biology

Scleral ring

Scleral ring is a biology topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Scleral ring rather than just read about it. In short: 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.

Key takeaways

Scleral ring belongs to biology; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Scleral ring to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Scleral ring from memory before moving on to harder problems.

Reference excerpt

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.

Terminology 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.

Function 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. Additionally, scleral rings may help the eye adjust to different viewing distances, also known as visual accommodation. Muscles are used to adjust the shape of the eye for accommodation, and the rings provide attachment sites for these muscles. In aquatic animals, the lens is squeezed in a different way to compensate for differences in light refraction underwater, and so the shape of the ring can be different than those in terrestrial animals. The ratio between the inner radius of the scleral ring and the radius of the orbit is correlated with ecology. It is larger in modern nocturnal birds than modern diurnal birds. This has been used as evidence for arguing that some dinosaurs were nocturnal.

Distribution

Reptiles A combination of scleral cartilage and ossicles are present, in which the cartilage acts as a cup around the posterior (rear) position of the eye and ossicles at the anterior (front) position of the eye form the ring. Within lepidosaurs (snakes, lizards, tuatara, and relatives), scleral rings have been found in all major lineages except Serpentes, or snakes, and two families within Anguimorpha: Dibamidae and Rhineuridae, which are both legless lizard families. All of these clades that lack a scleral ring share either a burrowing lifestyle or lack of limbs, indicating a possible correlation among these traits and loss of the scleral ring. Lizards typically have 14 ossicles in the ring, though this can vary. Mosasaurs, a clade of extinct marine squamates, also have scleral rings, as do members of the non-squamate Ichthyopterygia. Within Archelosauria (turtles, birds, crocodilians, and relatives), only birds and turtles retain the scleral rings. Fossil evidence shows that extinct marine crocodiles living in the Mesozoic had scleral rings, so the trait was lost over time. Scleral rings of varying lengths, curvatures, numbers of ossicles, and thickness are found in all birds. Birds typically have 12-18 ossicles, with 14 being the most common number.

Fish While all fish have scleral cartilage, teleost fish are the only family to retain scleral rings, with the rings being absent in the more basal clades Cladistia, Chondrostei, Lepisosteiformes, and Amiiformes. Teleost fish typically have only one or two ossicles per ring, and fish with no ossicles still retain cartilage. Most teleosts do not have ossicles, but this can vary even within groups. As a general trend, more basal groups (such as Elopomorpha and Osteoglossomorpha) tend to have no ossicles, while more derived groups (such as Percomorpha) are likely to have a variable number of ossicles (zero to two). In the Arthrodira, the typical number is 4. More active fish are more likely to have scleral rings, indicating that the rings help keep the eye stable during rapid swimming.

Extinct animals In the Osteolepis, the parietal eye also has a scleral ring.

Homology The scleral rings of fishes and those of reptiles and birds are not homologous. The scleral rings of fishes are on the eyeball's equator, and in modern teleosts number in 1 or 2, though in extinct species can go up to 4. It develops via ossification of the equatorial region of cartilaginous cup. The scleral rings in birds and reptiles are on the corneal limbus, and typically 10 to 18 in number. It develops directly from the mesenchyme, anteriorly to the cartilaginous cup.

Gallery

Illustrations

Worked examples

Example 1 — a first encounter with Scleral ring

Start with the simplest possible case. Write down what Scleral ring claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Scleral ring before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Scleral ring ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Scleral ring

In research: Scleral ring appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Scleral ring in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Scleral ring is common in secondary-school and first-year university syllabi. It links to neighbouring topics Dinosaur anatomy, Eye, Reptile anatomy, so understanding it makes those chapters shorter.
In everyday life: Look for Scleral ring outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Scleral ring in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Scleral ring means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Scleral ring out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Scleral ring in simple terms?

Why does Scleral ring matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Scleral ring?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Scleral ring.