ODOAP: Eyeball - Sclera (Prelims)

Fascial sheath of eyeball (Tenon's capsule)

Thin membrane surrounding the eyeball, separating it from orbital fat and forming a socket; sleeves form around the tendons of the extrinsic eye muscles.

Episcleral space

A potential space between the inner surface of the fascial sheath and the sclera.

Suspensory ligament of Lockwood

A hammock-like thickening of the inferior part of the fascial sheath that provides support for the eyeball from below.

Medial check ligament

A strong expansion of the medial rectus tendon sleeve attached to surrounding bones, limiting the muscle’s action.

Lateral check ligament

A strong expansion of the lateral rectus tendon sleeve attached to the zygomatic bone, limiting the muscle’s action.

Superior expansion

A thinner expansion from the superior rectus tendon to the levator palpebrae superioris, possibly coordinating upward gaze.

Inferior expansion

A thinner expansion from the inferior rectus tendon to the inferior tarsal plate, aiding eyelid alignment when looking downward.

Periorbita

The periosteal lining of the orbital cavity.

Orbital septum

A fibro-fatty membrane extending from the orbital rim to the eyelids, acting as a barrier and anchor for structures.

extrinsic muscles of the eye

The six muscles that move the eyeball

their tendons pierce the fascial sheath and each tendon has a corresponding fascial sleeve.

Ciliary nerves

vessels

Nerves and vessels that pierce the sclera around the optic nerve.

Vortex (vorticose) veins

Veins draining the uveal tract, passing through the sclera to venous channels.

Enucleation of the eyeball

Removal of the eyeball;

the fascial sheath should be preserved to provide a socket for a prosthesis.

Fibrous layer

Outer layer consisting of the sclera and the cornea; provides protection and shape.

Sclera

Posterior five-sixths of the eyeball;

opaque, tough fibrous coat that shapes the eye and anchors extraocular muscles.

Episclera

Outermost layer of the sclera;

loose connective tissue merging with Tenon's capsule;

vascular anteriorly.

Scleral stroma

Dense fibrous tissue with collagen fibrils;

largely responsible for scleral opacity and mechanical properties.

Lamina fusca

Innermost scleral layer, lightly pigmented by melanocytes;

grooves for ciliary vessels and nerves;

attaches to choroid via connective tissue.

Lamina cribrosa

Sieve-like area in the posterior sclera where optic nerve fibers pass;

a weak spot susceptible to deformation in glaucoma.

Canal of Schlemm (sinus venosus sclerae)

Circular canal just posterior to the limbus, involved in aqueous humor drainage;

lined by endothelium.

Anterior apertures

Openings in the sclera at rectus muscle insertions for branches of the anterior ciliary arteries (two per rectus, except lateral rectus has one).

Middle apertures

Openings about 4 mm posterior to the equator for the exit of vortex veins.

Posterior apertures

Small and numerous openings around the optic nerve transmitting long and short ciliary nerves and vessels.

Corneoscleral junction (limbus)

The junction between the cornea and sclera;

boundary region important for corneal and scleral continuity.

Fovea centralis relation to visual axis

Fovea is temporal and slightly inferior to the posterior pole, so the visual axis does not coincide with the optic axis.

Visual axis

Line from the nodal point through the fovea to the cornea; direction of gaze, not necessarily aligned with the optic axis.

Axis of the eyeball

Line joining the anterior and posterior poles; the geometric axis of the eye.

Anteroposterior diameter

Approx. 24 mm; vertical ~23 mm and horizontal ~23.5 mm.

Myopia/hyperopia relation to axial length

Longer axial length predisposes myopia (focal point in front of retina); shorter axial length predisposes hyperopia (focal point behind retina).

Function of Fascial Sheath

Provides a socket for the eyeball, positions and supports the globe in the orbit, and allows smooth coordinated movements with extraocular muscles.

Anterior Attachment (FS)

Firmly attached to sclera about 1.5 mm posterior to corneoscleral junction.

Posterior Attachment (FS)

Fuses with meninges around the optic nerve and sclera near optic nerve exit.

Ciliary nerves.

Ciliary vessels.

Vorticose (vortex) veins.

Structures Piercing the Sheath

Coarse Trabeculae