L22 - Fibrous Layer (the Sclera) Flashcards
(21 cards)
What are the three layers of the eyeball?
Fibrous layer (sclera, limbus, cornea), vascular layer (iris, ciliary body, choroid), nervous layer (pigment and neural layers).
What are the main components of the fibrous layer?
Sclera, limbus, cornea.
What are the main functions of the sclera?
Protection of intraocular contents from trauma and mechanical displacement; preserve the shape of the eyeball and maintain the exact position of the optic system; provide a rigid insertion for the extraocular muscles.
How much of the globe does the sclera form?
Posterior 5/6 of the connective tissue coat of the globe.
What covers the sclera externally and internally?
Externally by Tenon’s capsule (fascial sheath) and anteriorly by conjunctiva; internally by the choroid (with a potential suprachoroidal space in between).
What is the scleral sulcus?
An indentation (furrow) on the inner surface of the most anterior point of the sclera near the limbus.
What is the scleral spur?
A circular flange of the most anterior part of the sclera lying deep to Schlemm’s canal.
What is Schlemm’s canal?
A unique, ring-shaped, endothelium-lined vessel that encircles the cornea.
What is the lamina cribrosa?
A sieve-like region of the sclera where optic nerve fibres pass; perforated by the optic nerve (scleral foramen).
What are scleral apertures?
Three sets of openings in the sclera for passage of nerves and vessels.
What are the three main histological layers of the sclera?
Episclera, scleral stroma, lamina fusca.
What are the characteristics of the episclera?
Outermost layer; loose connective tissue; highly vascular; connected to Tenon’s capsule by fine strands; merges with underlying scleral stroma.
What are the characteristics of the scleral stroma?
Thick, dense connective tissue (collagen type I and III); continuous with corneal stroma at limbus; collagen fibrils vary from 25 to 230 nm and are arranged in irregular bundles; contains occasional elastic fibres.
How does the arrangement of collagen fibrils differ in the scleral stroma?
Outer regions: bundles run approximately parallel to the surface; inner regions: bundles run in all directions; random arrangement and interweaving contribute to strength and flexibility.
Why is the sclera opaque?
Due to the number of glycosaminoglycans (GAGs), lower water content (68%), and variation in size/distribution of collagen fibrils causing light scattering.
What are the characteristics of the lamina fusca?
Innermost layer; thin, irregular layer of melanocytes; many grooves for passage of ciliary vessels and nerves; separated from choroid by the perichoroidal space; weak attachment to choroid.
What is the blood supply of the anterior sclera?
Three vascular layers (conjunctival vessels, superficial episcleral plexus, deep vascular plexus) supplied by anterior ciliary arteries.
What is the blood supply of the posterior sclera?
Small branches from long and short posterior ciliary arteries.
What are the characteristics of conjunctival vessels over the sclera?
Most superficial, freely mobile; arteries are tortuous, veins are straight.
What are the characteristics of the superficial episcleral plexus?
Straight, radial configuration, mobile over deeper layers; maximal congestion in episcleritis.
What are the characteristics of the deep vascular plexus?
Lies in the superficial part of the sclera, immobile; congestion here is associated with scleritis (purplish hue).
