The head is organized around a series of functional spaces, and this organization is reflected in the structure of the skull.  The orbits are bony cavities on either side of the root of the nose.  They are involved in vision and contain the eyeballs and optic nerves, extraocular muscles that move the eyes, and associated nerves and blood vessels. 

Each orbit is formed by 7 bones and forms a 4-sided pyramid with a posterior apex and an anterior base (Fig. 2; Gr 7.1B, 7.36A; N4, 84).  The orbit has medial and lateral walls, a roof, and a floor. 

The medial wall is formed mainly by the orbital plate of the ethmoid bone (lamina papyracea) along with contributions from the frontal process of the maxilla and the lacrimal and sphenoid bones.  Small anterior and posterior ethmoidal foramina are present near the junction of the roof and medial wall (frontoethmoid suture).  Anteriorly the medial wall has a depression for the lacrimal sac (fossa for the lacrimal sac) (Gr 7.36A, 7.37A; N4, 82).  The bone of the orbital plate of the ethmoid separating the cavity of the orbit from the spaces of the ethmoidal air cells (ethmoidal paranasal sinus) is paper thin (Gr 7.38A-B; N5, 48), and life-threatening orbital infections (orbital cellulitis) may result from ethmoidal sinusitis.  The medial walls of the two orbits are separated by the upper portion of the nasal cavities in addition to the ethmoidal air cells (Fig. 3). 

The lateral wall of the orbit is formed by the frontal process of the zygomatic bone and the greater wing of the sphenoid bone.  The lateral wall doesn’t extend as far anteriorly as the medial wall of the orbit, and so provides a surgical approach to the eye (Gr 7.2).  The lateral wall is the thickest, strongest wall, which is important because it is most exposed to direct trauma.

Although the medial walls of the orbits are nearly parallel, the lateral walls diverge from each other at approximately a 90° angle (Fig. 2).  Consequently, the axes of the orbits form a 45° angle with each other.  Since the optical axes (axes of the eyeballs) are parallel, they differ from the orbital axes.  Since most of the extraocular muscles attach near the apex of the orbit, this divergence of the optical and orbital axes has significant implications for how the muscles move the eyeballs.

The roof of the orbit is formed mainly by the orbital part of the frontal bone with a contribution near the apex from the lesser wing of the sphenoid.  Anterolaterally in the roof is a depression for the orbital part of the lacrimal gland (fossa for the lacrimal gland, lacrimal fossa) (Gr 7.37A).  The roof separates the orbit from the floor of the anterior cranial fossa (Fig. 3; Gr 7.74, 7.76; N5).  The frontal sinus extends posteriorly into the orbital plate of the frontal bone  for a variable distance (Gr 7.68, 7.69, 7.71A-B; N5, 8, 48), and frontal sinusitis has been reported to result in infections of the orbit and abscesses of the frontal lobe of the brain (e.g., see http://emedicine.medscape.com/article/212946-overview ).  The relatively thin bone of the roof of the orbit also means that a sharp object, such as a knife or a pencil, that penetrates the orbit in a superior direction may penetrate the frontal lobe. 

The floor of the orbit is formed mainly by the orbital surface of the maxilla with smaller contributions from the zygomatic bone laterally and the orbital process of the palatine bone posteromedially (Fig. 2).  The floor features the infraorbital groove leading into the infraorbital canal for the infraorbital nerve and vessels.  The floor separates the orbit from the maxillary sinus (Fig. 3; Gr 7.74).  A blow to the eyeball (e.g., from a handball) may violently displace the orbital contents inferiorly, fracturing the floor of the orbit (blowout fracture).  Usually the infraorbital nerve and vessels (Gr 7.39A, 7.72B, 7.74A, 7.89C; N48, 83) prevent the eyeball  itself from entering the maxillary sinus, but the inferior rectus muscle may become wedged in a fracture, limiting eye movements ( http://www.emedicine.com/oph/topic229.htm ).  Hemorrhage may occur from a damaged infraorbital artery.  An inferior blowout fracture is unlikely in children because the small size of the maxillary sinus means that the floor of the orbit is relatively stronger than in adults. 

The base of the orbit is located anteriorly (Gr 7.36, 7.38, 7.92D & E, 7.93A; N4, 84).  It is formed by the orbital opening bounded by the thick bone of the orbital margin, which provides some protection to the orbital contents.   The supraorbital margin is the boundary of the squamous and orbital parts of the frontal bone.  It has a supraorbital notch (in some cases a foramen) for the supraorbital nerve and vessels.  Just superior to each supraorbital margin is a ridge, the superciliary arch, deep to the eyebrows.  The ridge is generally more prominent in adult males.

The infraorbital margin is formed by the maxilla and zygomatic bone.  The infraorbital foramen, which is the opening for the infraorbital canal, is located inferior to its midpoint.

The apex of the orbit is the optic canal, which is located between the body and the roots of the lesser wing of the sphenoid bone (Fig. 2; Gr 7.36A, 7.38B; N4).  The optic nerve leaves the orbit via the optic canal, and the ophthalmic artery enters the orbit from the middle cranial fossa through it.  The optic canal is closely related to the sphenoid sinus and posterior ethmoidal air cells (Gr 7.38, 7.68A, 7.69A; N48, 83), and the optic nerve may be infected from these paranasal sinuses or injured during surgery on them.

The optic canal is just medial to the superior orbital fissure, which lies between the greater and lesser wings of the sphenoid bone (Fig. 2; Gr 7.36; N4, 86).  The superior orbital fissure connects the orbit with the middle cranial fossa.  It provides passage for most of the nerves and vessels entering or leaving the orbit, including cranial nerves III, IV, VI, and V1 and the ophthalmic veins. 

The inferior orbital fissure separates the floor of the orbit from its lateral wall (Fig. 2; Gr 7.36, 7.44A; N4).  It connects the orbit with the pterygopalatine fossa posteriorly and with the infratemporal fossa more laterally.  The infraorbital nerve and artery (Gr 7.47, 7.48A; N44, 69) and the zygomatic branch of the maxillary nerve (Gr 7.70, 7.72B; N44) enter the orbit from the pterygopalatine fossa via the inferior orbital fissure.  A vein connecting the pterygoid venous plexus of the infratemporal fossa and the inferior ophthalmic vein traverses the fissure, providing an indirect route for the spread of infection into the cranial cavity (Gr 7.16A, 7.40C; N70, 85).

Anterior and posterior ethmoidal foramina for vessels and nerves of the same name are present in or near the frontoethmoidal suture on the medial wall of the orbit (Fig. 2; Gr 7.36A, 7.38, Table 7.6 [p. 647]; N4, 85, 86).  The anterior and posterior ethmoidal arteries are branches of the ophthalmic artery, and the corresponding nerves are branches of the nasociliary nerve from V1.  The fossa for the lacrimal sac opens into the bony nasolacrimal canal, which contains the membranous nasolacrimal duct that drains the collected lacrimal fluid into the inferior meatus of the nasal cavity (Gr 7.37, 7.67; N82).  That’s why someone’s nose runs when s/he cries, as from joy at the prospect of attending gross anatomy class.

The periosteum lining the orbit is given the special name “periorbita.”  Extensions of the periorbita into the upper and lower eyelids from the supraorbital and infraorbital margins, respectively, form the orbital septum (Gr 7.39B; N81).  The orbital septum is clinically important because it helps separate the preseptal (periorbital) space from the postseptal (orbital) space.  A less serious periorbital infection (preseptal cellulitis) rarely spreads into the orbit.  An orbital infection (orbital cellulitis) is more serious and usually requires hospitalization and intravenous antibiotics (e.g., see http://www.emedicine.com/oph/topic205.htm ). 

The eyeball occupies most of the anterior portion of the orbit.  The eyeball is essentially a sphere with an anterior bulging segment, the cornea (Fig. 4; Gr 7.36B-C, 7,38C, 7.39; N83, 87, 88).  The eyeball consists of three layers:  an outer fibrous layer, a middle vascular layer, and an inner nervous layer (Fig. 4; Gr 7.41, 7.42; N87).  The cornea is the transparent anterior 1/6 of the fibrous layer and admits light into the eye.  It is avascular (N90, 91) but richly innervated with sensory fibers by the nasociliary nerve (V1).  The sclera is the tough, opaque posterior 5/6 of the fibrous layer.  It is the “white of the eye.”  The sclera is the fibrous skeleton of the eyeball and gives attachment to the extraocular muscles that move the eye. 

The middle vascular layer of the eyeball, or uvea, is formed by the choroid, ciliary body, and iris (Gr 7.41, 7.42; N90, 91).  The choroid is the large posterior part of the uvea and lines most of the sclera.  It has a rich vascular supply which, importantly, supplies the layer of photoreceptor cells in the outer part of the nervous retina. 

The ciliary body connects the choroid and the circumference of the iris (Fig. 4; Gr 7.41; N87, 88, 90).  The ciliary body forms a complete ring around the the transparent, elastic, biconvex lens, which it suspends from ciliary processes by the suspensory ligament of the lens (ciliary zonule).  Contraction of the circularly arranged smooth muscle fibers of the ciliary muscle under preganglionic parasympathetic control by the oculomotor nerve (III) draws the ciliary processes closer to the lens.  This reduces tension on the suspensory ligament and allows the natural elasticity of the lens to make it more rounded for near vision (accommodation).  The lens loses elasticity as we age and may remain too flattened to allow a good focus of the image on the retina for near objects.  The inability to see near objects clearly with age is presbyopia. 

The ciliary processes also form aqueous humor, which circulates from the posterior chamber through the pupil into the anterior chamber of the eye (Gr 7.42B; N87, 88).  The aqueous humor nourishes the inner part of the cornea and, along with the gelatinous vitreous humor (vitreous body) behind the lens (Gr 7.41; N87), helps maintain intraocular pressure.  The aqueous humor is absorbed into the scleral venous sinus (of Schlemm) at the iridocorneal junction in the anterior chamber (N87, 88, 91). 

Increased intraocular pressure due to decreased absorption of aqueous humor (glaucoma) compresses the retina and retinal arteries and may result in blindness.  Unfortunately, most patients don’t notice the gradual visual loss, which initially is at the periphery of the visual field, until significant damage has occurred.  A less common form of glaucoma (closed-angle glaucoma) is painful and causes sudden visual loss, but in most patients glaucoma is symptomless early in its course.  For a patient’s guide, see http://www.merckmanuals.com/home/sec20/ch233/ch233a.html    
The iris is a thin, circular, adjustable diaphragm with a central opening, the pupil (Fig. 4).  The iris separates the anterior chamber of the eye located in front of it from the posterior chamber located behind it.  The iris is responsible for eye color due to its distribution of the pigment melanin.  It contains two smooth muscles that control the size of the pupil and, therefore, the amount of light entering the eye.  The sphincter pupillae (constrictor pupillae) muscle constricts the pupil under preganglionic parasympathetic control by the oculomotor nerve.  The postganglionic parasympathetic fibers are from cell bodies in the ciliary ganglion.  The dilator pupillae muscle dilates the pupil under control of postganglionic sympathetic fibers from cell bodies in the superior cervical sympathetic ganglion. 

When the oculomotor nerve is compressed (e.g., due to a transtentorial herniation), the parasympathetic fibers, which are located peripherally in the nerve, are affected first and the pupil dilates (mydriasis) due to paralysis of the sphincter pupillae muscle.  When the sympathetic innervation is interrupted, as in Horner’s syndrome, the pupil constricts (miosis).

The inner layer of the eyeball is the retina (Figs. 4, 5; Gr 7.41A; N87, 119).  It consists of a posterior optic part and an anterior non-visual part, which meet at the ora serrata, the scalloped posterior border of the ciliary body.  The optic retina is formed by an inner light-receptive neural layer and an outer pigment layer.  The neural layer comprises multiple cell layers with the layer of photoreceptor cells—rods and cones—located most peripherally (i.e., farthest from the vitreous humor) so that light has to traverse the other layers to reach it (Fig. 5).  The innermost layer of the optic retina is formed by ganglion cells, and their axons converge at the optic disc (optic papilla) to form the optic nerve (Fig. 5).  The optic disc lacks rods and cones and, therefore, is the blind spot of the eye—light rays that land on the optic disc are not seen.

Just lateral to the optic disc is a small, yellowish oval area, the macula lutea.  It contains a central depression, the fovea centralis (Fig. 4), which has the more superficial layers of the retina (i.e., those closest to the vitreous humor) pulled aside to expose its packed cone cells to light.  The fovea centralis is the point of greatest visual acuity in the eye. 

The outer pigment layer of the optic retina borders the choroid layer.  The pigment layer is a single layer of cells that reduces the scattering of light inside the eye.  It originally formed the outer layer of the embryonic optic cup, and the neural retina formed the inner layer (Fig. 6).  In a detached retina, the neural and pigment layers of the optic retina separate, re-creating the intraretinal space of the optic cup (e.g., see http://emedicine.medscape.com/article/798501-overview ).  Although the inner cells of the neural layer, those closest to the vitreous humor, receive their nourishment from branches of the central artery of the retina, which enters the eye at the optic disc, the layer of rods and cones is nourished by diffusion through the pigment layer from vessels of the choroid.  Consequently a detached retina (e.g., following a blow to the eye) results in loss of the photoreceptor cells of the affected area with loss of vision.

The extraocular muscles include one muscle that elevates the upper eyelid and six muscles that move the eye to direct the pupil toward the object to be viewed.  The muscle of the upper eyelid is the levator palpebrae superioris (Gr 7.38A, 7.39, 7.40A-B; N81, 83, 84, 86).  It takes origin from the lesser wing of the sphenoid bone above the optic canal and passes anteriorly to insert into a dense fibrous plate in the upper eyelid, the superior tarsal plate.  The levator palpebrae superioris elevates the upper eyelid under voluntary (GSE) control of the oculomotor nerve.  The levator has a small inferior offshoot composed of smooth muscle, the superior tarsal muscle (of Müller) (Gr 7.39B; N81).  The superior tarsal muscle is under sympathetic (GVE) control.  Therefore, a lesion of the oculomotor nerve results in complete ptosis and inability to voluntarily raise the upper eyelid, but Horner’s syndrome results in partial ptosis while retaining the ability to voluntarily raise the upper eyelid. 

The four recti (straight) muscles share an origin from a fibrous ring that surrounds the optic canal and part of the superior orbital fissure.  This common tendinous ring (anulus tendineus), therefore, forms a muscular cone into which initially pass most of the structures entering the orbit (Gr 7.38A [left side], 7.39A, 7.40B, 9.5; N83, 84, 86).  The exceptions are the frontal, lacrimal, and trochlear nerves and the ophthalmic veins, which enter the orbit outside of the muscular cone.

The superior, inferior, medial, and lateral rectus muscles all pass from the common tendinous ring to attach into the sclera of the anterior half of the eyeball (Fig. 7; Gr 7.40A-B, Table 7.7 [p. 648]; N84).  The medial and lateral recti occupy the same horizontal plane and produce the simple adduction and abduction movements of the pupil, respectively, that one would expect.   The superior and inferior recti muscles lie in the same vertical plane, but this is in the orbital axis rather than the optical axis.  Consequently, the individual action of the superior rectus is the move the pupil superiorly and medially from the anatomical position, while the inferior rectus moves the pupil inferiorly and medially (Gr Table 7.8 [p. 649]).  All of the recti except the lateral rectus are innervated by the oculomotor nerve (III).  The lateral rectus muscle, the abductor of the pupil, is innervated by the abducent nerve (VI).

The superior oblique muscle takes origin from the body of the sphenoid medial to the superior part of the common tendinous ring.  The muscle passes forward along the superomedial corner of the orbit to the trochlea (Fig. 7; Gr 7.37A; N83, 84).  The trochlea is a small piece of cartilage suspended from the orbital margin at the trochlear fovea (or trochlear spine) by a sling of fibrous connective tissue.  The tendon of the superior oblique hooks around the trochlea to turn posterolaterally and pass between the superior rectus muscle and the eyeball.  It inserts into the sclera of the superior lateral quadrant of the posterior half of the eyeball.  The superior oblique muscle has an individual action of moving the pupil inferiorly and laterally from the anatomical position (Gr Table 7.8 [p. 649]).  It is innervated by the trochlear nerve (IV), which receives its name from the trochlea.

The inferior oblique muscle takes origin from the anteromedial portion of the floor of the orbit (Gr 7.37A, 7.39, 7.40A; N84, 120).  It passes posterolaterally under the inferior rectus muscle to insert into the inferior lateral quadrant of the posterior half of the eyeball.  The inferior oblique, acting individually, moves the pupil of the eye superiorly and laterally.  It is innervated by the oculomotor nerve.

Although the individual action of each extraocular muscle starting from the anatomical position is described above, multiple muscles are involved in every eye movement, and this fact is used in testing the muscles clinically (see Moore Figs. 7.34E & 7.35).  Thus, the superior oblique muscle depresses the pupil of the already adducted eye, and the inferior oblique elevates it.  The superior rectus elevates the abducted eye, and the inferior rectus depresses it.  Part of the physical examination also is testing the coordinated movements of the two eyes that are necessary for binocular vision.  For example, having the patient look directly to the left requires coordinated contractions of the left lateral rectus and right medial rectus muscles.  You will study how the central nervous system controls these movements in Neuroscience. 

Before leaving a consideration of eye movements, it is important to mention one other important structure that is difficult to demonstrate in dissection.  The eyeball from the optic nerve anteriorly to near the corneoscleral junction is enclosed by a connective tissue layer, the fascial sheath of the eyeball (bulbar sheath, Tenon’s capsule), which is pierced by the tendons of extraocular muscles (Fig. 8; Gr 7.39B; N83).  The fascial sheath forms a socket within which the eye moves by the contraction of the extraocular muscles.  The sheath is reinforced inferiorly between the anterior portions of the medial and lateral rectus muscles as the suspensory ligament of the eyeball.  If the eye has to be surgically removed, an attempt is made to spare the fascial sheath and suspensory ligament to form a socket for the artificial eye.

The optic nerve is formed by the axons of ganglion cells.  It is enclosed by extensions of the cranial meninges and subarachnoid space as it passes to the apex of the orbit and traverses the optic canal (Gr 7.39B-C, Table 7.6 Transverse Section [p. 647]; N87).  The central artery of the retina, which is a branch of the ophthalmic artery, pierces the inferior surface of the optic nerve and enters the eye at the optic disc (Gr 7.41A, 7.42, Table 7.6 [p. 647]; N87, 90, 91).  The central vein of the retina accompanies the distal part of the artery but leaves it en route to the cavernous sinus.  Due to the extension of the subarachnoid space around the optic nerve, an increase in intracranial pressure reduces venous return from the retina and inhibits axoplasmic transport to produce swelling of the optic disc (papilledema).  There are other causes of a swollen optic disc (e.g., optic neuritis), but it is often the first sign of increased intracranial pressure.  Papilledema of unknown origin is a contraindication to lumbar puncture, which could cause a fatal brain herniation in the presence of a mass lesion (see http://www.merckmanuals.com/professional/sec09/ch107/ch107e.html ).   

The ophthalmic division of the trigeminal nerve (V1) is the sensory nerve of the orbital region.  It leaves the trigeminal ganglion and is embedded in the lateral wall of the cavernous sinus as it passes toward the superior orbital fissure (Gr 7.18, 7.22, 7.23A, Table 9.6 [p. 807]; N44, 86, 121).  The frontal nerve is the largest of the three terminal branches of the ophthalmic nerve (Gr 7.38A, 7.39A; N44, 86).  It enters the orbit above the common tendinous ring and divides into supraorbital and supratrochlear nerves at a variable distance into the orbit (Gr 7.10, 7.37A, 7.38A; N44, 86).  The supraorbital nerve divides into medial and lateral branches.  It was seen previously ascending into the forehead from the supraorbital notch or foramen.  The supratrochlear nerve passes just above the trochlea to enter the forehead medial to the supraorbital nerve.

The lacrimal nerve is the smallest of the three branches of V1 (Gr 7.38, 7.39A, 7.70, 7.72B,  Table 9.6 [p. 807]; N44, 86).  It enters the orbit above the common tendinous ring and passes along the superolateral corner of the orbit, above the lateral rectus muscle.  The lacrimal nerve supplies the lacrimal gland and the skin of the lateral portion of the upper eyelid.  It receives postganglionic parasympathetic nerve fibers from cell bodies in the pterygopalatine ganglion that reach it via the zygomatic branch of the maxillary nerve (V2) (N44, 132).  These parasympathetic fibers are secretomotor fibers.  The postganglionic sympathetic innervation from cell bodies in the superior cervical sympathetic ganglion also reaches the lacrimal gland via the zygomatic nerve.  These sympathetic fibers traverse the pterygopalatine ganglion from the deep petrosal nerve without synapsing and are vasoconstrictive.

The nasociliary nerve is the third terminal branch of the ophthalmic nerve (Gr 7.38, 7.39A; N44, 86).  It enters the orbit through the part of the superior orbital fissure enclosed by the common tendinous ring and crosses medially above the optic nerve.  The nasociliary nerve sends a branch to the ciliary ganglion, the sensory root, just after entering the orbit (N130, 131).  These are sensory (GSA) nerve fibers from the eyeball and traverse the ciliary ganglion without synapsing to reach the eye via the short ciliary nerves.  The sensory root of the ciliary ganglion also usually carries postganglionic sympathetic fibers that join the ophthalmic nerve from the internal carotid plexus in the cavernous sinus.  These sympathetic fibers traverse the ganglion without synapsing to supply the dilator pupillae muscle of the iris and the smooth muscle of blood vessels.  Alternatively, the sympathetic fibers may reach the ciliary ganglion in a separate sympathetic root arising in the cavernous sinus (N130).

The nasociliary nerve gives off 3-4 long ciliary nerves as it crosses the optic nerve.  These carry sensory fibers from the eye and also may carry postganglionic sympathetic nerve fibers (N131).  The sensory fibers supplied to the eye by the nasociliary nerve are tested in the corneal reflex.  The examiner touches the cornea with a wisp of cotton, resulting in reflex closure of the eyelids.  The afferent limb of the reflex arc, therefore, is by cranial nerve V1 and the efferent limb is by the facial nerve (VII), which supplies the orbicularis oculi muscle. 

The posterior ethmoidal branch of the nasociliary nerve passes medially between the superior oblique and medial rectus muscles to enter the posterior ethmoidal foramen (Gr 7.38B; N86).  It supplies the mucoperiosteum of the posterior ethmoidal air cells and sphenoid sinus.  It may continue across the cribriform plate in the anterior cranial fossa and enter the upper portion of the nasal cavity to help supply general sensation to the nasal mucosa.  The posterior ethmoidal nerve sometimes is absent. 

The terminal branches of the nasociliary nerve are the anterior ethmoidal and infratrochlear nerves (Gr 7.38; N44, 86).  The anterior ethmoidal nerve traverses the anterior ethmoidal foramen to supply the anterior ethmoidal air cells and frontal sinus.  It continues across the cribriform plate and descends into the anterosuperior part of the nasal cavity, where it supplies the nasal mucosa.  It leaves the nasal cavity between the nasal bone and lateral nasal cartilage as the external nasal nerve (N42, 44).  The infratrochlear nerve exits the orbit between the trochlea and medial rectus muscles (Gr 7.10A; N44, 86).  It is sensory to the lacrimal sac, medial portions of both eyelids, and skin over the root of the nose.

The oculomotor (III), trochlear (IV), and abducent (VI) nerves are the motor nerves of the orbit (N120).  The oculomotor nerve passes forward embedded in the lateral wall of the cavernous sinus to reach the superior orbital fissure.  It divides into superior and inferior branches/divisions as it enters the orbit within the common tendinous ring (Gr 7.38A, 7.39A, 9.5B, Table 9.4 [p. 804]; N86, 120).  The superior branch innervates the levator palpebrae superioris and superior rectus muscles.  The inferior branch of the oculomotor nerve sends a branch medially under the optic nerve to the medial rectus muscle and branches that descend to reach the inferior rectus and inferior oblique muscles.  The inferior branch also gives of the motor (parasympathetic) root of the ciliary ganglion (N120, 131).  The motor root carries preganglionic parasympathetic nerve fibers of the oculomotor nerve to the ciliary ganglion, where they synapse on the cell bodies of postganglionic neurons.  The postganglionic parasympathetic fibers pass through the short ciliary nerves to the eye, where they innervate the ciliary and sphincter pupillae muscles.

The slender trochlear nerve courses forward in the lateral wall of the cavernous sinus to the superior orbital fissure (Gr 7.22, 7.23, 7.38; N86, 103, 120).  Just before reaching the orbit, it crosses above the oculomotor nerve and enters the orbit above the common tendinous ring.  The trochlear nerve pierces the superior surface of the superior oblique muscle. 

The abducent nerve pierces the dura of the posterior cranial fossa over the clivus and traverses the cavernous sinus inferolateral to the internal carotid artery (Gr 7.28, 7.22, 7.23; N86, 103).  It enters the orbit within the common tendinous ring and passes onto the medial surface of the lateral rectus muscle, which it supplies (Gr 7.38, 7.39A). 

The motor innervation of the orbit is easily remembered by the formula LR6SO4AO3.  In other words, the lateral rectus muscle is innervated by cranial nerve VI, the superior oblique by cranial nerve IV, and all of the other extraocular muscles are innervated by cranial nerve III.

The ophthalmic artery is the blood supply to structures in the orbit (Fig. 9; Gr Table 7.6 [p. 647]; N85).  It branches from the internal carotid artery medial to the anterior clinoid process and enters the orbit via the optic canal below the optic nerve.  The ophthalmic artery ascends lateral to the optic nerve and passes medially above it to a point near the medial wall of the orbit, where it courses anteriorly.  Its branches include the central artery of the retina, lacrimal artery, long and short posterior ciliary arteries, supraorbital artery, posterior and anterior ethmoidal arteries, and its terminal branches, the supratrochlear and dorsal nasal arteries.  

The central artery of the retina is the most important branch of the ophthalmic artery (Fig. 9; Gr 7.41, 7.42; N90, 91).  It branches from the ophthalmic artery below the optic nerve and pierces it.  From the center of the optic nerve it enters the eye through the optic disc and sends branches across the inner surface of the retina.  The central artery of the retina supplies blood to all layers of the nervous retina except the layer of rods and cones, which are nourished by diffusion from vessels of the choroid layer.  Occlusion of the central artery of the retina by a thrombus from the carotid artery or heart results in instant and usually permanent blindness in the affected eye.  Temporary occlusion of the central artery of the retina is one cause of  temporary blindness lasting from seconds to hours and known as amaurosis fugax (e.g., http://ijahsp.nova.edu/articles/vol4num2/Bacigalupi.pdf).   

The lacrimal artery ascends anteriorly on the lateral side of the orbit to the reach the lacrimal gland and neighboring structures (Fig. 9; Gr 7.38A, Table 7.6 [p. 647]; N85).  The long and short posterior ciliary arteries pierce the sclera of the posterior part of the eye and are distributed through the choroid layer (N90, 91).  The supraorbital artery usually arises from the ophthalmic artery just after it crosses above the optic nerve.  It was seen earlier leaving the orbit with the supraorbital nerve. 

The posterior and anterior ethmoidal arteries join nerves to enter foramina of the same name in the medial wall of the orbit (N85).  Both supply ethmoidal air cells and cross the cribriform plate in the anterior cranial fossa to enter the nasal cavity.  The anterior ethmoidal artery usually gives off an anterior meningeal artery before leaving the cranial cavity.  The posterior and anterior ethmoidal arteries supply the upper mucosa of the nasal cavity (N40).

The supratrochlear artery is one terminal branch of the ophthalmic artery (N69, 85).  It leaves the orbit above the trochlea with the supratrochlear nerve to supply the forehead.  The dorsal nasal artery is the other terminal branch of the ophthalmic artery.  It exits the orbit to supply the dorsum of the nose.  The dorsal nasal artery often anastomoses with the angular artery, providing collateral circulation between the internal and external carotid arteries that may compensate if slow occlusion of the internal carotid artery occurs.  

The venous drainage of the orbit, except the nervous retina, is mainly via the superior and inferior ophthalmic veins (Gr 7.16, 7.40C; N85, 91).  The superior ophthalmic vein is formed by the union of the supraorbital and angular veins.  As it passes posteriorly in the upper medial part of the orbit, the superior ophthalmic vein receives tributaries mostly corresponding to branches of the ophthalmic artery.  In addition it receives vorticose veins (Gr 7.42B; N91) from the upper two posterior quadrants of the eyeball and often the inferior ophthalmic vein as tributaries.  The superior ophthalmic vein drains into the cavernous sinus, providing a route for the spread of infection from the middle third of the face (danger area) to the cavernous sinus. 

The inferior ophthalmic vein begins by the confluence of small veins in the floor of the orbit with usually a contribution from the angular vein (Gr 7.16A, 7.40C; N85).  It receives the vorticose veins from the lower two posterior quadrants of the eyeball (N91).  The inferior ophthalmic vein communicates through the inferior orbital fissure with the pterygoid plexus of veins (N70, 85), providing an alternative route for the spread of infection to the cavernous sinus.  The inferior ophthalmic vein often drains into the superior ophthalmic vein before that larger vein leaves the orbit; however, sometimes the inferior ophthalmic vein drains directly into the cavernous sinus. 

The central vein of the retina drains blood from the neural layer of the optic retina (Gr 7.41, 7.42, Table 7.6 Transverse Section [p. 647]; N90).  It leaves the eyeball at the optic disc to enter the optic nerve with the central artery of the retina.  The vein leaves the optic nerve and usually drains directly into the cavernous sinus.  Occlusion of the central vein of the retina often results in slow, painless visual loss.  See http://emedicine.medscape.com/article/1223746-overview