tubercle on the zygomatic bone by the lateral palpebral ligament.

Levator Palpebrae Superioris

• Origin: from the posterior part of the roof of the orbit, just above the optic foramen.
• Insertion: into the anterior surface of the superior tarsus, and possibly a few fibers into the skin of the upper eyelid.
• Innervation: the oculomotor nerve [III].

Superior Tarsal Muscle (Smooth Muscle)

• This is the superior tarsal muscle.
• It is innervated by postganglionic sympathetic fibers from the superior cervical ganglion.

7. ORBITAL GLANDS

Tarsal Glands (Meibomian Glands)

• Embedded in the tarsal plates of both the upper and lower eyelids.
• They are modified sebaceous glands (oil glands).
• They empty onto the free margin (edge) of each eyelid.
• Their secretion is an oily substance that:
  • Increases the viscosity (thickness) of tears.
  • Decreases the rate of evaporation of tears from the eyeball surface.
• Blockage and inflammation of a tarsal gland = chalazion - located on the inner surface of the eyelid.

Sebaceous Glands and Sweat Glands (of Eyelash Follicles)

• Associated with the eyelash follicles.
• Blockage and inflammation of either of these = a stye - located on the edge of the eyelid.

8. FISSURES AND FORAMINA

Optic Canal

• Viewed from the front, the round opening at the apex of the pyramid-shaped orbit is the optic canal.
• It opens into the middle cranial fossa.
• Bounded medially by the body of the sphenoid and laterally by the lesser wing of the sphenoid.
• Structures passing through the optic canal:
  • Optic nerve
  • Ophthalmic artery

Superior Orbital Fissure

• Just lateral to the optic canal, a triangular-shaped gap between the roof and lateral wall of the orbit.
• Allows structures to pass between the orbit and the middle cranial fossa.
• Structures passing through the superior orbital fissure:
  • Superior branch of the oculomotor nerve [III]
  • Inferior branch of the oculomotor nerve [III]
  • Trochlear nerve [IV]
  • Abducent nerve [VI]
  • Lacrimal branch of the ophthalmic nerve [V1]
  • Frontal branch of the ophthalmic nerve [V1]
  • Nasociliary branch of the ophthalmic nerve [V1]
  • Superior ophthalmic vein

Inferior Orbital Fissure

• A longitudinal opening separating the lateral wall from the floor of the orbit.
• Borders: the greater wing of the sphenoid + the maxilla, palatine, and zygomatic bones.
• Allows communication between:
  • The orbit and the pterygopalatine fossa (posteriorly)
  • The orbit and the infratemporal fossa (in the middle)
  • The orbit and the temporal fossa (posterolaterally)
• Structures passing through the inferior orbital fissure:
  • Maxillary nerve [V2] and its zygomatic branch
  • Infra-orbital vessels
  • A vein communicating with the pterygoid plexus of veins

Infra-orbital Foramen

• Starting posteriorly and crossing about two-thirds of the inferior orbital fissure, there is a groove (the infra-orbital groove) that continues forward across the floor of the orbit.
• This groove connects with the infra-orbital canal, which opens onto the face at the infra-orbital foramen.
• Structures passing through: the infra-orbital nerve (part of the maxillary nerve [V2]) and infra-orbital vessels, as they exit onto the face.

Other Openings (Medial Wall)

• Located at the junction between the superior and medial walls.
• These provide exits from the orbit into the ethmoid bone for the anterior and posterior ethmoidal nerves and vessels.

• In the lower part of the medial wall, anteriorly, there is a canal.
• A visible depression for the lacrimal sac is formed by the lacrimal bone and the frontal process of the maxilla.
• This depression continues into the nasolacrimal canal, which leads to the inferior nasal meatus.
• Contained within the nasolacrimal canal is the nasolacrimal duct, which is part of the lacrimal apparatus.

9. LACRIMAL APPARATUS

1. The lacrimal gland and its ducts
2. The lacrimal canaliculi
3. The lacrimal sac
4. The nasolacrimal duct

Lacrimal Gland

• Located anteriorly in the superolateral (upper outer) region of the orbit.
• The levator palpebrae superioris muscle divides it into two parts:
  • Larger orbital part: sits in a depression (the lacrimal fossa) in the frontal bone.
  • Smaller palpebral part: sits inferior to the levator palpebrae superioris in the superolateral part of the eyelid.
• Numerous ducts empty the glandular secretions into the lateral part of the superior fornix of the conjunctiva.

How Tears Flow

1. Fluid is continually secreted by the lacrimal gland.
2. As the eyelids blink, the fluid is moved across the surface of the eyeball from lateral to medial.
3. Fluid accumulates medially in the lacrimal lake.
4. From the lacrimal lake, fluid is drained by the lacrimal canaliculi - one canaliculus for each eyelid.
5. The opening through which fluid enters each canaliculus is the lacrimal punctum.

Lacrimal Canaliculi and Lacrimal Sac

• Passing medially, the lacrimal canaliculi eventually join the lacrimal sac.
• The lacrimal sac lies between the anterior and posterior lacrimal crests, posterior to the medial palpebral ligament and anterior to the lacrimal part of the orbicularis oculi muscle.
• When the orbicularis oculi contracts during blinking, the small lacrimal part of the muscle may dilate the lacrimal sac, drawing tears into it through the canaliculi from the conjunctival sac.

Nasolacrimal Duct

• From the lacrimal sac, tears drain down through the nasolacrimal duct, which runs through the nasolacrimal canal.
• It drains into the inferior nasal meatus (the lower passage inside the nose) - this is why your nose runs when you cry.

10. EYEBALL

1. Anterior chamber
2. Iris and pupil
3. Posterior chamber
4. Lens
5. Postremal (vitreous) chamber
6. Retina

Anterior and Posterior Chambers

• The space directly posterior to the cornea and anterior to the iris (the colored part of the eye).
• The central opening in the iris is the pupil.

• Posterior to the iris and anterior to the lens. It is smaller than the anterior chamber.

• Is secreted into the posterior chamber.
• Flows through the pupil into the anterior chamber.
• Is absorbed into the scleral venous sinus (canal of Schlemm) - a circular venous channel at the junction between the cornea and the iris.
• Supplies nutrients to the avascular (no blood vessels) cornea and lens.
• Maintains intra-ocular pressure.
• If this balance is disturbed and fluid increases, intra-ocular pressure rises - this condition is called glaucoma, which can cause visual problems.

Lens and Vitreous Humor

• Separates the anterior one-fifth from the posterior four-fifths of the eyeball.
• A transparent, biconvex, elastic disc attached around its circumference to muscles on the outer wall of the eyeball.
• This lateral attachment allows the lens to change shape and thus change its refractive power (ability to bend light) to maintain visual focus (visual acuity).
• Clinical term for opacity (cloudiness) of the lens: cataract.

• The posterior four-fifths of the eyeball (from lens to retina) is the postremal (vitreous) chamber.
• It is filled with a transparent, jelly-like substance - the vitreous body (vitreous humor).
• Unlike aqueous humor, vitreous humor cannot be replaced if lost.

Walls of the Eyeball

Outer Fibrous Layer

• Sclera posteriorly: covers the posterior and lateral parts, about five-sixths of the eyeball surface. It is an opaque layer of dense connective tissue - the "white of the eye." It is pierced by vessels and nerves (including the optic nerve posteriorly) and provides attachment for the eye muscles.
• Cornea anteriorly: continuous with the sclera, covers the anterior one-sixth of the eyeball. It is transparent, allowing light to enter the eyeball.
• The fascial sheath (Tenon's capsule) covers the surface of the sclera externally from the entrance of the optic nerve to the corneoscleral junction. Internally the sclera is loosely attached to the choroid.

Middle Vascular Layer

• The posterior two-thirds of the vascular layer.
• A thin, highly vascular (lots of blood vessels), pigmented layer.
• Smaller vessels lie adjacent to the retina; larger vessels are more peripheral.
• Firmly attached to the retina internally and loosely attached to the sclera externally.

• Extends from the anterior border of the choroid.
• A triangular-shaped structure between the choroid and the iris.
• Forms a complete ring around the eyeball.
• Has two components:
  • Ciliary muscle: smooth muscle fibers arranged longitudinally, circularly, and radially. Controlled by parasympathetics from the oculomotor nerve [III]. When they contract, they decrease the size of the ring formed by the ciliary body.
  • Ciliary processes: longitudinal ridges projecting from the inner surface of the ciliary body. Zonular fibers extend from them, attaching to the lens, suspending it in position - these collectively form the suspensory ligament of the lens.
• How accommodation (focus adjustment) works: When the ciliary muscle contracts, the ring of the ciliary body gets smaller. This reduces tension on the suspensory ligament of the lens. The lens then becomes more rounded (relaxed) - this is how the eye focuses for near vision.
• The ciliary processes also contribute to the formation of aqueous humor.

• The colored part of the eye, projecting outward from the ciliary body.
• Has a central opening: the pupil.
• Contains two sets of smooth muscle fibers that control the size of the pupil:
  • Sphincter pupillae: circularly arranged fibers. Innervated by parasympathetics. Contraction constricts (decreases) the pupillary opening.
  • Dilator pupillae: radially arranged fibers (myoepithelial cells). Innervated by sympathetics. Contraction dilates (increases) the pupillary opening.

Inner Layer - Retina

• Optic part of the retina (posterior and lateral): sensitive to light.
• Nonvisual part (anterior): covers the internal surface of the ciliary body and iris.
• The junction between these two parts is an irregular line called the ora serrata.

• Pigmented layer (outer): firmly attached to the choroid. Continues anteriorly over the internal surface of the ciliary body and iris.
• Neural layer (inner): can be subdivided into its various neural components. Only attached to the pigmented layer around the optic nerve and at the ora serrata. In a detached retina, it is the neural layer that separates.

• Optic disc: where the optic nerve leaves the retina. Lighter than surrounding retina. Branches of the central retinal artery spread from this point. There are no light-sensitive cells here, so it is called the blind spot.
• Macula lutea: lateral to the optic disc. A small area with a slight yellowish color. Contains a central depression called the fovea centralis.
  • The fovea centralis is the thinnest area of the retina.
  • It has fewer rods (cells for dim light, no color) and more cones (cells for bright light and color).
  • Visual sensitivity is highest at the fovea centralis.

11. OCULAR MUSCLES - Names and Functions

Intrinsic Muscles of the Eye (Inside the eyeball)

Extrinsic/Extra-ocular Muscles (Move the eyeball)