|Anatomy and Embryology of the Eye
▃▃Anatomy of Eyeball (AN41.1)
•Anteroposterior diameter: at birth—17.5 mm; in adults—24 mm.
•Horizontal diameter: 23.5 mm.
•Vertical diameter: 23 mm.
Being flattened in vertical diameter, its shape resembles an oblate spheroid.
Eyeball consists of three coats (tunics, Fig. 1.1):
1.Outer fibrous coat.
2.Intermediate vascular coat (uveal tissue).
3.Innermost nervous layer (retina).
Outer Fibrous Coat
It consists of cornea and sclera. Cornea is the anterior 1/6th, transparent and avascular part of the fibrous coat. Sclera is the posterior 5/6th opaque part of the fibrous coat. The anterior part of the sclera is covered by a mucous membrane, the conjunctiva, which is reflected over the inner surface of the eyelids. The junction of sclera and cornea is called the sclerocorneal junction or limbus. Conjunctiva is firmly adherent at the limbus.
Intermediate Vascular Coat (Uveal Tissue)
1.Iris: It is the anterior most part of the uveal tissue.
2.Ciliary body: It extends from iris to ora serrata and is subdivided into two parts: anterior (pars plicata, 2 mm) and posterior (pars plana, 4 mm).
3.Choroid: It is the posterior most part of the uveal tissue. It lies in contact with sclera on its outer surface and with retina on its inner surface.
Innermost Nervous Layer (Retina)
It extends from optic disc to ora serrata. It ends abruptly just behind the ciliary body as a dentate border called ora serrata. It is concerned with visual functions.
The eyeball is divided into anterior and posterior segments (Fig. 1.2) by the lens, which is suspended from the ciliary body by fine delicate fibrils called zonules (suspensory ligaments of lens). Both anterior and posterior chambers communicate with each other through the pupil.
This includes structures anterior to the lens, that is, cornea, iris, lens, and part of ciliary body. The anterior segment is divided by the iris into an anterior chamber and a posterior chamber.
Anterior Chamber (OP6.5)
•Boundaries: The posterior surface of cornea forms the anterior boundary, while the iris, part of ciliary body, and surface of lens in the pupillary area forms the posterior boundary of the anterior chamber.
•Depth: It is 2.5 mm in center. It is shallower in hypermetropes and deeper in myopes.
•It contains a clear watery fluid called aqueous humor (0.25 mL).
•Angle: Angle of anterior chamber is a peripheral recess of the anterior chamber through which drainage of aqueous humor takes place.
•It is a triangular space and contains aqueous humor.
•Boundaries: The posterior surface of iris and part of ciliary body forms the anterior boundary, while the lens, zonules, and ciliary body forms the posterior boundary.
This includes structures posterior to the lens, that is, vitreous humor, retina, choroid, and optic nerve.
Vitreous humor is a gel-like material which fills the cavity behind the lens. The detailed anatomy of various parts of the eyeball and ocular adnexa (eyelids, lacrimal apparatus, and orbit) is described in respective chapters.
▃▃Blood Supply of Eyeball
The eye has two separate systems of blood vessels (Table 1.1):
1.Retinal vessels: These supply part of the retina.
2.Ciliary blood vessel: These supply rest of the eye.
•Lacrimal artery: It supplies the lacrimal gland.
•Central retinal artery: Supplies the retina.
•Short posterior ciliary arteries (10–20 in number): Supply the uveal tract.
•Long posterior ciliary arteries (two in number).
•Muscular arteries: Supply four recti muscles which give rise to anterior ciliary arteries.
Short posterior ciliary arteries pierce the sclera in a ring around the optic nerve and give rise to the intrascleral circle of Zinn. These end as choriocapillaris and supply the entire choroid.
Long posterior ciliary arteries pierce sclera in horizontal meridian and pass forward in suprachoroidal space (between sclera and choroid), without giving any branch, reach ciliary muscle and form circulus arteriosus iridis major (major arterial circle of iris) with anterior ciliary arteries. It supplies ciliary processes and iris. The recurrent branches from the circle supply the anterior part of choriocapillaris. Branches from this major arterial circle run radially through the iris, which form a circular anastomosis near the pupillary margin called the circulus arteriosus iridis minor (minor arterial circle of iris). Anterior ciliary arteries give branches to the conjunctiva, sclera, and limbus.
Central retinal artery supplies the inner layers of retina. Outer layers of retina are nourished by diffusion from choriocapillaris.
Small anastomoses between vessels of uveal origin and central retinal artery connect the uveal and retinal circulations.
Occlusion of one of the ciliary arteries usually does not produce dramatic effects because of arterial ring and manifold arterial supply to choriocapillaris.
Venous drainage of inner retina occurs via central retinal vein to superior ophthalmic vein after which the blood passes into cavernous sinus and out of the skull through internal jugular vein. Outer retinal layers are drained by vortex veins which drain into the superior ophthalmic vein.
Table 1.1 Blood supply of the eyeball
Part of the eye
•Iris and ciliary body
•Long posterior ciliary arteries and anterior ciliary arteries.
•Short posterior ciliary arteries.
Vortex veins drain blood from whole of uveal tract except outer part of ciliary muscle which is drained by anterior ciliary veins.
♢outer layers of retina
♢inner layers of retina
By diffusion from choriocapillaris
central retinal artery.
via central retinal vein into cavernous sinus.
Uveal tract drains through three groups of ciliary veins, namely, short posterior ciliary veins, vortex veins, and anterior ciliary veins.
Short posterior ciliary veins receive blood only from sclera. They do not receive any blood from the choroid.
Vortex veins (venae vorticosae): Small veins from uveal tract join to form four vortex veins, namely, superior temporal, inferior temporal, superior nasal, and inferior nasal veins.
These pierce sclera behind the equator and drain into superior and inferior ophthalmic veins which, in turn, drain into cavernous sinus. As ophthalmic veins communicate with cavernous sinus, they act as a route by which infections can spread from outside to inside the cranial cavity. Vortex veins drain blood from whole of the uveal tract except outer part of ciliary muscle, which is drained by anterior ciliary veins. Veins from the outer part of ciliary body form a plexus, ciliary venous plexus, which drain into anterior ciliary veins. These receive blood from only outer part of ciliary muscle.
▃▃Innervation of Eyeball
Eyeball is innervated by both sensory and motor nerves.
■■Sensory Nerve Supply
It is derived from ophthalmic division of the trigeminal nerve (V1). It is purely a sensory nerve. It divides into three branches (Fig. 1.5):
Long ciliary nerve (a branch of nasociliary nerve) is sensory to eyeball but may also contain sympathetic fibers for pupillary dilatation. Nasociliary nerve also gives off sensory root to ciliary ganglion.
■■Motor Nerve Supply
These nerves supply extrinsic (extra ocular muscles) and intrinsic muscles.
Nerve Supply of Extrinsic Muscles (Extra Ocular Muscles)
Mnemonic—LR6 (SO4)3, that is, lateral rectus is supplied by 6th nerve (Abducens nerve). Superior oblique is supplied by 4th nerve (Trochlear nerve), and rest of the muscles (i.e., superior rectus [SR], inferior rectus [IR], medial rectus [MR], and inferior oblique) are supplied by 3rd nerve (oculomotor nerve).
Oculomotor nerve (III) divides into (Fig. 1.6):
•Superior branch: It innervates SR and levator palpebrae superioris (LPS).
•Inferior branch: It innervates MR, IR, inferior oblique (IO), and a branch to ciliary ganglion.