Superficially, the smoothtooth blacktip shark resembles the blacktip reef shark (C. melanopterus). It is rather robust in build, with a short and blunt snout. The large nostrils are preceded by well- developed, triangular flaps of skin. The small, circular eyes are equipped with nictitating membranes.
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The nictitating membrane (mid-blink) of a bald eagle Fully developed nictitating membranes are found in fish, amphibians, reptiles, birds and mammals, but are rare in primates.Paul Miller, Why do cats have an inner eyelid as well as outer ones? Scientific American. 20 Nov 2006.
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The Pondicherry shark has a robust build and a moderately long, pointed snout. The large and circular eyes are equipped with nictitating membranes. Each nostril is broad with a small, narrow nipple- shaped lobe on the anterior rim. The arched mouth lacks conspicuous furrows or enlarged pores at the corners.
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The juvenile plumage develops at about three months and is blackish finely barred with brown, with the head and neck being especially dark. The facial feathers gradually thin to expose the bluish skin. The adult plumage has developed by about fifteen months. The eyes of an emu are protected by nictitating membranes.
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Ventral view of head. As its common name suggests, the daggernose shark has a very long, flattened snout with a pointed tip and an acutely triangular profile from above. The eyes are circular and minute in size, with nictitating membranes (a protective third eyelid). The nostrils are small, without prominent nasal skin flaps.
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The barbeled houndshark has an extremely slender body and horizontally oval eyes equipped with internal nictitating membranes. A pair of tiny spiracles is present behind the eyes. Each nostril is preceded by a slender barbel. The mouth is long and strongly arched, with very long furrows at the corners extending onto both jaws.
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The nictitating membrane of a masked lapwing as it closes over the left eye, originating from the medial canthus The nictitating membrane (from Latin nictare, to blink) is a transparent or translucent third eyelid present in some animals that can be drawn across the eye from the medial canthus for protection and to moisten it while maintaining vision. Some reptiles, birds, and sharks have full nictitating membranes; in many mammals, a small, vestigial portion of the membrane remains in the corner of the eye. Some mammals, such as cats, camels, polar bears, seals and aardvarks, have full nictitating membranes. Often called a third eyelid or haw, it may be referred to in scientific terminology as the plica semilunaris, membrana nictitans, or palpebra tertia.
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The body of the creek whaler is spindle-shaped and rather stocky. The long snout has a narrowly parabolic shape and large nostrils preceded by small, nipple-shaped flaps of skin. The eyes are circular and of medium size, and are equipped with nictitating membranes. The arched mouth has very short furrows at the corners.
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Contrary to its common name, the graceful shark's spindle-shaped body has been described as "tubby". The wedge-like snout is short and pointed. The eyes are rather large and circular, and equipped with nictitating membranes (protective third eyelids). The mouth has short, indistinct furrows at the corners and contains 31-33 upper and 29-33 lower tooth rows.
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The largest known specimen of the longnose pygmy shark is a male long. Its body is slender and moderately compressed from side to side. The long, bulbous snout comprises about half the head length, and comes to a blunt, conical tip. The eyes are large, lack nictitating membranes (protective third eyelids), and are followed by large spiracles.
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Identifying traits of the sicklefin weasel shark include its spiracle and short mouth. The sicklefin weasel shark is a slender-bodied species reaching in length. The snout is fairly long and rounded, with the nostrils preceded by short flaps of skin. The large, oval eyes are equipped with nictitating membranes and are followed by minute spiracles.
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Reaching in length, the Australian weasel shark has a slender, spindle-shaped body and a moderately long head with a thick, rounded snout. The large, oval eyes possess nictitating membranes and have notched posterior rims. Tiny spiracles are located behind and above the eyes. The large nostrils are preceded by fairly long, triangular flaps of skin.
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The hardnose shark is slender, with a long snout and elongated rear tips on the dorsal fins. The hardnose shark is a slim-bodied species with a long, narrow, and pointed snout. Unlike in other Carcharhinus species, its rostral (snout) cartilages are highly calcified, hence the name "hardnose". The circular eyes are rather large and equipped with protective nictitating membranes.
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The nervous shark has a rather stout, spindle-shaped body and a short, broadly rounded snout. The anterior margin of each nostril is extended into a slender nipple-shaped lobe. The moderately large eyes are horizontally oval in shape and equipped with nictitating membranes. The mouth lacks conspicuous furrows at the corners and contains 25–30 upper and 23–28 lower tooth rows.
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Upper teeth Lower teeth C. longimanus most distinguishing characteristics are its long, wing-like pectoral and dorsal fins. The fins are significantly larger than most other shark species, and are conspicuously rounded. The shark's nose is rounded and its eyes are circular, with nictitating membranes. C. longimanus has a 'typical', although somewhat flattened requiem shark body, often with a mildly humpbacked aspect.
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Distinguishing traits of the smalltail shark include its long snout and its broad first dorsal fin. The smalltail shark is a slender-bodied species with a fairly long, pointed snout. The leading margin of each nostril is enlarged into a narrow, pointed lobe. The large, circular eyes are equipped with nictitating membranes, and behind them is a series of prominent pores.
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Characteristic traits of the coral catshark include its very thin body and pattern of dark and light spots. The coral catshark has a very slender, cylindrical, firm body and a short, narrow head. The snout is short and slightly flattened, with a blunt tip. The eyes are horizontally oval and protected by rudimentary nictitating membranes; behind are spiracles of moderate size.
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The bramble shark has a "thick", cylindrical body and a somewhat flattened head. The snout is blunt and shorter than the width of the mouth, with widely spaced nostrils that are preceded by small flaps of skin. The eyes lack nictitating membranes; the tiny spiracles are located well behind them. The wide, curved mouth bears very short furrows at the corners.
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Bulky and soft-bodied, the false catshark has a broad head with a short, rounded snout. The nostrils have large flaps of skin on their anterior rims. The narrow eyes are over twice as long as high, and are equipped with rudimentary nictitating membranes; behind the eyes are large spiracles. The huge mouth is arched and bears short furrows at the corners.
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Sharks have eyelids, but they do not blink because the surrounding water cleans their eyes. To protect their eyes some species have nictitating membranes. This membrane covers the eyes while hunting and when the shark is being attacked. However, some species, including the great white shark (Carcharodon carcharias), do not have this membrane, but instead roll their eyes backwards to protect them when striking prey.
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The horizontally oval eyes are placed high on the head and are equipped with rudimentary nictitating membranes (protective third eyelids). Beneath the eye is a prominent ridge, and behind it is a tiny spiracle. The mouth forms a short, wide arch, and bears somewhat long furrows at the corners. Tooth rows number 24–32 in the upper and 36–42 in the lower jaw.
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The large, circular eyes are equipped with nictitating membranes. The furrows at the corners of the mouth are barely evident. There are 32–35 upper and 29–31 lower tooth rows; each upper tooth has a slender, upright cusp and fine serrations that become coarser near the base, while the lower teeth are narrower and more finely serrated. The five pairs of gill slits are long.
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The common thresher can be distinguished from other thresher sharks by the appearance of its head and the coloration above its pectoral fins. Teeth The common thresher is a fairly robust shark with a torpedo-shaped trunk and a short, broad head. The dorsal profile of the head curves evenly down to the pointed, conical snout. The eyes are moderately large and lack nictitating membranes.
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Red fox (left) and corsac fox (right) yawning The red fox has an elongated body and relatively short limbs. The tail, which is longer than half the body length (70 per cent of head and body length), is fluffy and reaches the ground when in a standing position. Their pupils are oval and vertically oriented. Nictitating membranes are present, but move only when the eyes are closed.
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The larger Figaro species, F. boardmani, reaches long, while the smaller F. striatus reaches long. Members of this genus closely resemble Galeus, with firm, slender bodies and short heads. The eyes are horizontally oval and equipped with rudimentary nictitating membranes (protective third eyelids); a narrow ridge is found beneath each eye and a tiny spiracle behind. The anterior rims of the nostrils are enlarged into triangular flaps.
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A Southern Hemisphere porbeagle showing the white patch on the rear tip of the first dorsal fin, which is unique to the species. The porbeagle is a very stout-bodied shark with a fusiform (spindle-like) shape. The long, conical snout tapers to a sharp point, and is supported by enlarged, highly calcified rostral cartilages. The eyes are large and black, without nictitating membranes (protective third eyelids).
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The maximum reported length of the blotchy swellshark is . It has a firm, stout body with a soft, distensible abdomen, and a short, broad, flattened head. The snout is proportionately long and rounded, with large nostrils divided by short, triangular flaps of skin in front. The small, horizontally oval eyes are placed high on the head and equipped with rudimentary nictitating membranes (protective third eyelids).
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Reaching long, the balloon shark has a stout, firm body and a broad, flattened head. The snout is short and rounded, with each nostril divided by a narrow lobe on its anterior rim. The horizontally oval eyes are equipped with rudimentary nictitating membranes (protective third eyelids) and placed rather high on the head. Above and below each eye are ridges, and behind is a small spiracle.
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The sharptooth houndshark is a stout-bodied species with a short, thick, and blunt snout. The nostrils are widely spaced and preceded by lobe-like flaps of skin that do not reach the mouth. The horizontally oval eyes have ridges underneath and are equipped with nictitating membranes. The large mouth bears long, deep furrows at the corners, with those on the lower jaw almost meeting in the middle.
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The medium-sized, circular eyes are equipped with nictitating membranes (protective third eyelids). The mouth has very short, subtle furrows at the corners and contains 13-15 (typically 14) tooth rows on either side of both jaws. The upper teeth are distinctively broad, triangular, and slightly oblique with strong, coarse serrations, while the lower teeth are narrower and upright, with finer serrations. The five pairs of gill slits are fairly long.
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When the animal completely submerges, the nictitating membranes cover its eyes. In addition, glands on the nictitating membrane secrete a salty lubricant that keeps the eye clean. When a crocodilian leaves the water and dries off, this substance is visible as "tears". The ears are adapted for hearing both in air and underwater, and the eardrums are protected by flaps that can be opened or closed by muscles.
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Birds can actively control their nictitating membrane.Frans C. Stades, Milton Wyman, Michael H. Boevé, Willy Neumann, Bernhard Spiess. Ophthalmology for the Veterinary Practitioner. 105–106 In birds of prey, the membrane also serves to protect the parents' eyes from their chicks while they are feeding them, and when peregrine falcons go into their dives, they will blink repeatedly with their nictitating membranes to clear debris and spread moisture across the eyes.
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Like other members of its genus, the northern river shark has a stocky body with a high back. The head is wide and flattened, with a broadly rounded snout and minute eyes equipped with nictitating membranes. Each nostril is divided into a very large incurrent opening and a small excurrent opening by a triangular skin flap. The sizable mouth is broadly arched, with short furrows at the corners.
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The large, horizontally oval eyes are placed moderately high on the head and bear rudimentary nictitating membranes (protective third eyelids); there are prominent ridges underneath. The mouth forms a long, wide arch with very short furrows at the corners. The teeth are tiny with typically three cusps in males and five cusps in females; the central cusp is the longest. The upper teeth are exposed when the mouth is closed.
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Reaching in length, the northern sawtail catshark has a firm, thin body with a mostly cylindrical cross-section. The head is short, narrow, and flattened, with a bluntly pointed snout. The eyes are horizontally oval and equipped with rudimentary nictitating membranes (protective third eyelids); beneath each is a narrow ridge, and behind is a tiny spiracle. The anterior rims of the nostrils are enlarged into triangular, outward-pointing flaps.
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The eyes, ears and nostrils are arranged so that they can remain above water when the rest of the body submerges. The nostrils and ears have valves that close underwater while nictitating membranes cover the eyes. Their lips can close behind the incisors, allowing for chewing in water. Beavers typically spend 5–6 minutes underwater per dive but they can hold their breath for as long as 15 minutes.
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The Borneo shark is slim- bodied, with a long, pointed snout and oblique, slit-like nostrils preceded by narrow, nipple-shaped flaps of skin. The eyes are rather large and circular, and equipped with nictitating membranes. The corners of the sizable mouth bear short, indistinct furrows, and immediately above are a series of enlarged pores that are unique within the genus. There are 25–26 upper and 23–25 lower tooth rows.
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The markings of the banded houndshark become indistinct with age. The banded houndshark is a moderately slender-bodied species growing up to long. The snout is short, broad, and rounded; the widely separated nostrils are each preceded by a lobe of skin that does not reach the mouth. The horizontally oval eyes are placed high on the head; they are equipped with rudimentary nictitating membranes (protective third eyelids) and have prominent ridges underneath.
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The milk shark has a slender build with a long, pointed snout, large, round eyes with nictitating membranes (protective third eyelids), and no spiracles. On each side of the head behind the corner of the jaw, there are usually seven to 15 enlarged pores. The nostrils are small, as are the adjacent triangular skin flaps. There are long furrows at the corners of the mouth on both the upper and lower jaws.
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Upper teeth Lower teeth Slim and streamlined, the silky shark has a fairly long, rounded snout with barely developed flaps of skin in front of the nostrils. The circular, medium-sized eyes are equipped with nictitating membranes (protective third eyelids). Short, shallow furrows are present at the corners of the mouth. Fourteen to 16 and 13–17 tooth rows are found on either side of the upper and lower jaws, respectively (typically 15 for both).
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The eyes are small and equipped with nictitating membranes (protective third eyelids). The sizable, arched mouth has very short furrows at the corners. It has 26-29 upper and 27-29 lower tooth rows. The teeth are tall and upright; those in the upper jaw are wide and triangular with serrated edges, while those in the lower jaw are narrow and spear-like with serrations only near the tip, and tiny cusplets at the base in very young individuals.
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Comparison of an owl (left) and hawk (right) remex. The serrations on the leading edge of an owl's flight feathers reduce noise Owl eyes each have nictitating membranes that can move independently of each other, as seen on this spotted eagle-owl in Johannesburg, South Africa. Most owls are nocturnal, actively hunting their prey in darkness. Several types of owls, however, are crepuscular—active during the twilight hours of dawn and dusk; one example is the pygmy owl (Glaucidium).
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Sawtail catsharks reach maximum lengths of between . They have slender, firm bodies and narrow, slightly flattened heads with short to long, pointed snouts. The nostrils are divided into incurrent and excurrent openings by triangular flaps of skin on their anterior rims. The horizontally oval eyes are placed mostly on the sides of the head and equipped with rudimentary nictitating membranes (protective third eyelids); below each eye is a subtle ridge, and behind is a small spiracle (auxiliary respiratory opening).
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The teeth are numerous and small with multiple cusps; the upper teeth are exposed when the mouth is closed. The large, oval eyes are placed somewhat on the upper surface of the head, and have rudimentary nictitating membranes (protective third eyelids) and ridges underneath. There are two dorsal fins, the first much larger than the second. The first dorsal originates over the forward half of the pelvic fin bases, while the second is located over the anal fin.
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The pygmy ribbontail catshark has a thin body with a short, rounded snout and elongated, oval eyes bearing rudimentary nictitating membranes. Each nostril is preceded by a short, triangular flap of skin. The mouth is wide and V-shaped, without furrows at the corners and containing numerous rows of small, multi-pointed teeth that become more comb-like towards the sides. There are papillae on the roof of the mouth and the edges of the gill arches.
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Northern gannets have streamlined bodies adapted for plunge-diving at high speed, including powerful neck muscles, and a spongy bone plate at the base of the bill. The nostrils are inside the bill and can be closed to prevent water entry; the eyes are protected by strong nictitating membranes. There are subcutaneous air sacs in the lower body and along the sides. Other air sacs are located between the sternum and the pectoral muscles and between the ribs and the intercostal muscles.
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Woodpeckers tighten their nictitating membrane a millisecond prior to their beak impacting the trunk of a tree to prevent shaking-induced retinal injury.Wygnanski-Jaffe T, Murphy CJ, Smith C, Kubai M, Christopherson P, Ethier CR, Levin AV. (2007) Protective ocular mechanisms in woodpeckers Eye 21, 83–89. The membrane can be used to protect the eye while attacking prey, as in sharks. Nictitating membranes can protect eyes from ultraviolet radiation, as they do in polar bears to prevent snow blindness.
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The circular eyes, located at the forward outer corners of the cephalofoil, are equipped with protective nictitating membranes. The relatively small, arched mouth contains 15–16 upper and 14 lower tooth rows on each side, and sometimes also a single row of tiny teeth at the upper and/or lower symphyses (jaw midpoints). The teeth are small and smooth-edged, with angled triangular cusps. Five pairs of gill slits are seen, with the fifth pair over the pectoral fin origins.
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The North American river otter has a delicate sense of touch in the paws in addition to great dexterity. North American river otters characteristically approach within a few feet of a boat or a person on shore because they're near-sighted, a consequence of vision adapted for underwater sight. North American river otters have transparent nictitating membranes to protect their eyes while swimming. The right lung of the North American river otter is larger than the left, having four lobes compared with two for the left.
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The pigeye shark is a very robust-bodied species with a short, broad, and rounded snout. The small and circular eyes are equipped with nictitating membranes. The anterior rims of the nostrils bear medium-sized flaps of skin. The mouth forms a wide arch and has barely noticeable furrows at the corners. There are 11–13 (usually 12) upper and 10–12 (usually 11) lower tooth rows on each side; in addition, there are single rows of tiny teeth at the upper and lower symphyses (jaw midpoints).
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Large American crocodile skull Adult American crocodile American crocodile is a highly fecund species (38 clutch of eggs; fecundity over 20 after 15 years old) with a high adult survival rate and long life span. Like all true crocodilians, the American crocodile is a quadruped, with four short, stocky legs; a long, powerful tail; and a scaly hide with rows of ossified scutes running down its back and tail. Its snout is elongated and includes a strong pair of jaws. Its eyes have nictitating membranes for protection, along with lacrimal glands, which produce tears.
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The sizable, horizontally oval eyes are a reflective green in life and lack nictitating membranes (protective third eyelids); they are followed a short distance behind by much smaller spiracles (accessory respiratory openings). The nostrils are anteriorly placed and preceded by short flaps of skin. The mouth is wide and evenly arched, with thin lips and short but deep furrows around the corners. There are around 34 tooth rows in either side of both jaws; each tooth has three (occasionally up to five) slender cusps, with the central one the longest.
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The eyes, placed at the ends of the cephalofoils, are proportionately smaller than in other hammerheads and equipped with nictitating membranes (protective third eyelids). The nostrils are positioned just inside of the eyes, each with a well-developed groove running towards the center of the cephalofoil. The mouth is strongly curved, containing on either side 15-16 upper tooth rows and 15-17 lower tooth rows. The teeth have single narrow cusps with smooth or weakly serrated edges, that are angled in the upper jaw and upright in the lower jaw.
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The kingfishers have excellent vision; they are capable of binocular vision and are thought in particular to have good colour vision. They have restricted movement of their eyes within the eye sockets, instead using head movements to track prey. In addition, they are capable of compensating for the refraction of water and reflection when hunting prey underwater, and are able to judge depth under water accurately. They also have nictitating membranes that cover the eyes to protect them when they hit the water; the pied kingfisher has a bony plate which slides across the eye when it hits the water.
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The horizontally oval eyes are equipped with rudimentary nictitating membranes (protective third eyelids) and placed rather high on the head, with a thick ridge running under each. The mouth is wide and arched, with short furrows at the corners extending onto both jaws; the upper teeth are exposed when the mouth is closed. There are 18-30 and 13-26 tooth rows on either side of the upper and lower jaws respectively. The teeth have a narrow central cusp with a pair of small lateral cusplets; those of adult males are slightly more curved than those of females.
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Each nostril is split into tiny incurrent and excurrent openings by a flap of skin in front; the flap has a three-lobed shape with the central lobe forming a long, conical barbel. The barbels are thicker than in the leopard catshark, and do not reach the mouth. The eyes are horizontally oval and placed rather high on the head, with rudimentary nictitating membranes (protective third eyelids) and a thick ridge running underneath. The sizable mouth forms a broad arch, with short furrows extending from the corners onto both the upper and lower jaws; the upper teeth are exposed when the mouth is closed.
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The plica semilunaris of conjunctiva is a vestigial remnant of a nictitating membrane in humans. The nictitating membranes in cats and dogs do not have many muscle fibers, so they are not usually visible; chronic visibility should be taken as a sign of poor condition or ill health. It can, however, be seen clearly when gently opening the eye of the healthy animal when it is asleep, or pushing down/applying pressure on the eyeball, which will cause it to appear. In some breeds of dogs, the nictitating membrane can be prone to prolapse of the gland of the third eyelid, resulting in a condition called cherry eye.
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The air pressure from such a dive could possibly damage a bird's lungs, but small bony tubercles on a falcon's nostrils are theorized to guide the powerful airflow away from the nostrils, enabling the bird to breathe more easily while diving by reducing the change in air pressure. To protect their eyes, the falcons use their nictitating membranes (third eyelids) to spread tears and clear debris from their eyes while maintaining vision. A study testing the flight physics of an "ideal falcon" found a theoretical speed limit at for low-altitude flight and for high-altitude flight. In 2005, Ken Franklin recorded a falcon stooping at a top speed of .
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