The
sclera, also known as the
white part of the eye, is the opaque (usually white, though certain animals, such as horses and lizards, can have black sclera), fibrous, protective, outer layer of the
eyeEyes are organs that detect light, and send electrical impulses along the optic nerve to the visual and other areas of the brain. Complex optical systems with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system...
containing
collagenCollagen is the main protein of connective tissue in animals and the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content. It is naturally found exclusively in metazoa, including sponges. In muscle tissue it serves as a major component of endomysium...
and elastic fiber. It is derived from the
neural crestThe neural crest, a transient component of the ectoderm, is located in between the neural tube and the epidermis of an embryo during neural tube formation...
. In children, it is thinner and shows some of the underlying pigment, appearing slightly blue. In the elderly, however, fatty deposits on the sclera can make it appear slightly yellow.
The sclera forms the posterior five sixths of the connective tissue coat of the globe.
The
sclera, also known as the
white part of the eye, is the opaque (usually white, though certain animals, such as horses and lizards, can have black sclera), fibrous, protective, outer layer of the
eyeEyes are organs that detect light, and send electrical impulses along the optic nerve to the visual and other areas of the brain. Complex optical systems with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system...
containing
collagenCollagen is the main protein of connective tissue in animals and the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content. It is naturally found exclusively in metazoa, including sponges. In muscle tissue it serves as a major component of endomysium...
and elastic fiber. It is derived from the
neural crestThe neural crest, a transient component of the ectoderm, is located in between the neural tube and the epidermis of an embryo during neural tube formation...
. In children, it is thinner and shows some of the underlying pigment, appearing slightly blue. In the elderly, however, fatty deposits on the sclera can make it appear slightly yellow.
The sclera forms the posterior five sixths of the connective tissue coat of the globe. It is continuous with the dura mater and the cornea, and maintains the shape of the globe, offering resistance to internal and external forces, and provides an attachment for the extraocular muscle insertions. The sclera is perforated by plenty of nerves and vessels passing through the posterior scleral foramen, the hole that is formed by the optic nerve. At the optic disk the outer two-thirds of the sclera continues with the dura mater (outer coat of the brain) via the dural sheath of the optic nerve. The inner third joins with some choroidal tissue to form a plate (lamina cribrosa) across the optic nerve with perforations through which the optic fibers (fasciculi). The thickness of the sclera varies from 1mm at the posterior pole to 0.3 mm just behind the rectus muscle insertions. The sclera's blood vessels are mainly on the surface, and together with the
conjunctivaThe conjunctiva is a clear mucous membrane consisting of cells and underlying basement membrane that covers the sclera and lines the inside of the eyelids...
(which lies on top) This is a thin layer covering the sclera. Along with the vessels of the conjunctiva, those of the sclera renders the inflamed eye bright red.
Histology
The collagen of the sclera is continuous with the cornea. From outer to innermost, the four layers of the sclera are:
- episclera
- stroma
- lamina fusca
- endothelium
The sclera is opaque due to the irregularity of the collagen fibers, as opposed to near-uniformn thickness and parallel arrangement of the corneal collagen. To avoid confusion, the cornea, which is an outer continuation from the sclera, has 5 layers. Moreover, the cornea bears more mucopolysaccharide (a carbohydrate that has among its repeating units a nitrogenous sugar, hexosamine) to embed the fibrils. The middle, thickest layer is also called the stroma.
LASIKLASIK or Lasik is a type of refractive surgery for correcting myopia, hyperopia, and astigmatism. LASIK is performed by ophthalmologists using a laser...
(laser-assisted in situ keratomileusis) surgery to correct vision is usually done in this region of the cornea. The outside layer contains all the blood vessels of the stroma and along with the conjunctival vessels, are the cause of the bright redness of the inflamed eye. The sclera, like the cornea, contains a basal endothelium, above which there is the lamina fusca, containing a high count of pigment cells.
Sometimes, very small gray-blue spots can appear on the sclera, a condition called scleral melanocytosis. They are not harmful.
Additional images
External links
- "Sagittal Section Through the Eyeball"