It is the cavity in the facial skeleton that holds the eye. The apex of the triangle is to the back, the base in front. The periosteum of the orbit is called the periorbita, and is the fascia of the eyeball. It is continuous with the dura at the optic canal and the superior orbital fissure. Through the inferior orbital fissure, it is continuous with the external skull periosteum.
Parts of the orbit:
Superior wall, or roof: formed by the frontal bone, and in the back part, by part of the sphenoid bone. There is a fossa for the lacrimal gland and a hole in the sphenoid bone, the lacrimal foramen.
Medial wall: is mostly made of the ethmoid bone, but most medially, also the lacrimal bone, and above, the frontal bone. The medial wall is very thin and has a lacrimal fossa for the lacrimal sac. The two sides are separated by the ethmoidal sinuses and the upper nasal cavity.
Inferior wall: is mostly the maxilla, but also the zygomatic and palatine bones. Between the inferior and lateral wall, there is a fissure, the inferior orbital fissure from the bottom edge of the sphenoid bone.
Lateral wall: in front, is the zygomatic bone, and toward the back of the head is the greater wing of the sphenoid bone.
Apex of the orbit: the optic canal is the “tip” of the triangle. The actual hole is in the sphenoid bone. The superior orbital fissure is to the outside of it
Eyelids and the Lacrimal Apparatus
Are thin skin on the outside and lined on the inside by palpebral conjunctiva, which, at the edges, folds onto the eyeball to become the bulbar conjunctiva. The bulbar conjunctiva is thin and transparent and is loose over the sclera, is attached to the periphery of the cornea. The folds between the two types are the conjunctival fornices (sup. and inf.).
The eyelids have tarsal plates which are thickenings of CT that help them hold their shape, and are deep to fibers of orbicularis oculi. These plates contain tarsal glands that secrete a lubricating lipid. It also shields against tears absorbing too much into the tissue. (When you cry a lot, it can’t do the job, so the fluid goes in to the tissue and your eyes swell.)
The eyelashes have sebaceous glands called ciliary glands.
Ligaments and dividers
Medial palpebral ligament connects the tarsal plates and some fibers of orbicularis oculi to the medial edge of the orbit.
Lateral palpebral ligament does the same on the lateral side, but only to the tarsal plates, not the muscle.
The orbital septum is a membrane from the tarsal plates to the orbit all the way around the top or bottom part of the eyelid/orbit that is joined to the periosteum that contains fat and blocks the spread of infection.
Lacrimal glands secrete the fluid and are found in the aforementioned fossae for the lacrimal glands, which is upper and outside in the orbit. It has superior and inferior parts, divided by the lateral end of the tendon of levator palpebrae superioris.
Lacrimal ducts move the fluid to the eye. There can be up to twelve, and they open into the superior conjunctival fornix which is the line where the conjunctiva folds onto the eye. The conjunctival sac is the space between the palpebral and bulbar parts.
The inner corner of the eye collects liquid (where you see tears “well up”) and is called the lacrimal lake. The punctum is the little hole dot you see at the inner corner of the eye. It drains tears into the lacrimal canaliculi , which drain into the lacrimal sac (the bulge under the punctum) and then into the nasal cavity through the nasolacrimal duct. (Weird, you think you cry from the inside but actually it’s from the outside of the eye inward, where tears collect.)
Innervation of the system is by CN VII which sends parasympathetic and sympathetic signals
The parasympathetic path is through the greater petrosal nerve and the nerve of the pterygoid canal, to the pterygopalatine ganglion to synapse with the post-ganglionic fibers. This causes production and release of fluid.
The sympathetic system causes vasoconstriction, and they come from the superior cervical ganglion through the carotid plexus, joining the deep petrosal nerve then follow with the above.
The zygomatic nerve brings both types of innervation as well, by way of the ophthalmic nerve.
Outer fibrous layer of the eyeball
Sclera - the opaque part covering the eyeball except for a little bit at the front. It is the white of the eye.
Cornea - the transparent part of the coat covering the front 1/6th of the eyeball.
Middle Vascular Layer of the eyeball
Choroid - just interior to the sclera, a vascular layer that ends in the front as the ciliary body. It is separable from the retina. The ciliary body is muscular and vascular and has ciliary processes in front that secrete the aqueous humor. The aqueous humor fills the following spaces:
Anterior chamber - between the cornea and the iris/pupil.
Posterior chamber - between the back of the iris/pupil and the lens.
Iris - lies on the lens and is the contractile diaphragm around the pupil. The two muscles controlling the size of the pupil are the dilator pupillae and the sphincter pupillae (parasympathetic). The pupillary light reflex involves CN II and III.
Inner Layer of the Eyeball (retina)
Neural layer and pigment layer
Continued anteriorly as the ciliary and iridial parts of the retina.
Fundus - The back of the eye, containing the optic disk, which is the opening for the optic nerve to enter the eyeball. It, along with the macula lutea form the blind spot. The is the specialized area dense in cones and surrounds the fovea, the center of it, and the most acute visual spot (no capillaries).
The functional part of the retina stops at the ora serrata which is just behind the ciliary body.
The retina is supplied by the central artery and central vein of the retina except for the rods and cones, which are supplied by the choroid layer.
Refractive Media of the Eye
Cornea - is the outer fibrous clear part of the eyeball. It refracts light and is sensitive to touch. It has no blood vessels, getting nutrients from the aqueous humor and somewhat absorbing oxygen from the air.
Aqueous Humor - the fluid in the anterior and posterior chambers, produced by the ciliary processes, drains into the Canal of Schlemm.
Lens - sits in a fibrous capsule which is anchored by the suspensory ligament of the lens to the ciliary body in front and by fibers to the ciliary processes. It focuses by being pulled by the fibers which are attached to the ciliary muscle in the ciliary body. It naturally is pulled into far vision, but parasympathetic stimulation contracts the ciliary body and lets the lens fold into near vision. Therefore, in the absence of stimulus the eye is set to far vision (“relax your eyes” like those stupid 3D pictures).
Vitreous Humor - the watery fluid behind the lens in the vitreous body, which is a mesh of jelly. Its pressure holds the retina down and supports the lens.
Muscles of the Orbit
Levator Palpebrae Superioris lifts the superior eyelid and is fan-shaped into where it merges with an aponeurosis in the tarsal plate. Works opposite orbicularis oculi. (Oculomotor nerve)
Recti Muscles arise from a fibrous ring, the common tendinous ring which runs around the optic canal. (They fan out and around the eyeball.) They attach to the sclera on the anterior part.
Medial Rectus Muscles obviously pull the eye inward. Oculomotor nerve
Lateral Rectus Muscles obviously pull the eye sideways. Abducent nerve
Superior Rectus Muscle pulls the eye upward and inward (look up). Oculomotor nerve
Inferior Rectus Muscles pull the eye down and inward. (look down). Oculomotor nerve
Superior oblique comes from the body of the sphenoid bone, passes through a tendon that bends it, and then goes into the sclera. It puts the eye outward and downward (diagonal to the outer corner). Trochlear nerve
Inferior Oblique - runs from the floor of the orbit to the sclera and pulls the eye diagonally outward and upward. Oculomotor nerve.
A note: eye motion
Abduction is to the side, while Adduction is to the center.
Depression is down, while elevation is up.
If we go around the eye, starting at the middle and going upward and outward (which is clockwise on one side and counterclockwise on the other), the muscles controlling movement to that direction are: MR, SR, IO, LR, SO, IR. This does not help the attachments, however, either to the eye or to the tendon ring.
The nerves are: oculomotor moves everything except two: the abducens nerve moves the lateral rectus, which is the abducer (duh), and the trochlear nerve moves the superior oblique because it does not emerge from the central tendon like the others, so its nerve is different.
Fascial Sheath of the Eyeball
It wraps the eyeball from the optic nerve to the cornea.
The tendons of the muscles enter it and reflect. (?)
Ligaments - (badly explained, find another book)
The medial and lateral rectus muscles expand into triangular sheaths that are called medial and lateral check ligaments that attach to the lacrimal (medial) and zygomatic (lateral) bones. This limits how far you can roll the eye inward and outward.
The suspensory ligament comes from the check ligaments blending with the fascia of inferior rectus and inferior oblique.
Innervation of the Orbit
The optic nerve supplies vision, of course.
CN III, Oculomotor, supplies levator palpebrae superioris, superior rectus, medial rectus, inferior rectus, and inferior oblique.
CN IV (trochlear, remember rye looking up) supplies superior oblique.
CN VI, Abducens, supplies lateral rectus. (Think, a child abductor is a sneaky guy, he’s always sneaking looks to the side.)
CN V1 (Ophthalmic) passes through the sup. orbital fissure to supply some of the orbit.
The Lacrimal Nerve comes from the wall of the cavernous sinus to the gland, conjunctiva, and skin of the eyelid. With it run CN V2 which from the zygomatic part brings the secretory fibers.
The frontal nerve divides into supraorbital and supratrochlear.
The nasociliary nerve is the sensory nerve to the eye.
It gives off the infratrochlear nerve, to the eyelids, conjunctiva, skin of the nose, and lacrimal sac.
It gives off the ethmoidal nerves to the mucous membranes of the ethmoid and sphenoid sinuses and nasal cavities.
The ciliary ganglion is found in the gap between the optic nerve and lateral rectus in the back of the orbit. It gives off:
The short ciliary nerves which carry parasympathetic and sympathetic to the ciliary body and iris.
The long ciliary nerves which are branches of CN V1 and carry postsynaptic sympathetic fibers to dilator pupillae and afferents from the iris and cornea.
Vasculature of the Orbit
Ophthalmic - comes off the internal carotid to cross optic foramen to get to orbital cavity. Gives off, in order: (these are probably too small to see and not important but I’m not sure)
Posterior Ciliary - pierces near the optic nerve to serve choroid and photoreceptors
Anterior Ciliary - from muscular rectus branches of ophthalmic artery, entering the sclera at the rectus attachments and to the ciliary body and iris.
Central artery of the retina - in dura of optic nerve and runs with it, piercing it near the eye and fans out over the retina, but does not supply the photoreceptors.
Lacrimal - along top of lateral rectus to lacrimal gland, conjunctiva, and eyelids
Supraorbital - from supraorbital foramen to forehead and scalp
Posterior Ethmoid - through posterior ethmoidal foramen to the ethmoidal cells
Anterior Ethmoid - through anterior ethmoidal foramen to anterior cranial fossa - ethmoidal cells, frontal sinus, nasal cavity, skin of nose.
Dorsal Nasal - dorsal part of nose
Ends as Supratrochlear - supraorbital margin to forehead and scalp.
Maxillary artery gives off infraorbital, which runs down infraorbital groove to face.
Veins of the Orbit
Superior and inferior ophthalmic veins go through the superior orbital fissure and drain to the cavernous sinus.
Central vein of the retina goes either directly into cavernous sinus or through one of the ophthalmic veins.
Vorticose veins from the vascular layer pierce the sclera and drain to the inferior ophthalmic vein.
Scleral venous sinus encircles the anterior chamber and collects aqueous humor.
Internal Aspect of the Cranial Base, Moore 4th ed. pp. 842-847
Anterior Cranial Fossa
Body and lesser wings of Sphenoid bone
The orbital parts of the frontal bone suport the frontal lobes of the brain and on the other side make the roof of the orbits. You can see the orbital gyri left dents
Frontal crest is the midline - a bony point of the frontal bone.
The foramen cecum and crista galli (a ridge of bone) of the ethmoid bone are in the groove.
Along the sides, there are anterior and posterior ethmoidal foramina, which pass the vessels and nerves with the same name.
The cribriform plate of the ethmoid lies to the sides of the crista galli. Holes in the plate let the olfactory nerves pass to the olfactory bulbs.
Middle Cranial Fossa
Greater wings of sphenoid and squamous parts of temporal bones on the sides.
Petrous parts of temporal bones in the back. The petrous crests are the boundaries between the middle and posterior fossae.
Lesser wing of sphenoid in front.
The optic canals pass in the sphenoid bone.
Sella Turcica is a bony formation on the upper surface of the sphenoid bone. It has the anterior and posterior clinoid processes as “corners.” (They are raised parts of the sphenoid bone.)
Tuberculum sellae - is the olive shaped swelling in front of the hypophyseal fossa
Dorsum sellae - piece of bone of the sphenoid posterior and medial to the posterior clinoid processes.
Lesser wings of the sphenoid on the inside end in the anterior clinoid processes.
There are four foramina that poke through the greater wing of the sphenoid bone, making a C - shaped sweep:
Superior Orbital Fissure - between the two wings, most frontward. Lets the ophthalmic veins and nerves to the orbit pass.
Foramen rotundum - is most medial, along the side of the hypophysial fossa and lets the maxillary nerve (CN V2) pass.
Foramen ovale - is a little to the side of the rotundum. It opens into the infratemporal fossa to let the mandibular nerve (CN V3) pass.
Foramen spinosum - more to the side. Lets the middle meningeal vessels and meningeal branch of mandibular nerve through.
The foramen lacerum isn’t in the C-shape pattern, but a bit behind, tucked at the corners of the posterior clinoid processes and is an artifact in a dry skull but covered by cartilage in life. Some things cross it, and a groove for the greater petrosal nerve extends outward from it.
Posterior Cranial Fossa
Contains in life:cerebellum, pons, medulla.
Dorsum sellae in front
Petrous ridge and mastoid parts of temporal bone in the front laterally
The clivus is a groove running down the middle from the back of the dorsum sellae and ending at the foramen magnum.
Behind the foramen magnum, there is a line, the internal occipital crest that splits down the middle. In front of it are the cerebellar fossae. The crest ends in the internal occipital protuberance
There are grooves for the transverse sinus and the sigmoid sinus in the temporal and occipital bones.
The jugular foramen is at the base of the petrous ridge. It lets the sigmoid sinus drain into and become the IJV, and also CN IX, X, and XI.
The internal acoustic meatus is between the temporal and occipital bones, and transmits the labyrinthine artery and facial and vestibulocochlear nerves.
The hypoglossal canal is in the foramen magnum’s wall.
The condylar canal is behind the condyles and lets a small vein pass to the vertebral veins in the neck from the sigmoid sinus.
The Dural Sinuses, Moore 4th pp. 879-884
Dural Venous Sinuses
They are endothelium lined spaces between the periosteal and meningeal layers of the dura that collect blood from large veins from the surface of the brain and they all eventually bring blood to the internal jugular.
Superior Sagittal Sinus is in the cerebral falx from the crista galli to the internal occipital protuberance, where there is a confluence of sinuses (superior sagittal, straight, occipital, and transverse). It receives superior cerebral veins and communicates through lateral venous lacunae.
Arachnoid Granulations are folds of arachnoid that poke into the venous sinuses and drain CSF. They may form pits in the bone.
Inferior Sagittal Sinus runs in the bottom of the pit of the cerebral falx and ends in the straight sinus.
Straight Sinusis formed by the inferior sagittal sinus joining the great cerebral vein. It joins the confluence of sinuses (continuation of inf. sag.)
The Transverse Sinuses run from the confluence of sinuses around the back of the head and upward and become the sigmoid sinuses as they get close to the petrous ridges of the temporal bones.
The Sigmoid Sinuses follow grooves in the temporal and occipital bones then turn anteriorly and down to become the IJV after they cross through the jugular foramen.
Occipital Sinus runs in the cerebellar falx and ends at the top at the confluence of sinuses. It gets some blood from the internal vertebral venous plexuses.
Cavernous Sinus is along the sides of the sella turcica on the sphenoid bone. The sellar compartments (one on each side) are really a plexus of small veins.
They extend from the superior orbital fissure in the front to the apex of the petrous part of the temporal bone in back.
They get blood from the ophthalmic veins, superficial middle cerebral vein, and sphenoparietal sinus.
They communicate with each other through channels running in the front and back of the pituitary, the intercavernous sinuses.
They drain backwards to the petrosal sinuses and emissary veins to the pterygoid plexuses.
The internal carotid artery runs in the plexus, the carotid plexus of sympathetic nerves, and CN VI.
The lateral wall of the cavernous sinus contains:
Trigeminal Nerve 1st part.
The Superior Petrosal Sinusesrun from the back of the cavernous sinus plexus to the transverse sinuses, meeting where they become the sigmoid sinus. They lie in the cerebellar tentorium, which is attached to the petrous ridge.
The Inferior Petrosal Sinuses begin at the same place, the back of the cavernous sinuses, and run between the petrous part of the temporal bone and the basilar part of the occipital bone and empty into the origin of the IJV.
The Basilar Plexus connects the inferior petrosal sinuses and the internal vertebral plexuses.
Emissary veins connect the dural sinuses with external veins. Usually blood flows from the brain outward.
The frontal emissary vein runs through the foramen cecum and connects the superior sagittal sinus with the frontal sinus and nasal cavity veins.
The parietal emissary vein passes through the parietal foramen (top of head) to connect the sup. sagittal sinus to external veins of the scalp.
The mastoid emissary vein passes through the mastoid foramen and connects the sigmoid sinus to either the occipital or posterior auricular vein.
The posterior condylar emissary vein may exist, connecting the sigmoid sinus through the condylar canal to the suboccipital plexus.
Vasculature of the Dura
The middle meningeal artery branches from the maxillary artery and enters the foramen spinosum where it runs sideways and forward, splitting into two branches on the greater wing of the sphenoid.
Anterior branch runs above the pterion and curves toward the top of the skull. It is susceptible to injury.
The posterior branch runs up and back over the back of the skull.
Veins accompany the arteries and are susceptible to injury. The middle meningeal veins drain into the pterygoid venous plexus
Sara’s note: the pterygoid venous plexus is not described in the text except that things drain into it. Look where it is on the picture.
Nerve Supply of the Dura
Anterior and middle cranial fossae areas of the dura are mostly parts of the trigeminal nerve.
The anterior meningeal branches come off the ethmoid nerves, which are a part of CN V1, and the meningeal branches of the maxillary and mandibular nerves supply the anterior cranial fossa.
CN V2 and CN V3 give branches that supply the middle cranial fossa
The posterior cranial fossa is mostly supplied by the tentorial nerve which is a branch of the ophthalmic nerve, by sen