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1. Describe the structure and relative location of the bony orbit. Identify the major foramina, fissures, and fossae associated with the orbit and what is transmitted through these structures.

The bony orbit is formed by seven bones that form a pyramid-shaped space. The base of the pyramid is the orbital rim, which is composed of the frontal bone, maxilla, and zygomatic bone. The long axis of the orbit projects posteromedially, which will have important ramifications for the functions of extra-ocular muscles, the muscles which move the eye.

The orbit is located adjacent to many important regions of the skull. It is located inferior to the anterior cranial fossa,  and anterior to the middle cranial fossa. The orbit is located lateral to the nasal cavity and several of the paranasal sinuses, which are membrane-lined cavities contained by bone. Paranasal sinuses drain into the nasal cavity and are named for the bones that contain them. Thus, the orbit is inferolateral to the frontal sinus, lateral to the ethmoidal air cells, and superior to the maxillary sinus. The orbit also is anterior to the pterygopalatine fossa, an important space for branches of the maxillary nerve (V2) and artery.

The orbit is filled with orbital fat, which surrounds and protects the eye, extra-ocular mm., neurovasculature, and lacrimal gland.

The bony orbit can be divided into four areas: roof, medial wall, lateral wall, and floor.

Roof:

• Formed mostly by the frontal bone

  • Trochlear fovea (medial)

    • Trochlea - cartilaginous pulley for tendon of superior oblique m.

  • Lacrimal fossa (lateral)

    • Lacrimal gland - produces tears

• Minor portion of the sphenoid, lesser wing towards the apex

  • Hosts the Optic canal, which conveys:

    • Optic n. (CN II), & 

    • Ophthalmic a.

• Separates orbit from the anterior cranial fossa (superior)

• Frontal (paranasal) sinus found superomedially

Lateral wall:

• Zygomatic bone anteriorly

• Sphenoid, greater wing posteriorly

• Separated from the roof by the superior orbital fissure, which conveys:

  • Oculomotor n. (CN III), 

  • Trochlear n. (CN IV), 

  • Ophthalmic n. (V1) brs., 

  • Abducens n. (CN VI), &

  • Superior ophthalmic v.

Floor:

• Maxilla: majority of floor

• Zygomatic bone: anterolaterally

• Palatine bone: posteromedially

• Separated from the lateral wall by the inferior orbital fissure, which:

  • Conveys the infra-orbital n. & zygomatic n. (brs. of V2), &

  • Connects the pterygopalatine fossa with the orbit.

• Maxillary sinuses located inferiorly

Medial wall:

• The frontal bone forms the superior-most portion of the medial wall

• Below the frontal bone, the following bones form the remainder of the medial wall, from anterior to posterior:

  • Maxilla

  • Lacrimal bone

    • Nasolacrimal fossa (tear drainage)

  • Ethmoid bone: forms majority of medial wall

    • Ethmoidal air cells medial

  • Sphenoid

2. Detail the components of the palpebrae (eyelids) and associated structures.

The eyelids (palpebrae) sit anterior to the orbit, and serve to protect and cover the eye.

The tissues (superficial-to-deep) of the eyelids include:

• Skin: continuous with the conjunctiva; eyelash follicles

• Subcutaneous tissue: thin layer with little adipose tissue

• Palpebral fibers of orbicularis oculi m.

• Tarso-fascial layer: composed of the orbital septum, superior & inferior tarsal plates, medial & lateral palpebral ligaments, tarsal glands, & the aponeurosis of levator palpebrae superioris m. (superior eyelid only)

• Palpebral conjunctiva

The orbicularis oculi m. is a muscle of facial expression, which controls the palpebral fissure, the opening through which the eye encounters the world, and associated regions. Orbicularis oculi m. is tripartite, consisting of:

• Orbital part - outermost part; voluntarily controlled; this part can tightly shut the palpebral fissure

• Palpebral part - innermost part; involuntarily controlled; this part facilitates blinking

• Lacrimal part - deep & medial; facilitates tear movement 

The orbicularis oculi m., like all muscles of facial expression, is innervated by the facial n. (CN VII).

The tarso-fascial layer of the eyelid is an association of connective tissues that gives structure and support to the features superficial to the bony orbit. 

The major constituents of the tarso-fascial layer include:

• Orbital septum:

  • a weak, membranous sheet, continuous with the periosteum of the bones of the orbital rim

  • separates the contents of the orbit from the contents of the palpebrae

• Superior & inferior tarsal plates:

  • provide structure the palpebrae

• Medial & lateral palpebral ligaments:

  • bands of tissue that anchor the superior and inferior palpebrae to the medial and lateral aspects of the orbit

• Aponeurosis (tendon) of levator palpebrae superioris m.:

  • attaches to superior tarsal plate

  • tarsal m. (autonomic) 

• Tarsal (Meibomian) glands:

  • modified sebaceous glands

  • secrete meibum to prevent tear evaporation

The conjunctiva is a delicate mucous membrane that lines the eyelids and covers the anterior aspect of the eyeball (excepting the cornea and pupil). The bulbar conjunctiva covers the eyeball, and the palpebral conjunctiva lines the eyelids. The conjunctival sac is the potential space between bulbar and palpebral conjunctiva into which tears are secreted and conducted. The conjunctiva is continuous with the skin of the palpebrae at the margins of the palpebral fissure.

3. Describe the components of the lacrimal system, diagram the route of lacrimal fluid from the lacrimal gland to the nasal cavity, and distinguish between somatic and autonomic innervation of the lacrimal gland. 

The lacrimal system begins with the lacrimal gland, located in the superolateral margin of the orbit. The lacrimal gland rests in the lacrimal fossa of the frontal bone. The lacrimal gland secretes lacrimal (tear) fluid into the superolateral margin of the conjunctival sac. Tear fluid is aided by the meibum secretions of the tarsal glands, which helps to prevent evaporation. Tear fluid is conducted across the conjunctiva and accumulates in the lacrimal lake at the medial canthus.  Tear fluid is conducted from the lacrimal lake into the lacrimal sac, the proximal (superior) dilation of the nasolacrimal duct. The lacrimal sac lies in the lacrimal groove of the lacrimal bone. The nasolacrimal duct conducts tear fluid into the inferior nasal meatus.

Innervation 

The lacrimal n. is a branch of the ophthalmic division (V1) of the trigeminal n. that is somatic afferent from the lacrimal gland and conjunctiva and skin of the upper eyelid. 

Preganglionic parasympathetic fibers that serve the lacrimal gland originate from the facial n. (CN VII) and synapse in the pterygopalatine ganglion. Postganglionic fibers from the pterygopalatine (sphenopalatine) ganglion make their ways to the lacrimal gland by hitchhiking on branches of the maxillary division of the trigeminal n. (V2). Parasympathetic stimulation enhances lacrimation. Sympathetic fibers are sourced from the superior cervical ganglion. 

4. Describe the basic layers and supporting apparatus of the eye and its various components.

The eye consists of three layers: fibrous layer, vascular layer (uvea), & the nervous layer (retina). The three layers of the eye surround the inner part of the eye.

Layers of the Eye

The fibrous layer of the eye is important in providing structure and strength to the eye. This layer consists of the:

• Sclera

  • Covers the majority of the eye

  • Often referred to as the “white of the eye”

  • Insertion site for extra-ocular and intrinsic eye muscles

• Cornea 

  • Covers the anterior portion of the eye

  • Completely avascular

  • Sensitive to touch

  • Main refractive index of the eye

The vascular layer (uvea) transmits & refracts light, and is composed of 3 structures: 

• Iris

  • Diaphragm located on the anterior surface of the lens

  • Portion of the uvea with color

  • Pupil (aperture for transmitting light; central in iris; appears black)

    • Controlled by two muscles: 

      • Sphincter pupillae mm. (contracts pupil, parasympathetically-controlled)

      • Dilator pupillae mm. (dilates pupil, sympathetically-controlled)

• Ciliary body

  • Anchors lens

  • Ciliary m. (smooth) contracts for accommodation

  • Secretes aqueous humour & portions of vitreous humour

• Choroid

  • Richly vascular & pigment

  • Thin, middle layer of posterior eye

The nervous layer (retina) is responsible for modulating the special sense of sight, and is thus:

• Composed of 10 layers that contain specialized light receptors (rods & cones), ganglion cells & other cells

• Axons of ganglion cells exit the retina as the optic n. (CN II)

  • Central retinal a. supplies optic n. & retina

    • Bundled within optic n.

    • Branch of the ophthalmic a.

The inner part of the eye consists of the lens, anterior segment, and posterior segment. The anterior segment contains aqueous humour, whereas the posterior segment contains the vitreous humour.

5. Describe the attachments, actions, and innervations of the extra-ocular muscles.

The extra-ocular muscles are muscles found within the orbit that have proximal attachments on either the common anular tendon (tendinous ring), or osseous features within the orbit and distal attachments to either superior palpebra (e.g. levator palpebrae superioris m.), or the sclera (all other extra-ocular mm.). With the exception of the levator palpebrae superioris m., all extra-ocular muscles affect gaze, and often their actions are complicated by the asymmetry of the orbit.

Extra-ocular muscles, excepting levator palpebrae superioris m., are often referred to as extrinsic eye muscles. Extra-ocular muscles are innervated by one of the following cranial nerves: oculomotor n. (CN III), trochlear n. (CN IV), or abducens n. (CN VI).

Please consult the following table for the attachments, actions, and innervations of the extra-ocular muscles. 

Gaze, the direction of the pupil, may: elevate (move upwards), depress (move downwards), adduct (move medially), abduct (move laterally), intort (superior pole slightly rotates medially), extort (superior pole slightly rotates laterally), or some combination of the preceding. 

6. Diagram the somatic and autonomic innervation of the orbit & eye.

The orbit and its contents are served the optic n. (CN II), oculomotor n. (CN III), trochlear n. (CN IV), ophthalmic n. (V1), abducens n. (VI), facial n. (VII), and autonomic perivascular plexuses of the internal carotid a. Parasympathetic functions of the eye (constriction of the pupil & accommodation) are served by the ciliary ganglion. Sympathetic functions of the eye (dilation of the pupil and constriction of vasculature) are served by fibers from the superior cervical ganglion of the sympathetic trunk. 

Optic n. (CN II):

The optic n. (CN II) arises from the optic chiasm of the diencephalon and enters the orbit through the optic canal. The optic n. conveys special somatic afferent fibers for vision.

Oculomotor n. (CN III):

The oculomotor n. (CN III) arises from the midbrain between the posterior cerebral a. & superior cerebellar a., travels within the lateral wall of the cavernous sinus, divides into superior and inferior divisions, and these divisions enter the orbit through the superior orbital fissure. 

The oculomotor n. efferently innervates most of the extra-ocular mm., specifically the:

• Levator palpebrae superioris m.

• Superior rectus m.

• Medial rectus m.,

• Inferior rectus m.,

• Inferior oblique m., and also

• transmits preganglionic parasympathetic fibers to the:

  • Ciliary ganglion to serve

    • Sphincter pupillae muscles (constricts pupils)

    • Ciliary muscles (accommodation)

Trochlear n. (CN IV):

The trochlear n. (CN IV) is the smallest cranial nerve and the only nerve to arise from the dorsal aspect of the brainstem. The trochlear n.: 

• travels within the lateral wall of the cavernous sinus

• enters the orbit through the superior orbital fissure, and 

• efferently innervates the superior oblique m. 

Abducens n. (CN VI): 

The abducens n. (CN VI) arises from the brainstem, and 

• travels through the cavernous sinus

• enters the orbit through the superior orbital fissure efferently innervates the lateral rectus m. (i.e. the ABDUCens ABDUCts the eye).

Ophthalmic n. (V1) branches:

The ophthalmic n. (v1) is one of three divisions of the trigeminal n. (CN V), and it afferently serves the orbit and eye. 

The ophthalmic n. consists of several important branches, including the:

• Frontal n.:

  • Largest branch of V1

    • A prominent feature of the superior orbit, found superior to the levator palpebrae superioris m.

  • Afferently serves the medial upper eyelid (skin & conjunctiva), skin of the forehead, and frontal sinus mucosa

  • Gives rise to:

    • Supra-orbital n.

      • Lateral & larger

      • May be seen exiting the cranium via the supra-orbital foramen or notch

    • Supratrochlear n.

      • Medial & smaller 

• Lacrimal n. 

  • Smallest branch of V1. 

  • Enters the orbit through the superior orbital fissure, superior to lateral rectus m.

  • Afferently serves the lacrimal gland.

• Nasociliary n. 

  • Branches from V1 in the superior orbital fissure

  • Gives rise to several branches, which afferently serve the paranasal sinuses, nasal cavity, conjunctiva, skin of eyelids, eye (directly & through the ciliary ganglion), as well as transmits postganglionic sympathetic fibers from superior cervical ganglion via perivascular plexuses to the dilator pupillae muscles of the eye

Ciliary ganglion

The ciliary ganglion is a peripheral parasympathetic ganglion for intra-ocular muscles, typically located on the lateral aspect of the optic n. (CN II) near the apex of the orbit. The ciliary ganglion receives preganglionic parasympathetic fibers from the oculomotor n. (CN III).

The ciliary ganglion communicates with the eye via the short ciliary nn. which transmit three types of fibers between the eye and the ciliary ganglion:

• Afferent fibers from the eye, through the ganglion, to the nasociliary n. (V1),

• Postganglionic parasympathetic fibers from the ganglion (preganglionic fibers from oculomotor n.) to sphincter pupillae and ciliary muscles, and

• Postganglionic sympathetic fibers (from superior cervical ganglion via perivascular plexuses) to the intra-ocular vessels (vasoconstriction).

Facial n. (CN VII):

The facial n. serves a variety of features and functions. It is best known for efferent innervation of the muscles of facial expression. In the orbit, the facial n. has a very narrow function in that it provides preganglionic parasympathetic fibers to the pterygopalatine (sphenopalatine) ganglion, which among other things, serves the lacrimal gland by means of postganglionic fibers that hitchhike to the gland on branches of the maxillary n. (V2). Parasympathetic stimulation increases lacrimation.

7. Diagram the branches of ophthalmic artery and veins.

The ophthalmic artery is the main blood supply to the orbit and eye. 

The ophthalmic a. branches from the internal carotid a. (ICA) after the ICA emerges from the lumen of the cavernous sinus and enters the orbit through the optic canal. It typically crosses to the medial wall of the orbit. The bulk of the ophthalmic a. may be found between the bellies of the superior oblique m. and the medial rectus m. The branches of the ophthalmic a. typically accompany their namesake V1 branches on their way to serve target structures (e.g. lacrimal a., ant./post. ethmoidal aa., supra-orbital a., supratrochlear a., etc.). 

The first branch of the ophthalmic a., the central retinal artery, is bundled within the dural sheath of the optic n. (CN II).

The superior ophthalmic v. drains blood from the eye, superior orbit, ethmoidal cells, forehead, and the angular v. of the face. The superior ophthalmic v. traverses the superior optic fissure en route to the cavernous sinus. The inferior ophthalmic v. collects blood from the face and inferior orbit. The inferior ophthalmic v. may join the superior ophthalmic v. in traversing the superior orbital fissure, and it also may have parts that traverse the inferior orbital fissure to communicate with the deep face. Both the superior and inferior ophthalmic vv. may transport blood (and potential infection) from the face to the cavernous sinus.