2.1. General eye features

The outer coats of the eye are the cornea and sclera; their juncture is the limbus (Fig 2.1). The interior of the eye is divided into the anterior and posterior segments. The anterior segment includes the cornea, iris, ciliary body and lens as well as the spaces of the anterior and posterior chambers filled with aqueous humor. The posterior segment includes the retina, choroid and optic nerve head as well as the vitreous compartment filled with vitreous humor.

Figure 2.1

Illustration of the human eye. Reproduced with permission from NEI-NIH ftp://ftp.nei.nih.gov/eyean/eye12-72.tif.

2.2. Vascular supply and drainage

The arterial input to the eye is provided by several branches from the ophthalmic artery, which is derived from the internal carotid artery in most mammals (Fig 2.2, left). These branches include the central retinal artery, the short and long posterior ciliary arteries, and the anterior ciliary arteries. Venous outflow from the eye is primarily via the vortex veins and the central retinal vein, which merge with the superior and inferior ophthalmic veins that drain into the cavernous sinus, the pterygoid venous plexus and the facial vein (Fig 2.2, right). In some species (e.g., rodents and lagomorphs), the orbital veins form a sinus.

Figure 2.2

Arterial (left) and venous (right) connections to the systemic circulation. Reproduced from Anatomy of the Human Body, Gray, H., 2th Edition, Lea & Febiger, Philadelphia, 1954.

The iris and ciliary body are supplied by the anterior ciliary arteries, the long posterior ciliary arteries and anatosmotic connections from the anterior choroid (Fig 2.3 and 2.4). The anterior ciliary arteries travel with the extraocular muscles and pierce the sclera near the limbus to join the major arterial circle of the iris. The long posterior ciliary arteries (usually two) pierce the sclera near the posterior pole, then travel anteriorly between the sclera and choroid to also join the major arterial circle of the iris. The major arterial circle of the iris gives off branches to the iris and ciliary body. Most of the venous drainage from the anterior segment is directed posteriorly into the choroid and thence into the vortex veins.

Figure 2.3

Diagrams of the blood vessels of the eye. Course of vasa centralia retinæ: α. Arteria. α_l. Vena centralis retinæ. β. Anastomosis with vessels of outer coats. γ. Anastomosis with branches of short posterior (more...)

Figure 2.4

Bisected vascular cast of the rabbit eye (left) and rabbit anterior segment. Reproduced with permission from Investigative Ophthalmology & Visual Science.

The retina is supplied by the central retinal artery and the short posterior ciliary arteries (Fig 2.3). The central retinal artery travels in or beside the optic nerve as it pierces the sclera then branches to supply the layers of the inner retina (i.e., the layers closest to the vitreous compartment). There are marked species differences in the inner retinal vascularization, with primates having a complex 4-zone arrangement and an avascular zone at the fovea, lagomorphs having a rather simple narrow band of superficial vessels, rodents having a wagon-wheel spoke-like arrangement and guinea pigs having no inner retinal vessels (Fig 2.5 and 2.6). Retinal venules and veins coalesce into the central retinal vein, which exits the eye with the optic nerve parallel and counter-current to the central retinal artery.

Figure 2.5

Fundus photographs of the human (left), rabbit (middle) and rat (right) retina.. Reproduced with permission from Investigative Ophthalmology & Visual Science. Reproduced with permission from American Physiological Society and Investigative Ophthalmology (more...)

Figure 2.6

Primate foveal avascular zone. Reproduced with permission from Society for Neuroscience.

The short posterior ciliary arteries (typically 6-12) pierce the sclera around the optic nerve then arborize to form the arterioles of the dense outer layer of conduit vessels of the choroid (Fig 2.3, 2.4, 2.7). The arterioles the give off roughly perpendicular terminal arterioles that supply lobules of choriocapillaries that comprise the sheet-like layer of the choriocapillaris adjacent to Bruch’s membrane, the retinal pigment and the outer segments of the photoreceptors (Fig 2.7). The choriocapillaris lobules drain into venules that join the larger venules of the outer conduit layer that coalesce into the 4-5 vortex veins that pierce the sclera at the equator.

Figure 2.7

Vascular casts of the cat choroid showing arrangement of conduit vessels to choriocapillaris (left) and sheet-like structure of choriocapillaris viewed from the retina (right). Reproduced with permission from Investigative Ophthalmology & Visual (more...)

The vascular supply of the optic nerve is complex (Fig 2.8). The optic nerve has three zones referenced to the lamina cribosa, the connective tissue extension of the sclera through which the optic nerve axons and the central retinal artery and vein pass. The prelaminar (i.e., inside the eye relative to the lamina cribosa) optic nerve is supplied by collaterals from the choroid and retina circulations. The laminar zone is supplied by branches from the short posterior ciliary and pial arteries. The post laminar zone is supplied by the pial arteries. Venous drainage is via the central retinal vein and pial veins. For the optic nerve vessels, the laminar zone marks the transition from exposure to the IOP to the cerebral fluid pressure within the optic nerve sheath.

Figure 2.8

Schematic of optic nerve circulation. Reproduced with permission from Progress in Retinal and Eye Research, Elsevier.

2.3. Innervation

Autonomic innervation of the ocular circulations is restricted to the vessels of the uvea (i.e., the choroid, ciliary body and iris) and optic nerve; the retina appears to lack sympathetic and parasympathetic nerves^7–9. The postganglionic sympathetic nerves originate in the superior cervical ganglion. Parasympathetic innervation originates in the pterygopalatine ganglion via the facial nerve. In the choroid of primates and some species of birds, a network of ganglion cells has been identified similar to the enteric nervous system of the gut, but their functional significance is unclear^10,11.