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Scleral ring
The scleral ring (also sclerotic ring) is a hardened ring of plates, often derived from bone, found in the eye of animals in several groups of vertebrates. Mammals, amphibians, snakes, and crocodilians lack scleral rings. The ring is in the fibrous outer layer of the eye, called the sclera.
Scleral rings can be made of cartilaginous material (scleral cartilage) or bony material (scleral ossicles), or often a combination of both, that comes together to form a ring. The arrangement, size, shape, and number of ossicles vary by group. They are believed to have a role in supporting the eye, especially in animals whose eyes are not spherical, or which live underwater.
The structure is referred to as scleral ring or the sclerotic ring. The individual bones in the ring are the scleral ossicles if bony, or scleral cartilage if cartilaginous. The Latin name is Anulus [Annulus] ossicularis sclerae.
In some species, there may be a cartilaginous cup in the posterior half of the eye. Any ossification in the cup is called Ossicula posteriora sclerae. In birds, there is sometimes an ossification around the optic nerve, called the os opticus.
Because the word sclerotic often implies pathology of the sclera (see "sclerosis", an unrelated medical condition), some authors have urged using only "scleral ring" and avoiding "sclerotic ring", to avoid confusion and to increase the utility of character comparisons.
Rudolf Leuckart claims that the scleral ring was already noted in De arte venandi cum avibus. It was first discovered by Volcher Coiter in the 1570s, then rediscovered by Malpighi in the eye of an eagle. A 1931 thesis made an exhaustive study.
Scleral rings may help support inner structures of the eye, especially in animals that do not have round eyes. Animals that move rapidly, including both fast flying birds and fast swimming fish have the most robust scleral rings, indicating that these thick rings are used to protect the eye during intense changes in pressure in the air and in the water.
In birds, the eye is generally of a hemispherical shape on the posterior half. The anterior half may be of 3 shapes: flat (as in most diurnal birds), globose (as in Falconiformes and many Passeriformes), and tubular (as in owls). In all cases, the eye shape diverges significantly from an ellipsoidal shape in the anterior end, and the corneal limbus is usually curved inwards. This is hypothesized to require mechanical reinforcement by the scleral ring. A cartilaginous cup reinforces the posterior end.
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Scleral ring
The scleral ring (also sclerotic ring) is a hardened ring of plates, often derived from bone, found in the eye of animals in several groups of vertebrates. Mammals, amphibians, snakes, and crocodilians lack scleral rings. The ring is in the fibrous outer layer of the eye, called the sclera.
Scleral rings can be made of cartilaginous material (scleral cartilage) or bony material (scleral ossicles), or often a combination of both, that comes together to form a ring. The arrangement, size, shape, and number of ossicles vary by group. They are believed to have a role in supporting the eye, especially in animals whose eyes are not spherical, or which live underwater.
The structure is referred to as scleral ring or the sclerotic ring. The individual bones in the ring are the scleral ossicles if bony, or scleral cartilage if cartilaginous. The Latin name is Anulus [Annulus] ossicularis sclerae.
In some species, there may be a cartilaginous cup in the posterior half of the eye. Any ossification in the cup is called Ossicula posteriora sclerae. In birds, there is sometimes an ossification around the optic nerve, called the os opticus.
Because the word sclerotic often implies pathology of the sclera (see "sclerosis", an unrelated medical condition), some authors have urged using only "scleral ring" and avoiding "sclerotic ring", to avoid confusion and to increase the utility of character comparisons.
Rudolf Leuckart claims that the scleral ring was already noted in De arte venandi cum avibus. It was first discovered by Volcher Coiter in the 1570s, then rediscovered by Malpighi in the eye of an eagle. A 1931 thesis made an exhaustive study.
Scleral rings may help support inner structures of the eye, especially in animals that do not have round eyes. Animals that move rapidly, including both fast flying birds and fast swimming fish have the most robust scleral rings, indicating that these thick rings are used to protect the eye during intense changes in pressure in the air and in the water.
In birds, the eye is generally of a hemispherical shape on the posterior half. The anterior half may be of 3 shapes: flat (as in most diurnal birds), globose (as in Falconiformes and many Passeriformes), and tubular (as in owls). In all cases, the eye shape diverges significantly from an ellipsoidal shape in the anterior end, and the corneal limbus is usually curved inwards. This is hypothesized to require mechanical reinforcement by the scleral ring. A cartilaginous cup reinforces the posterior end.