Sclera

The sclera (Latin: sclera) is a white outer layer surrounding the eyeball's posterior five-sixths. Posteriorly the sclera is thickest. While traveling anteriorly, it becomes thinner. The conjunctiva covers the sclera from the front and further continues to the eyelids. Posteriorly, the sclera is connected to the facial sheath by connective tissue.

Structure of the sclera

The outer surface of the sclera is smooth, with an exception in the place where the tendons of the orbital muscles attach to the sclera. The sclera is approximately 1 mm thick in the posterior part in adults, but around the equator, it is 0.6 mm. The thinnest it is immediately behind the tendinous insertions of the recti muscles. In the place of the corneoscleral junction, the sclera is around 0.8 mm thick

In the frontal part, the sclera is continuous with the cornea. The connection site is called the corneoscleral junction or sclerocorneal junction. Behind the junction is a canal called the scleral venous sinus (the canal of Schlemm), which collects the aqueous humour from the eye's anterior chamber and lies in the scleral sulcus. Behind the canal, a scleral spur can be found. The scleral spur is a triangular scleral tissue that acts as a site of attachment for the ciliary muscle.

The sclera has a posterior pole that is perforated by the optic nerve. The perforation place is known as the posterior scleral foramen, where some part of the outer sclera is continuous with the dural and arachnoid sheaths of the optic nerve. The perforation site is about 3 mm medial and 1 mm above the posterior pole.

During childhood, the sclera is thin, so the pigment cells show through, making the sclera a bluish tinge, while in the elderly, the sclera can look yellowish due to the fat deposition.

The fibers of the optic nerve also go through the inner third of the sclera. This piercing through the sclera forms a structure called the lamina cribrosa. The lamina cribrosa is used as a passage also by the central retinal artery and vein.

The sclera has other openings:

  • Four anterior openings are located where the rectus muscles are attaching to the sclera. These openings work as a passage for the anterior ciliary artery.
  • Four to five middle openings are located posterior to the equator of the eye and act as a passage for the vorticose or vortex veins.
  • Posterior openings of the sclera are located around the posterior scleral foramen and act as a passage for the long and short ciliary arteries, veins, and nerves.

Layers of the sclera

The sclera can be divided into three layers: the episclera, the scleral stroma, lamina fusca.

The episclera

The episclera is the outer layer of the sclera. On the superficial part, the episclera is connected to the Tenon’s capsule. The deeper is connected to the scleral stroma. In the frontal part, the episclera contains the arterial episcleral plexus made by the branches of the anterior ciliary arteries. The plexus lies between the extrinsic muscle insertions and the corneoscleral junction. Usually, the plexus cannot be seen, but in a case of inflammation, the plexus can get congested and create the appearance of “red eyes.” The episclera becomes thinner as it moves to the back of the eye.

The scleral stroma

The scleral stroma is made of the dense irregular connective tissue responsible for creating the sclera’s white colour. Within the tissue, an irregular arrangement of collagen fibers can be seen. This arrangement is responsible for the opacity of the sclera. If the colour of the sclera changes, that may indicate a pathological process.

Lamina fusca

Lamina fusca is the inner layer of the sclera. The lamina fusca is slightly brown due to the presence of melanocytes. The lamina fusca covers the choroid. The space between the lamina fusca and choroid is known as the perichoroidal space. The long and short posterior ciliary arteries and nerves go through this space.

Function of the sclera

The sclera protects the inner contents of the eye from mechanical trauma. The sclera has a rigid structure that is responsible for the shape of the eyeball and keeps the eye structures in their place.

Vasculature and innervation of the sclera

Blood supply and venous drainage

The episcleral plexus supply the anterior part of the sclera. The posterior part is supplied by the branches of the long and short posterior ciliary arteries (arising from the ophthalmic artery).

The venous drainage happens through the vorticose veins into the superior and inferior ophthalmic veins.

Innervation

The anterior part of the sclera is innervated by the long ciliary nerve, while the posterior part by the short ciliary nerve.