The arterial supply to the orbit derives its origin from the internal carotid artery (ICA). In the head and the neck, the common carotid bifurcates at the C4-C5 region. This bifurcation leads to the external carotid artery and the ICA. The external carotid artery is an important vessel because of its blood supply to the orbit, dura, and cranial nerves. The important branches involved in irrigating the orbit include the following:
Facial artery and angular artery branch - Supplies orbit and medial lid
Internal maxillary artery
Middle meningeal artery - Major vessel supplying the orbit in the absence of the ophthalmic vessel
Anterior deep temporal artery
The Arterial System
Internal carotid artery
In addition to supplying the anterior part of the brain, eye, and adnexa, the internal carotid artery (ICA) sends branches to the forehead and the nose. The course and relations of this vessel may be divided into 4 portions: cervical, petrous, cavernous, and cerebral. The length of the ICA varies according to the length of the neck and the point of bifurcation of the common carotid. Although rare, it sometimes arises from the arch of the aorta. Instead of being straight, the course of the ICA is often very tortuous.
Branches from the ICA are numerous, and they are not within the scope of this article. This article primarily discusses the branches and the tributaries that irrigate the orbit. Interestingly, the cervical portion of the ICA gives off no branches. The ICA provides collateralization with the middle meningeal artery and lacrimal and ethmoidal anastomoses.
The ophthalmic artery (arteria ophthalmica) is the major blood supply of the orbit.  It arises from the ICA (first branch of the ICA) as the ICA is emerging from the cavernous sinus on the medial side of the anterior clinoid process. It enters the orbital cavity through the optic foramen, below and lateral to the optic nerve. The ophthalmic artery passes over the nerve (in 85% of cases) to reach the medial wall of the orbit.
The artery then proceeds forward horizontally, beneath the lower border of the superior oblique muscle, and divides into 2 terminal branches, frontal and dorsal nasal. As the artery crosses the optic nerve, it is accompanied by the nasociliary nerve and is separated from the frontal nerve by the superior rectus muscle and the superior levator palpebral muscle. The ophthalmic artery rarely arises from the middle meningeal artery. Most branches of the ophthalmic artery arise in the posterior one third of the orbit and pass anteriorly.
As outlined below, the branches of the ophthalmic artery are divided into an orbital group, distributing vessels to the orbit and surrounding parts, and an ocular group, distributing vessels to the muscles and bulb of the eye.
The orbital group consists of the following (see the image below):
Posterior ethmoidal artery
Anterior ethmoidal artery
Internal palpebral artery
The ocular group consists of the following:
Long ciliary artery
Short ciliary artery
Anterior ciliary artery
Central retinal artery
One of the largest branches derived from the ophthalmic artery, the lacrimal artery (arteria lacrimalis) arises close to the optic foramen and, not infrequently, is given off before the ophthalmic artery enters the orbit. Interestingly, the lacrimal artery has a variable origin.
This artery runs along the lateral wall of the orbit, accompanying the lacrimal nerve along the upper border of the lateral rectus muscle, and supplies the lacrimal gland.
The superior and inferior lateral palpebral arteries, which are terminal branches of the lacrimal artery, irrigate the upper and lower eyelids, respectively, and the conjunctiva. These vessels run medially and anastomose with the medial palpebral arteries, forming an arterial circle.
The muscular branches of the lacrimal artery supply the superior and lateral recti muscles. The glandular branches irrigate the lacrimal gland.
The lacrimal artery may also give off 1 or 2 zygomatic branches. One zygomatic branch passes through the zygomatico-temporal foramen, to reach and irrigate the temporal fossa, and anastomoses with the deep temporal arteries. Another branch appears on the cheek through the zygomatico-facial foramen and anastomoses with the transverse facial artery.
A recurrent meningeal branch of the lacrimal artery passes backward through the lateral part of the superior orbital fissure or through the meningeal foramen in the greater wing of the sphenoid bone, to the dura mater, and anastomoses with a branch of the middle meningeal artery. The lacrimal artery sometimes is derived from one of the anterior branches of the middle meningeal artery. This vessel establishes an anastomosis between the ICA and the external carotid artery.
The supraorbital artery (arteria supraorbitalis) arises from the ophthalmic artery, superiorly, as that vessel crosses over the optic nerve. The supraorbital artery passes upward on the medial borders of the superior rectus muscle and the levator palpebral muscle. The supraorbital nerve accompanies the supraorbital artery between the periosteum and the levator palpebral muscle to the supraorbital foramen.
As the supraorbital artery passes through the supraorbital foramen, it divides into a superficial branch and a deep branch, which supply the eyebrow and the forehead. It anastomoses with the frontal artery, the frontal branch of the superficial temporal artery, and the supraorbital artery of the opposite side. Within the orbit, the supraorbital artery supplies the superior rectus muscle and the levator palpebral muscle. Furthermore, it sends a branch across the pulley of the superior oblique muscle to supply the medial palpebral commissure.
The ethmoidal artery branches into the posterior and anterior ethmoidal arteries. The posterior ethmoidal artery, the smaller of the 2 branches, arises from the ophthalmic artery as it passes along the medial wall between the superior oblique muscle and the medial rectus muscle. This vessel passes through the posterior ethmoidal canal, supplying the posterior ethmoidal cells. In the orbit, this vessel can irrigate the superior oblique muscle, the superior and medial recti muscles, and the superior levator palpebral muscle.
The anterior ethmoidal artery accompanies the nasociliary nerve and exits the orbit through the anterior ethmoidal foramen. In the orbit, it supplies the superior oblique muscle. This vessel also supplies the anterior and middle ethmoidal cells, frontal sinus, lateral wall nose, and nasal septum.
Medial palpebral arteries
The superior and inferior medial palpebral arteries (arteriae palpebrales mediales, internal palpebral arteries) arise from the ophthalmic artery, opposite the pulley of the superior oblique muscle. These vessels leave the orbit to encircle the eyelids near their free margins, forming a superior arch and an inferior arch between the orbicularis oculi and the tarsi.
The superior medial palpebral artery anastomoses with the zygomatico-orbital branch of the temporal artery at the lateral angle of the orbit. This artery also anastomoses with the upper of the 2 lateral palpebral branches of the lacrimal artery.
The inferior medial palpebral artery anastomoses with the lower of the 2 lateral palpebral branches from the lacrimal artery at the lateral angle of the orbit, as well as with the transverse facial artery and, at the medial part of the eyelid, with a branch from the angular artery. From this last anastomosis, a branch passes to the nasolacrimal duct, ramifying in its mucous membrane, as far as the inferior meatus of the nasal cavity. The superior and inferior medial palpebral arteries supply the lacrimal sac.
One of the terminal branches of the ophthalmic artery, the frontal artery (arteria frontalis) leaves the orbit at its medial angle above the trochlea with the supratrochlear nerve, ascending on the forehead, and supplies the forehead and the scalp. The frontal artery anastomoses with the supraorbital artery and the contralateral vessels.
Dorsal nasal artery
The dorsal nasal artery (arteria dorsalis nasi, nasal artery), the other terminal branch of the ophthalmic artery, leaves the orbit above the medial palpebral tendon. After giving off a branch to the upper part of the lacrimal sac, it divides into 2 branches, one of which crosses the root of the nose. The dorsal nasal artery anastomoses with the angular artery, a terminal branch of the facial artery.
Central retinal artery
The central retinal artery (arteria centralis retinae) is the first branch, as well as one of the smallest branches, of the ophthalmic artery. It arises in the posterior one third of the ophthalmic artery. The central retinal artery travels for a short distance beneath the optic nerve, entering the dural sheath of the optic nerve about 5-15 mm behind the globe. It pierces the optic nerve obliquely and travels forward through the central optic nerve. This artery is the principal blood vessel that irrigates the retina.
The ciliary arteries (arteriae ciliares) are divided into 3 groups: short posterior, long posterior, and anterior ciliary arteries.
The short posterior ciliary arteries may number 6-12 vessels. Arising from the ophthalmic artery or its branches, the short posterior ciliary arteries travel anteriorly around the optic nerve to the posterior part of the eyeball, piercing the sclera around the entrance of the nerve. They supply the choroid and ciliary processes.
The 2 long posterior ciliary arteries pierce the sclera posteriorly, a short distance from the optic nerve.  They travel forward between the sclera and the choroid, to the ciliary muscle, where they divide into 2 branches. These branches form an arterial circle, the circulus arteriosus major, around the circumference of the iris.
Numerous converging branches from the circulus arteriosus major run in the substance of the iris to its pupillary margin, where they form a second arterial circle, the circulus arteriosus minor. The long and short posterior ciliary arteries give collateralization to all the recti muscles, except the lateral rectus muscle.
The anterior ciliary arteries are derived from the muscular branches. They travel to the front of the eye to form a vascular zone beneath the conjunctiva. They then pierce the sclera, a short distance from the cornea, and end in the circulus arteriosus major. The anterior ciliary arteries supply, in pairs, 3 recti muscles: superior, medial, and inferior. The lateral rectus muscle is supplied by a single anterior ciliary artery arising from the lacrimal artery. 
Anterior to the insertions of the 4 recti muscles, the anterior and posterior ciliary arteries anastomose with the long posterior ciliary vessels where the anastomosed vessels pierce the sclera, forming the intramuscular circle of the iris. 
The muscular branches (rami musculares) arise from a common trunk, leading to the superior (lateral) branch and the inferior (medial) branch. The muscular branches enter the sclera anterior to tendon insertions of the recti muscles and anastomose with the long posterior ciliary arteries.
The superior branch supplies the levator muscle, the superior rectus muscle, the superior oblique muscle, and a portion of the lateral rectus muscle (the lacrimal artery irrigates the remaining portion).
The inferior branch supplies the medial rectus muscle, the inferior rectus muscle, the inferior oblique muscle, and, sometimes, the lateral rectus muscle. This vessel gives off most of the anterior ciliary arteries. Additional muscular branches are given off by the lacrimal and supraorbital arteries or by the trunk of the ophthalmic artery. All the recti muscles have 2 anterior ciliaries, except the lateral rectus muscle.
Blood supply to the eyelid
The blood supply to the eyelid includes the following:
Marginal arcade - Anterior to the tarsus
Peripheral arcade - Between the levator muscle and the Muller muscle
The Venous System
The vortex veins provide drainage for the uveal tract (choroid, ciliary body, iris).  They pierce the sclera obliquely and open posterior to the equator. The superior vortex veins (lateral and medial) drain into the superior ophthalmic vein or its muscular or lacrimal branches. The 2 inferior vortex veins (lateral and medial) drain into the inferior ophthalmic vein.
Superior ophthalmic vein
The superior ophthalmic vein is the main venous channel for the superior orbit;  it drains to the cavernous sinus. The course of the superior ophthalmic vein begins at the inner angle of the orbit. It pursues the same course as the ophthalmic artery and receives tributaries corresponding to the branches of that vessel. It passes between the 2 heads of the lateral rectus muscle, through the medial part of the superior orbital fissure, and ends in the cavernous sinus.
Inferior ophthalmic vein
The inferior ophthalmic vein begins at the floor and medial wall of the orbit, travels backward, and divides into 2 branches. It provides a channel for inferior drainage.