Orbit and Eye
Written Learning Objectives
1. What are the bones forming 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, and form a pyramidal-shaped space. The bony orbit can be divided into four areas: roof, medial and lateral walls, and floor.
Roof:
Formed mostly by the frontal bone and a minor portion of the sphenoid towards the apex
Medial wall:
The frontal bone forms the superior portion of the medial wall
From anterior to posterior:
Maxilla
Lacrimal bone
Ethmoid bone: forms majority of medial wall
Sphenoid
Lateral wall:
Zygomatic bone anteriorly
Sphenoid posteriorly
Floor:
Maxilla: majority of floor
Zygomatic bone: anterolaterally
Palatine bone: posteromedially
Bony structures
Optic canal
Optic n. (CN II) & ophthalmic a.
Superior orbital fissure
Oculomotor n. (CN III), trochlear n. (CN IV), branches of ophthalmic n. (V1), abducens n. (CN VI), and superior ophthalmic v.
Inferior orbital fissure
Infra-orbital n. & zygomatic n. (V2)
Connection between pterygopalatine fossa & orbit
Lacrimal fossa
Nasolacrimal sac
2. Describe various components of the eyelid (palpebra) and its associated structures.
The eyelids (palpebrae) layered from superficial-to-deep, include: skin, subcutaneous tissue, palpebral fibers of orbicularis oculi m. and associated connective tissues, a tarso-fascial layer, and palpebral conjunctiva (which rests upon the orbital conjunctiva of the eye). The superior eyelid includes the aponeurosis of the levator palpebrae superioris m attaching to the tarso-fascial layer. The edge of the eyelids host eyelashes and associated tarsal glands.
In other words, the tissues (superficial-to-deep) of the eyelids include:
Skin: continuous with the conjunctiva
Subcutaneous tissue: thin layer with little adipose tissue
Palpebral fibers of orbicularis oculi m.
Tarso-fascial layer: composed of tarsal plates, medial & lateral palpebral ligaments, tarsal glands, aponeurosis of levator palpebrae superioris m. (superior eyelid only), superior & inferior tarsal mm.
Palpebral conjunctiva: split into marginal, tarsal, and orbital parts
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 fornices (superior and inferior) are the reflections of conjunctiva from bulbar to palpebral conjunctiva, and the conjunctival sac is the potential space between bulbar and palpebral conjunctiva.
3. What are the components of the lacrimal system? What is the route of lacrimal fluid?
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, and sits between the aponeurosis for levator palpebrae superioris m. (medial to the gland) and the lateral rectus m. (inferior to the gland). The lacrimal gland secretes into the conjunctival sac along the lateral margin of the superior fornix. Tear fluid is conducted across the conjunctiva and accumulates in the lacrimal lake in the medial canthus. Deep to the lacrimal lake is a mound of skin, the lacrimal caruncle. Superior and inferior to the lacrimal lake are lacrimal papillae, which host small lacrimal puncta (pores) into which tear fluid flows. Lacrimal (tear) fluid is conducted from the puncta via superior and inferior lacrimal canaliculi 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.
Route of lacrimal fluid:
lacrimal gland → conjunctival sac → lacrimal lake → lacrimal papillae → lacrimal puncta → lacrimal canaliculi (superior & inferior) → lacrimal sac → nasolacrimal duct → inferior nasal meatus
4. What is the parasympathetic pathway serving the lacrimal gland?
The lacrimal n. is a branch of the ophthalmic division (V1) of the trigeminal n. that is afferent from the lacrimal gland and conjunctiva and skin of the upper eyelid. While the lacrimal n. MAY serve as a conduit for some of the postganglionic parasympathetic fibers serving the lacrimal gland, it is not the source of those fibers.
Anatomists have classically understood (and taught) that the secretomotor postganglionic fibers to the lacrimal gland were conducted by the lacrimal n. from fibers transmitted from the pterygopalatine ganglion via the zygomaticotemporal brs. (i.e. pterygopalatine ganglion → zygomaticotemporal brs. → lacrimal n. → lacrimal gland). Recent studies show considerable variation in the course of these fibers, with the majority of cases demonstrating secretomotor postganglionic fibers from the pterygopalatine ganglion directly innervating the lacrimal gland via the zygomaticotemporal brs. Variants do exist that reflect the ‘classical condition.’
5. What are the three parts of the eyeball? Describe the supporting apparatus of the eyeball and its various components.
The fibrous layer of the eye is important in providing structure and strength to the eyeball. This layer consists of the sclera and cornea. The sclera covers the majority of the eyeball, and is often referred to as the “white of the eye.” It is the insertion site for extra-ocular and intrinsic eye muscles. The cornea covers the anterior portion of the eye (approximately ⅙th of the anterior eye - Moore et al., 7th edition), is completely avascular, sensitive to touch, and is the main refractive index of the eye.
The vascular layer of the eye (uvea) is composed of 3 structures: choroid, ciliary body, and iris. The iris is a diaphragm located on the anterior surface of the lens. The pupil, an aperture important in transmitting light, is located centrally in the iris. The iris is the portion of the uvea with color, while the pupil is black. The pupil is controlled by two muscles: sphincter pupillae (contracts pupil, parasympathetically-controlled) and dilator pupillae (dilates pupil, sympathetically-controlled).
The inner (nervous) layer of the eye is composed of the neural retina. The retina is composed of 10 layers that contain specialized receptors (rods and cones) as well as interneurons, glia, and ganglion cells. The axons of the ganglion cells exit the retina as the optic n. (CN II). The main source of blood supply to the retina is the central retinal a., which travels within the optic n. to the retina, and is a branch of the ophthalmic a.
6. What are the extra-ocular muscles of the eye and their attachments? What are the actions of these muscles? What nerves innervate these muscles?
Extra-ocular muscles separated by eye movements
The distal tendon of the superior oblique m. runs through the trochlea, a fibrocartilaginous loop (pulley) that attaches to the trochlear fossa of the frontal bone, before inserting on the posterosuperior sclera of the eye.
The common anular tendon (common tendinous ring) is the origin site for the rectus muscles. The ring encircles the optic n. (CN II), oculomotor n. (CN III), and abducens n. (CN VI).
7. What are the nerves that provide motor, sensory, and autonomic innervation to the orbit and its content?
Ophthalmic n. (V1) branches:
The frontal nerve is the largest branch of V1, and it afferently serves the medial upper eyelid (skin & conjunctiva), skin of the forehead, and frontal sinus mucosa. A prominent feature of the superior orbit, the frontal n. may be found superior to the levator palpebrae superioris m.
The frontal n. divides about midway through the orbit into the supra-orbital (lateral) and supratrochlear (medial) nn. The supratrochlear n. passes superior to the trochlea of the superior oblique m. to afferently serve the skin of the medial forehead, and skin and conjunctiva of the medial superior eye. The supra-orbital n. further divides into medial and lateral branches, which afferently serve the mucosa of the frontal sinus, skin of the lateral forehead, and skin and conjunctiva of the lateral superior eye. Branches of the supra-orbital n. may be seen exiting the cranium via the supra-orbital foramen or notch.
The lacrimal n. (V1) is the smallest branch of the ophthalmic n. The lacrimal n. enters the orbit through the superior orbital fissure, and may be found superior to the lateral rectus m., en route to the lacrimal gland.
The nasociliary n. branches from V1 in the superior orbital fissure. The nasociliary n. gives rise to:
long ciliary nn.,
anterior & posterior ethmoidal nn.,
infratrochlear n., and
a communicating branch to the ciliary ganglion transmitting afferent fibers from the eye via short ciliary nn.
The infratrochlear n. provides sensory innervation to conjunctiva, skin of eyelids, lacrimal caruncle and sac, and side of nose. This nerve can be seen exiting the orbit below the trochlea of the superior oblique m.
The anterior and posterior ethmoidal nn. exit through the anterior and posterior ethmoidal foramina, respectively.
The long ciliary nerves arise from the nasociliary n. (V1) in the vicinity of the optic n. (CN II). These branches carry afferent fibers from the eye and postganglionic sympathetic fibers to the dilator pupillae muscle of the eye.
The oculomotor n. (CN III) arises from the midbrain, 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 (and also through the common anular tendon). The superior and inferior divisions are typically separated by the nasociliary n. (of V1) and the optic n. (CN II).
The superior division of oculomotor n. efferently innervates:
levator palpebrae superioris m., and
superior rectus m.
The inferior division of oculomotor n. efferently innervates:
medial rectus m.,
inferior rectus m.,
inferior oblique m., and
transmits preganglionic parasympathetic fibers to:
ciliary ganglion (for sphincter pupillae muscles and ciliary muscles)
Short cilary nn.
The short ciliary nn. 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 (preganglionics from oculomotor) to the ciliary muscles and sphincter pupillae muscles, and
postganglionic sympathetic fibers (from perivascular plexuses) to the intra-ocular vessels.
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 has three conduits to/from the remainder of the PNS:
nasociliary n. (of V1) - transmits afferent fibers from the eye through the ganglion to the nasociliary n.,
oculomotor n. (CN III) - transmits preganglionic parasympathetic fibers to the ciliary ganglion, and
sympathetic - postganglionic sympathetic fibers are transmitted to (and through) the ciliary ganglion from perivascular plexuses of the internal carotid a. These fibers may also associate with the oculomotor n.
The ciliary ganglion communicates with the eye via the short ciliary nn.
Optic n. (CN II) travels through the optic canal to enter the orbit and pierce the sclera. This is the most obvious structure in this area. This cranial nerve is special somatic afferent and functions in vision.
The abducens n. (CN VI) arises from the brainstem, travels through the cavernous sinus, and enters the orbit through the superior orbital fissure (and also through the common anular tendon). The abducens n. innervates the lateral rectus m. (i.e. the ABDUCens ABDUCts the eye).
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 travels along the medial wall of the orbit to innervate the superior oblique m. This nerve is generally easiest to locate running deep to the posterior-most aspect of the superior oblique m.
8. Trace the branches of ophthalmic artery and veins. Identify the association of the neurovasculature of the orbit with the cavernous sinuses.
The ophthalmic a. branches from the internal carotid a. after the ICA emerges from the cavernous sinus. The ophthalmic a. enters the orbit through the optic canal and branches to supply the structures of the orbit. The bulk of the ophthalmic a. may be found between the superior oblique m. and the medial rectus m., along the medial border of the orbit, and the many smaller branches typically accompany their namesake V1 branches (e.g. lacrimal, ant./post. ethmoidal, supra-orbital, supratrochlear, etc.).
The central artery of the retina is the first branch of the ophthalmic a. The central artery of the retina enters the dural sheath of the optic n. (CN II) and travels within the nerve to the retina.
The superior ophthalmic v. drains blood from the eye, superior orbit, ethmoidal cells, forehead, and the angular v. of the face. The inferior ophthalmic v. collects blood from the face and inferior orbit. Both the superior and inferior ophthalmic vv. may transport blood from the face to the cavernous sinus.
The cavernous sinuses are relatively expansive dural venous sinuses lateral to the sella turcica, extending between the superior orbital fissures (anteriorly) and the apices of the petrous part of the temporal bones (posteriorly). Tributaries of these sinuses include ophthalmic vv., sphenoparietal sinuses, and superficial middle cerebral vv. The cavernous sinuses drain into the petrosal sinuses. Structures traversing the cavernous sinus include: the internal carotid a., abducens n. (CN VI), and internal carotid plexus of sympathetic nn. The oculomotor (CN III), trochlear (CN IV), and the ophthalmic (V1) and maxillary (V2) divisions of the trigeminal n. (CN V) are located in the lateral walls (meningeal layer of dura mater) of the cavernous sinuses. Cavernous sinus thromboses may endanger these structures.
