The stapes is the third bone of the three ossicles in the middle ear. The stapes is a stirrup-shaped bone, and the smallest in the human body. It rests on the oval window, to which it is connected by an annular ligament. The stapes is described as having a base, resting on the oval window, as well as a head that articulates with the incus. These are connected by anterior and posterior limbs (). The stapes articulates with the incus through the incudostapedial joint. The stapes is the smallest bone in the human body, and measures roughly 3 x 2.5mm, greater along the head-base span.
Structure
Stapes
Stapes | Structure
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what are the 3 smallest bones in the human body
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The stapes is the third bone of the three ossicles in the middle ear. The stapes is a stirrup-shaped bone, and the smallest in the human body. It rests on the oval window, to which it is connected by an annular ligament. The stapes is described as having a base, resting on the oval window, as well as a head that articulates with the incus. These are connected by anterior and posterior limbs (). The stapes articulates with the incus through the incudostapedial joint. The stapes is the smallest bone in the human body, and measures roughly 3 x 2.5mm, greater along the head-base span.
Stapes | Structure
The stapes or stirrup is a bone in the middle ear of humans and other mammals which is involved in the conduction of sound vibrations to the inner ear. This stirrup-shaped bone is connected to the oval window by its annular ligament, which allows the footplate to transmit sound energy through the oval window into the inner ear. The stapes is the smallest and lightest bone in the human body, and is so-called because of its resemblance to a stirrup ().
Stapes | Introduction
The stapes is one of three ossicles in mammals. In non-mammalian four-legged animals, the bone homologous to the stapes is usually called the columella; however, in reptiles, either term may be used. In fish, the homologous bone is called the "hyomandibular", and is part of the gill arch supporting either the spiracle or the jaw, depending on the species. The equivalent term in amphibians is the "".
Stapes | Animals
The stapedius is the smallest skeletal muscle in the human body. At just over one millimeter in length, its purpose is to stabilize the smallest bone in the body, the stapes.
Stapedius muscle | Introduction
The largest bone in the body is the femur or thigh-bone, and the smallest is the stapes in the middle ear.
Bone | Introduction
The stapes is commonly described as having been discovered by the professor Giovanni Filippo Ingrassia in 1546 at the University of Naples, although this remains the nature of some controversy, as Ingrassia's description was published posthumously in his 1603 anatomical commentary '. Spanish anatomist Pedro Jimeno is first to have been credited with a published description, in ' (1549). The bone is so-named because of its resemblance to a stirrup (), an example of a late Latin word, probably created in mediaeval times from "to stand" (), as stirrups did not exist in the early Latin-speaking world.
Stapes | History
For example, the lower branch of the squamosal bone is shortened to offset the tall dorsal process of quadratojugal which connects to it. On the other hand, the rear branch jugal bone lengthens to fill some of the space left by the shortening of the anterior process of the quadratojugal. In archosauriforms, the jugal even re-encloses the lower temporal fenestra. The stapes are long, thin, and solid, without a perforating hole (stapedial foramen) present in the more robust stapes of other reptiles.
Archosauromorpha | Skull
The stapes develops from the second pharyngeal arch during the sixth to eighth week of embryological life. The central cavity of the stapes, the obturator foramen, is due to the presence embryologically of the stapedial artery, which usually regresses in humans during normal development.
Stapes | Development
The skeleton consists of both fused and individual bones supported and supplemented by ligaments, tendons, muscles and cartilage. It serves as a scaffold which supports organs, anchors muscles, and protects organs such as the brain, lungs, heart and spinal cord. Although the teeth do not consist of tissue commonly found in bones, the teeth are usually considered as members of the skeletal system. The biggest bone in the body is the femur in the upper leg, and the smallest is the stapes bone in the middle ear. In an adult, the skeleton comprises around 14% of the total body weight, and half of this weight is water.
Skeleton | Humans
A much more detailed description of the first two ossicles followed in Andreas Vesalius' "De humani corporis fabrica" in which he devoted a chapter to them. Vesalius was the first to compare the second element of the ossicles to an anvil although he offered the molar as an alternative comparison for its shape. The first published description of the stapes came in Pedro Jimeno's "Dialogus de re medica" (1549) although it had been previously described in public lectures by Giovanni Filippo Ingrassia at the University of Naples as early as 1546. The term "ossicle" derives from , a diminutive of "bone" (; genitive ).
Ossicles | History
The morphology of the stapes suggests that the animal was capable of hearing high-frequency sound. "B. woodi" does not possess lateral line sulci or an ossified branchial system. The principal method of respiration was probably buccal (gulping air through mouth) rather than costal (expanding chest volume to take in air), indicated by the small straight ribs.
Balanerpeton | Introduction
Surgery may be complicated by a persistent stapedial artery, fibrosis-related damage to the base of the bone, or obliterative otosclerosis, resulting in obliteration of the base.
Stapes | Clinical relevance
These are the most unusual of the braincase bones, and were previously misidentified as stapes. While they are not stapes, it is difficult to be certain what they are. As mentioned, they are sutured to the tubera basioccipitalia, and so could not have moved freely, meaning that they were not bones for the transmission of sound in this manner. Medially, they are sutured to the posterodorsal margins of the tubera basioccipitalia by a strongly serrate and partially intergrown suture. The posterior surface of this forms a steeply inclined trough running from the suture to the distal extremity of the paraoccipital process.
Phantomosaurus | Opisthotic bones
The configuration of the stapes is intermediate between non-amniote tetrapods and amniotes. On the one hand, its connection to the otic notch is unusual, since true reptiles and other amniotes have lost an otic notch, forcing the tympanum and stapes to shift downwards towards the quadrate bone of the jaw joint. On the other hand, the thin, sensitive structure of "Seymouria"'s stapes is a specialization over most non-amniote tetrapods, which have a thick stapes useless for hearing yet useful for reinforcing the skull. The inner ear of "Seymouria baylorensis" retains a cochlear recess located behind (rather than below) the vestibule, and its anterior semicircular canal was likely encompassed by a cartilaginous (rather than bony) supraoccipital.
Seymouria | Skull
The bony palate is short, with its posterior end often located between the third molars. The alisphenoid strut, an extension of the alisphenoid bone of the skull that separates two foramina (openings) in the skull, is present in some individuals. The mastoid bone usually contains some openings. In the holotype, the head and body length is 130 mm, the tail length is 160 mm, the hindfoot length (without claws) is 31 mm, the ear length is 27 mm, and the skull length is 35.7 mm.
Nephelomys levipes | Introduction
The three small bones in the middle ear of mammals including humans, the malleus, incus, and stapes, are today used to transmit sound from the eardrum to the inner ear. The malleus and incus develop in the embryo from structures that form jaw bones (the quadrate and the articular) in lizards, and in fossils of lizard-like ancestors of mammals. Both lines of evidence show that these bones are homologous, sharing a common ancestor. Among the many homologies in mammal reproductive systems, ovaries and testicles are homologous. Rudimentary organs such as the human tailbone, now much reduced from their functional state, are readily understood as signs of evolution, the explanation being that they were cut down by natural selection from functioning organs when their functions were no longer needed, but make no sense at all if species are considered to be fixed.
Homology (biology) | In mammals
Situated between the incus and the inner ear, the stapes transmits sound vibrations from the incus to the oval window, a membrane-covered opening to the inner ear. The stapes is also stabilized by the stapedius muscle, which is innervated by the facial nerve.
Stapes | Function
The ossicles (also called auditory ossicles) are three bones in either middle ear that are among the smallest bones in the human body. They serve to transmit sounds from the air to the fluid-filled labyrinth (cochlea). The absence of the auditory ossicles would constitute a moderate-to-severe hearing loss. The term "ossicle" literally means "tiny bone". Though the term may refer to any small bone throughout the body, it typically refers to the malleus, incus, and stapes (hammer, anvil, and stirrup) of the middle ear.
Ossicles | Introduction
The stapes, the bone transmitting sound waves from the eardrum to the middle ear, is elongated and not pierced by a foramen. Pterygoid teeth are typically lacking.
Ichthyosaur | Skull
The holotype of "Archaeornithoides" was a very small individual. The head fragment as preserved measures just twenty-seven millimetres in length, indicating an original skull length of about five centimetres. The body length was estimated at fifty to sixty centimetres, making the "Archaiornithoides" type one of the smallest known non-avian dinosaurs. Adult length is uncertain. The snout of "Archaeornithoides" features a long antorbital fenestra, stretching over three quarters of the length of the maxilla. The maxilla bears at least eight teeth. These are small, conical and smooth, lacking wrinkles, serrations or carinae. The palatine bone seems to show the presence of a secondary fenestra.
Archaeornithoides | Description
"C. microcephala" is known from a single fossil, a winged gyne preserved in profile view. The body has an approximate length of with an essentially square head. The head is 1.5 times shorter than the mesosoma and has a sculpturing of small pits. The clypeus has a trapazohedral shape to the lower edge, and the eyes are placed at the middle point of the head. The antennae have segments that are 2 times as long as they are wide, with a scape which reaches to the back edge of the head. The mandibles are small, being under half the head length and each chewing side is toothed.
Cyrtopone | "C. microcephala"
Occasionally the joints between the ossicles become rigid. One condition, otosclerosis, results in the fusing of the stapes to the oval window. This reduces hearing and may be treated surgically.
Ossicles | Clinical relevance
The stapedius emerges from a pinpoint foramen in the apex of the pyramidal eminence (a hollow, cone-shaped prominence in the posterior wall of the tympanic cavity), and inserts into the neck of the stapes.
Stapedius muscle | Structure
The middle ear contains three tiny bones known as the ossicles: "malleus", "incus", and "stapes". The ossicles were given their Latin names for their distinctive shapes; they are also referred to as the "hammer", "anvil", and "stirrup", respectively. The ossicles directly couple sound energy from the eardrum to the oval window of the cochlea. While the stapes is present in all tetrapods, the malleus and incus evolved from lower and upper jaw bones present in reptiles. The ossicles are classically supposed to mechanically convert the vibrations of the eardrum into amplified pressure waves in the fluid of the cochlea (or inner ear), with a lever arm factor of 1.3.
Middle ear | Ossicles
Fused bones include those of the pelvis and the cranium. Not all bones are interconnected directly: There are three bones in each middle ear called the ossicles that articulate only with each other. The hyoid bone, which is located in the neck and serves as the point of attachment for the tongue, does not articulate with any other bones in the body, being supported by muscles and ligaments. There are 206 bones in the adult human skeleton, although this number depends on whether the pelvic bones (the hip bones on each side) are counted as one or three bones on each side (ilium, ischium, and pubis), whether the coccyx or tail bone is counted as one or four separate bones, and does not count the variable wormian bones between skull sutures.
Skeleton | Humans
The movement of the ossicles may be stiffened by two muscles. The stapedius muscle, the smallest skeletal muscle in the body, connects to the stapes and is controlled by the facial nerve; the tensor tympani muscle is attached to the upper end of the medial surface of the handle of malleus and is under the control of the medial pterygoid nerve which is a branch of the mandibular nerve of the trigeminal nerve. These muscles contract in response to loud sounds, thereby reducing the transmission of sound to the inner ear. This is called the acoustic reflex.
Middle ear | Muscles
The tympanic bullae are small and less than in length. The width of the basioccipital is less than . The width of the infraorbital canals is very narrow being less than . The width across the nasals is very narrow and their length is less than . They have medium-sized auditory bullae with a length of less than . The shield bullae have a shallow depth of less than . The skull also has a narrow basioccipital and is very broad across the carotid foramina. The mouth consists of a mandible whose height is less than with a mandible ramus depth of less than .
Omilteme cottontail | Anatomy
The bone flap represents the first and second epiparietal pairs, the "P1s" and "P2s", as is evidenced by the osteoderm points still being visible, those of the first pair pointing to the front near the midline, those of the second pair forming the corners of the flap. The third epiparietals, pointing to behind, together form a pincer-like construction around the notch. At each rear corner of the frill a very large osteoderm is present. As it spans the suture between the squamosal and the parietal, this has been called an epiparietosquamosal or "EPS".
Spiclypeus | Skeleton
The part of the skeleton that is ‘tucked under’ (the doublure) has no sutures crossing it to form a rostral plate.The thorax has 11 (rarely 10) segments, the side lobes (or pleurae) are furrowed, and the articulating facets distinct.
Phacopina | Description
In such animals, the scapula is usually a relatively simple plate, lacking the projections and spine that it possesses in mammals. However, the detailed structure of these bones varies considerably in living groups. For example, in frogs, the procoracoid bones may be braced together at the animal's underside to absorb the shock of landing, while in turtles, the combined structure forms a Y-shape in order to allow the scapula to retain a connection to the clavicle (which is part of the shell). In birds, the procoracoids help to brace the wing against the top of the sternum. In the fossil therapsids, a third bone, the true coracoid, formed just behind the procoracoid.
Scapula | In other animals
The clypeus is setose, meaning that it bears bristles or setae, except on the middle part. The mandibles are small and only contain two teeth. The antennae are moderately long, the scapes are very short but the pedicel is even shorter. The mesosoma measures and is setose. Several setae are seen around the metanotum (the dorsal sclerite) and propodeum. The coxae, the proximal segment and functional base of the leg, are setose (bearing bristle or setae), inflated and large. The metapleural gland opening is small and noticeable, located away from the propodeum and above the metacoxa (the hind of the coxa).
Sphecomyrma | "S. mesaki"
Otosclerosis is a congenital or spontaneous-onset disease characterized by abnormal bone remodeling in the inner ear. Often this causes the stapes to adhere to the oval window, which impedes its ability to conduct sound, and is a cause of conductive hearing loss. Clinical otosclerosis is found in about 1% of people, although it is more common in forms that do not cause noticeable hearing loss. Otosclerosis is more likely in young age groups, and females. Two common treatments are stapedectomy, the surgical removal of the stapes and replacement with an artificial prosthesis, and stapedotomy, the creation of a small hole in the base of the stapes followed by the insertion of an artificial prosthesis into that hole.
Stapes | Clinical relevance
The hindmost part of the surangular had a small foramen placed in the same position as similar openings in the mandibles of non-bird theropods and modern birds. The splenial bone was three-pronged (as in some modern birds, but unlike the simple splenial of "Archaeopteryx"), and its lower margin followed the lower margin of the mandible. There was a large rostral mandibular fenestra and a small, rounded caudal fenestra behind it. Though only five specimens preserve parts of the beak's keratinous covering, these show that there would have been differences between species not seen in the skeleton. The holotype of "C.
Confuciusornis | Skull
hoffmannii" while being a species being only half the length of the latter. Spaces within the braincase providing for the occipital lobe and cerebral hemisphere are narrow and shallow, suggesting that such brain parts were relatively small. The parietal foramen in "Mosasaurus", which is associated with the parietal eye, is the smallest in the Mosasauridae family. The trachea likely stretched from the esophagus to below the back end of the mandible's coronoid process, where it split into smaller pairs of bronchi that extended parallel to each other.
Mosasaurus | Skull
The scapes are short, measuring , and the pedicel (the second segment of the antenna) is The flagellomeres vary in length, ranging between . The clypeus is shallow and measures (when excluding the denticles). It is broad, and the distance between its lateral-most denticles is . It is also more concave-shaped than it is straight. The oral margin of the clypeus is lined with a row of 30 denticles, which are peg-shaped. They also have rounded apices. The row of clypeal margin denticles is composed of two short rows. Each row is composed of 15 denticles each. The mandibles are short and barely overlap medially.
Zigrasimecia | "Z. tonsora"
A thin alisphenoid strut (a piece of bone on the lower side of the skull separating two foramina) is present in specimens from Marojejy, but not in those from Anjanaharibe-Sud. The tegmen tympani, the roof of the tympanic cavity, is reduced. The root of the lower incisor is visible at the back of the mandible (lower jaw) as a slight protrusion; a true capsular process is absent. There are 13 thoracic (chest), 7 lumbar, 4 sacral, and 38 or 39 caudal (tail) vertebrae. The humerus (upper arm bone) lacks an entepicondylar foramen.
Northern voalavo | Skeleton
Writing in the journal La Nature, de Lapouge describes the bones in detail: "I think it unnecessary to note that these bones are undeniably human, despite their enormous size... The first is the middle part of the shaft of a femur, 14 cm length, almost cylindrical in shape, and the circumference of the bone is 16 cm... The second piece is the middle and upper part of the shaft of a tibia... The circumference is 13 cm at the nutrient foramen... the length of fragment is 26 cm... The third, very singular, was regarded by good anatomists as the lower part of a humerus...
Giant of Castelnau | Discovery
Similarly, the sacrum is usually counted as a single bone, rather than five fused vertebrae. There is also a variable number of small sesamoid bones, commonly found in tendons. The patella or kneecap on each side is an example of a larger sesamoid bone. The patellae are counted in the total, as they are constant. The number of bones varies between individuals and with age – newborn babies have over 270 bones some of which fuse together. These bones are organized into a longitudinal axis, the axial skeleton, to which the appendicular skeleton is attached.
Skeleton | Humans
Approximately in the middle of the parietal bones was a small hole known as a pineal foramen, which may have held a sensory organ known as a parietal eye. The pineal foramen is smaller in "Seymouria" than in other seymouriamorphs. The stapes, a rod-like bone which lies between the braincase and the wall of the skull, was tapered. It connected the braincase to the upper edge of the otic notch, and likely served as a conduit of vibrations received by a tympanum (eardrum) which presumably lay within the otic notch. In this way it could transmit sound from the outside world to the brain.
Seymouria | Skull
Air vibrations could not set up pulsations through the skull as in a proper auditory organ. The spiracle was retained as the otic notch, eventually closed in by the tympanum, a thin, tight membrane. The hyomandibula of fish migrated upwards from its jaw supporting position, and was reduced in size to form the stapes. Situated between the tympanum and braincase in an air-filled cavity, the stapes was now capable of transmitting vibrations from the exterior of the head to the interior. Thus the stapes became an important element in an impedance matching system, coupling airborne sound waves to the receptor system of the inner ear.
Tetrapod | Hearing
The ossicles are suspended in the head and loosely coupled to the skull. When the head moves, the ossicles move out of phase with the head, but still follow the same cyclic motion. This causes the stapes to move in and out of the oval window. When vibrations come from the mastoid, inertial bone-conduction is greatest below 800 Hz. Putting the bone vibrator on the forehead instead of the mastoid does not significantly create this affect.
Bone conduction auditory brainstem response | Inertial bone-conduction
The first three pairs in the praemaxilla are very small though; the describers assumed these were replacement teeth, recently erupted. The back of the skull and the cervical vertebrae are strongly crushed, obscuring most details. Eight dorsal vertebrae are preserved, with a total length of . The sternum was fan-shaped with a prominent keel. The wings are robustly built; the ulna is with longer than the wing metacarpal which has a length of . The wingspan of the type individual was estimated at 1.35 meter (4.43 ft). In comparison, the hindlimbs must have been weakly built, the metatarsal having a length of just .
Haopterus | Description
The resulting three-boned structure is still seen in modern monotremes, but in all other living mammals, the procoracoid has disappeared, and the coracoid bone has fused with the scapula, to become the coracoid process. These changes are associated with the upright gait of mammals, compared with the more sprawling limb arrangement of reptiles and amphibians; the muscles formerly attached to the procoracoid are no longer required. The altered musculature is also responsible for the alteration in the shape of the rest of the scapula; the forward margin of the original bone became the spine and acromion, from which the main shelf of the shoulder blade arises as a new structure.
Scapula | In other animals
It is occupied by the base of the stapes, the circumference of which is fixed by the annular ligament to the margin of the foramen.
Oval window | Introduction
"Pneumatoraptor" differs from other theropods in having a narrow shoulder blade that is nearly circular in cross section, as well as having a large opening in the bone to house a hollow air sac cavity. The bone is small, indicating that the animal was only about half the size of the related "Sinornithosaurus" or 0.73 m (2.2 ft). The shoulder girdle is L-shaped, showing it to be a member of the group Paraves, which also includes the dromaeosaurids, troodontids, and birds. While the "Pneumatoraptor" remains are too incomplete to tell which, if any, of these specific groups it belongs to, many of the bones features are similar to dromaeosaurids.
Pneumatoraptor | Description
In mammals, the articular and quadrate bones have migrated to the middle ear and are known as the malleus and incus. Along with the stapes, these are known as the ossicles and are a defining characteristic of mammals.
Quadrate bone | Function in mammals
A short bone is one that is cube-like in shape, being approximately equal in length, width, and thickness. The only short bones in the human skeleton are in the carpals of the wrists and the tarsals of the ankles. Short bones provide stability and support as well as some limited motion.
Anatomical terms of bone | Short bones
The trunk is elongate, with thin ribs extending from each vertebra. The presence of a large scapulocoracoid in "Hyloplesion" distinguishes it from "Microbrachis". The limb bones are small and robust, with the hindlimbs being quite larger than the forelimbs . Like other microbrachomorphs such as "Microbrachis", "Hyloplesion" has only three digits in the manus, a condition known as tridactyly.
Hyloplesion | Description
A trabecula (plural trabeculae, from Latin for "small beam") is a small, often microscopic, tissue element in the form of a small beam, strut or rod that supports or anchors a framework of parts within a body or organ. A trabecula generally has a mechanical function, and is usually composed of dense collagenous tissue (such as the trabecula of the spleen). They can be composed of other materials such as muscle and bone. In the heart, muscles form trabeculae carneae and septomarginal trabecula. Cancellous bone is formed from groupings of trabeculated bone tissue. In cross sections, trabeculae of a cancellous bone can look like septa, but in three dimensions they are topologically distinct, with trabeculae being roughly rod or pillar-shaped and septa being sheet-like.
Trabecula | Introduction
The stapes bone transmits movement to the oval window. As the stapes footplate moves into the oval window, the round window membrane moves out, and this allows movement of the fluid within the cochlea, leading to movement of the cochlear inner hair cells and thus hearing. If the round window were to be absent or rigidly fixed (as can happen in some congenital abnormalities), the stapes footplate would be pushing incompressible fluid against the unyielding walls of the cochlea. It would therefore not move to any useful degree leading to a hearing loss of about 60dB. This is, unsurprisingly, the same as for conditions where the stapes itself is fixed, such as otosclerosis.
Round window | Function
Sexual dimorphism is the difference of size in male and female skeletons. The two bone structures that are used to determine whether a person is male or female are the skull and pelvis. The bones can also be used to reveal the age of the person before they died. At birth the human cranial vault consists of seven separate bones; by age 5 these bones have knitted together at the cranial sutures. At about the age of 30 , the sutures generally begin to fill in with bone and smooth out. Another way to tell the age of bones is by bone osteons under a microscope or look for arthritis indicators on the bones.
Osteology | Introduction
This system had evolved independently within several different amphibian lineages. The impedance matching ear had to meet certain conditions to work. The stapes had to be perpendicular to the tympanum, small and light enough to reduce its inertia, and suspended in an air-filled cavity. In modern species that are sensitive to over 1 kHz frequencies, the footplate of the stapes is 1/20th the area of the tympanum. However, in early amphibians the stapes was too large, making the footplate area oversized, preventing the hearing of high frequencies. So it appears they could only hear high intensity, low frequency sounds—and the stapes more probably just supported the brain case against the cheek.
Tetrapod | Hearing
In some of the eyeless genera ("Ductina") the suture coincides with the marginal suture, and thus appears to be absent. The part of the skeleton that is "tucked under" (the doublure) has no sutures crossing it to form a rostral plate. The thorax has 11 (rarely 10) segments, the side lobes (or pleurae) are furrowed, and the articulating facets distinct. The tailshield (or pygidium) is always smaller than the headshield (or cephalon), a situation called micropygous. In the Phacopidae a merger of the anterior and the two pairs of neighbouring lobes of the glabella forms a frontal lobe that expands forward and can be inflated and overhanging the frontal border.
Phacopidae | Description
From reconstructions of the skeleton, "Coelurus" had a relatively long neck and torso due to its long vertebrae, a long slender hindlimb due to its long metatarsus, and potentially a small slender skull. The skull is unknown except for possibly a portion of lower jaw found at the same site as the rest of the known "Coelurus" material. Although it has the same preservation and coloring as the fossils known to belong to the "Coelurus" skeleton, it is very slender, which may mean it does not belong to the skeleton; this bone is 7.9 centimeters long (3.1 in) but only 1.1 centimeters tall (0.43 in).
Coelurus | Description
The holotype specimen consists of the remains of a small individual. Because most of the vertebral column is absent, its body length cannot be directly determined, but a comparison can be made with the previously smallest known ornithomimosaur, the 1.6 metres long "Shenzhousaurus", which has a thighbone length of 191 millimetres, while the femur length of "Hexing" is 135 millimetres. Such a small body size was among ornithomimosaurs up till now only known from juveniles but the holotype is not a young animal as is shown by the complete fusion of the skull bones, the neck ribs, the scapulocoracoid and the ankle bones.
Hexing | Description
The length of the holotype individual was in 2019 estimated at long. Such a body size is rather limited for a troodontid. The specimen may represent an adult or subadult individual, due to the lack of cranial and body proportions associated with juvenile individuals. Nevertheless, either assignation would not be much different; for instance, the estimated would become . The describing authors established some derived or apomorphic traits relative to the Paraves. The jugal bone is pneumatised. The rear branch of the lacrimal bone is short, with less than 15% of the length of the descending branch, measured from the inner corner downwards.
Hesperornithoides | Description
The cephalopharyngeal skeleton is 0.57–0.61 mm long. Mandibular sclerites (hardened portions of the exoskeletons) exist as long, narrow, mouth-hooks that appear triangular from a side-view. Dentary sclerites located next to the mandible are pointed down, and are lightly pigmented towards the back of the fly. Small sclerites are present between the mandibular ones, and an additional sclerite is found on the posterior end near the hypostome (a structure found near the mouth which allows an animal to anchor itself firmly on another to suck, as in the tick's hypostome). The third instar stage is 4.10–9.42 mm long and 0.76–2.13 mm wide.
Dryomyza anilis | Larva
The filaments are preserved with a gap between the bones, which several authors have noted corresponds closely to the expected amount of skin and muscle tissue that would have been present in life. The feathers are closest to the bone on the skull and end of the tail, where little to no muscle was present, and the gap increases over the back vertebrae, where more musculature would be expected, indicating that the filaments were external to the skin and do not correspond with subcutaneous structures. The filaments exhibit random orientations and are often wavy, which has been interpreted as evidence that they were soft and pliable in life.
Sinosauropteryx | Feathers
In advanced cynodonts, including the mammaliaforms, have lost the quadratojugal, with the diminutive quadrate connecting to the stapes to function as a hearing structure. In modern mammals, the quadrate bone evolves to become the incus, one of the ossicles of the middle ear. This is an apomorphy of the mammalian clade, and is used to identify the fossil transition to mammals.
Quadratojugal bone | In mammals
The hindwing of an average adult is generally 30 mm or larger.
Tonyosynthemis claviculata | Body
A sacral length of around 6.5 is deduced. Dames observed 20 caudal vertebrae; Wellnhofer posits 21. Dames also noted the similarity in the long, rod-like structures of the tail to those in the flying pterosaur "Rhamphorhynchus", considering them to be ossified tendons. Thin, pointed cervical ribs begin at the third vertebra. The longest are about 50 mm and total eleven in number. Gastralia, or belly ribs, are arranged in 10 parallel rows and their position is suggested as evidence of post-mortem shrinking of the body. The scapulae are separated by about 17 mm and are flattened, narrow, and positioned much in their natural position.
Specimens of Archaeopteryx | Trunk
The reason for this evolutionary change is still unknown; however, it may be to assist in grasping small objects and thin branches.
Sesamoid bone | Other animals
The stapes appears to be relatively constant in size in different ethnic groups. In 0.01-0.02% of people, the stapedial artery does not regress, and persists in the central foramen. In this case, a pulsatile sound may be heard in the affected ear, or there may be no symptoms at all. Rarely, the stapes may be completely absent.
Stapes | Variation
This species is likely now extinct.
Theliderma stapes | Conservation
There are five types of bones in the human body: long, short, flat, irregular, and sesamoid.
Bone | Types
The holotype specimen of "Bellubrunnus" represents a small individual, with a wingspan of less than a foot, and is one of the smallest pterosaur fossils known. Nearly every bone is preserved in BSP–1993–XVIII–2, although no soft tissues have been found, neither as impressions nor as organic remains. The entire skeleton is complete except for missing parts of the right foot and tail tip. Because the skeleton is preserved in ventral view, many details of the skull ("sk" in the skeletal diagram) are obscured by the lower jaws (dentary, "dt" + angular, "ar" in the skull diagram). Many skull bones are also crushed and distorted, and some such as the maxillae ("mx"), nasal bones ("ns"), and sclerotic rings ("scl") are displaced.
Bellubrunnus | Description
However, Hans Thewissen, discoverer of "Ambulocetus", has questioned these conclusions, suggesting that the smaller skeleton could be a partially digested meal. Even if the small skeleton is a fetus, Thewissen writes that it may not have been preserved in its normal in-vivo position. Authors pointed out in the original article, however, that the fetal skull has no tooth marks. This species is medium-sized with a skeleton in length and an estimated weight of . Males are slightly larger than females; of the two adult skeletons found, the one interpreted as male was about 12% larger than the one interpreted as female.
Maiacetus | Paleobiology
There are two hierarchical structures on the microscale. The first, at a scale of 100 μm to 1 mm, is inside the compact bone where cylindrical units called osteons and small struts can be distinguished. The second hierarchical structure, the ultrastructure, at a scale of 5 to 10 μm, is the actual structure of the osteons and small struts.
Mineralized tissues | The microscale
The bone has a length of seven centimetres. This manual element shows a prominent dorsomedial process, a feature shared with the much smaller "Ornitholestes" which occasioned the specific name. The process with "Ornitholestes" is much less distinctive though. On its upper end there is only one cotyle, from which von Huene deduced it must have been a metacarpal. However, several coelurosaurian groups lack a second cotyle on the first phalanx also. If "Rapator" had a build like "Australovenator", it would have attained a considerable size: a body length of nine metres (30 ft) has been estimated.
Rapator | Description
STMN 26-3 consists of an incomplete skeleton preserving a partial skull and mandible, partial presacral vertebrae, proximal–middle caudal vertebrae, nearly complete right scapula, both humeri, the proximal end of the left ulna, partial pubes, both ischia, both femora, the right tibia and fibula and pes, as well as remains of long, singular and unbranched filamentous integumentary structures. The holotype is from a subadult individual that probably measured 70 cm in length based on the proportions of the related South African species "Heterodontosaurus tucki". However, "Tianyulong" had unusual proportions compared to other heterodontosaurids. The head was large and the legs and tail were long, but the neck and forelimbs were short.
Tianyulong | Description
The arms are around half the length of the legs and over half the length of the presacral vertebral column. The hands are long, and tridactyl, with a reduced third finger in "Caudipteryx" and "Ajancingenia". There are between 5 and 8 sacral vertebrae. The pubis is vertical or subvertical, with a concave anterior edge. The tibia is 15%-25% longer than the femur. The tail is short, with the number of vertebrae reduced to 24 or so, and proximally very thick, with broad transverse processes. The ischium retains the primitive character of a prominent, triangular obturator process and lack the proximodorsal process that is found in birds.
Oviraptorosauria | Description
An enormous chandelier of bones, which contains at least one of every bone in the human body, hangs from the center of the nave with garlands of skulls draping the vault. Other works include piers and monstrances flanking the altar, a coat of arms of the House of Schwarzenberg, and the signature of František Rint, also executed in bone, on the wall near the entrance.
Sedlec Ossuary | Introduction
The forelimb bones are all elongated, with the degree of elongation increasing the farther the bones are from the body. The ulna is vestigial and fused to the radius. The forearm commonly between 31–33 mm in length. The thumb is not overly enlarged but is capable of free movement. The wrist is highly flexible. The fusing of the radius and ulna allow for strength to hold up the wing. The bones of the wing are controlled by a single muscle for each action. At the elbow, there is a small bone called the ulna sesamoid, it is similar to the patella.
Little pied bat | Skeletal structure
Within mammals and other synapsids the columella has evolved into the stapes, a homologous bone within the newly evolved inner ear. As the tympanic cavity evolved to reduce in size, the columella shortened in length. The stirrup-shaped articular processes of the columella inspired a new name for this auditory ossicle, the stapes. The auditory ossicles continue to function in conducting transmitting sound through the auditory pathway; however, they have lost their function in conducting low frequency ground vibrations. Later-arising reptiles with columella likely evolved stronger limbs and a more crawling posture, which removed the body from the ground and prevented the transmission of ground-conducted sounds.
Columella (auditory system) | Homology in mammals
An alisphenoid strut separates two foramina (openings) at the base of the skull, the buccinator-masticatory foramen and the foramen ovale accessorium. The pattern of grooves and foramina on the head indicates that the circulation of the arteries in the head of "T. ucucha" follows the primitive pattern. The tegmen tympani, the roof of the tympanic cavity, overlaps the suspensory process of the squamosal bone. At the back of the mandible (lower jaw), there is a capsular process to receive the root of the lower incisor, which is absent in "T. hylophilus".
Thomasomys ucucha | Description
Caudal to what will eventually become the stapes, Reichert's cartilage also forms the styloid process of the temporal bone. The cartilage between the hyoid bone and styloid process will not remain as development continues, but its perichondrium will eventually form the stylohyoid ligament.
Pharyngeal arch | Reichert's cartilage
A modified stapes operation, called a stapedotomy, is thought by many otologic surgeons to be safer and reduce the chances of postoperative complications. In stapedotomy, instead of removing the whole stapes footplate, a tiny hole is made in the footplate - either with a microdrill or with a laser, and a prosthesis is placed to touch this area, oval window. This procedure can be further improved by the use of a tissue graft seal of the fenestra, which is now common practice. Laser stapedotomy is a well-established surgical technique for treating conductive hearing loss due to otosclerosis. The procedure creates a tiny opening in the stapes (the smallest bone in the human body) in which to secure a prosthetic.
Stapedectomy | Stapedotomy
They were discovered on a layer of fungus on which they feed. From these specimens exact measurements could be made, and was found that the smallest individual is only 0.325 mm long. The largest individual is 0.352 mm long, and the average length of all the specimens is 0.338 mm. The body is elongated and oval in shape, yellowish-brown in colour, and its antennae are split into 10 segments.
Scydosella | Introduction
Two spots between the eyes indicate the presence of the ocelli (light-sensitive simple eyes). In its abdomen, there were eleven segments, being the third the widest. The ends of the segments were rounded on the sides. In the posterior part of the segments, a row of tiny crenulations was visible. The first six segments were uniform in size, the three following ones were somewhat shorter and the last two were the longest. The telson was a broad, flat spine, and it rose slightly in the middle. The appendages are unknown, although it has been suggested that they be less than seven pairs (what has been considered an overestimate).
Strabops | Description
The gyne is notably larger than the worker, at approximately long, and has a head that is wider than long when the mandibles are excluded. Also unlike the worker, the eyes are placed at the midpoint of the head, though they are small as the workers are. There are a number of denticles along the chewing margin on the large triangle shaped mandibles. While the scape extends beyond the back of the head, its total length is less than that of the head.
Paraneuretus | "P. dubovikoffi"
Such structures include a shelf at the rear of the surangular and a thick articular bone which has a tall retroarticular process with an inner prong pierced by a "chorda tympani" foramen.
Prestosuchus | Skull
The cage is circular and is approximately 4.5 meters in diameter and 5 meters in height. Two pieces of bone, that have been emptied of marrow, are stuck in the side of it. Several tapestries are hung on it.
Spider (Bourgeois) | The cage
The trapezoid bone (lesser multangular bone) is a carpal bone in tetrapods, including humans. It is the smallest bone in the distal row of carpal bones that give structure to the palm of the hand. It may be known by its wedge-shaped form, the broad end of the wedge constituting the dorsal, the narrow end the palmar surface; and by its having four articular facets touching each other, and separated by sharp edges. It is homologous with the "second distal carpal" of reptiles and amphibians.
Trapezoid bone | Introduction
In the tetrapod and higher clades from the lower-middle Famennian there are several defining changes on the basis of anatomy of "Ichthyostega", "Tulerpeton", and "Acanthostega". In the cranium, there is a stapes derived from the hyomandibular of fishes; a single bilateral pair of nasal bones, and a fenestra ovalis in the otic capsule of the braincase. The opening of the otic wall of the braincase can be considered a paedomorphic feature for tetrapods and is linked to the stapes functionally. The stapes was thought to be just a structural support between the palate and the stapedial plate of the braincase.
Skeletal changes of organisms transitioning from water to land | Head changes in aquatic tetrapods
A recent suggestion, based upon observations that men and women have the same number of ribs, speculates that the bone was the baculum, a small structure found in the penis of many mammals, but not in humans.
Eve | Creation
The width of a single human hair ranges from approximately 20 to 200 μm. The longest human chromosome is approximately 10 μm in length. Around 100 μm (0.1 millimeters) is the smallest thing visible to the naked eye.
Micrometre | Introduction
"Styloichthys" is characterized by large pores on the cosmine surface. These pores are often spoon-shaped and arranged in parallel grooves. There is a jagged margin between ethmosphenoid and otoccipital shields, an otoccipital with a wide flat surface carrying no vestibular fontanelle. The snout bends sharply downward and is slightly concave. The lower jaw has a ventrally protruding flange formed by prearticular and meckelian bone. Also characteristic of "Styloichthys" is the lyre-shaped trajectory of the supraorbital canal, a fenestra ventralis, small internasal cavities, three coronoids in the lower jaw, an eyestalk and a slender postorbital pila which connects the basipterygoid process and the side of the braincase wall.
Styloichthys | Description
In front of the clypeus, the bases of the long straight mandibles are placed close together. On the masticatory margin there are no denticles, with possibly three teeth present at the tip end of each mandible, though only two are visible. The thorax is slightly elongated, with a mesoscutum that is a little wider than long and attaching with no overhang of the sclerites to the propodeum. The petiole is rounded and narrow, without a spine on the upper surface.
Odontomachus paleomyagra | Description
There are 22 bones in the skull. Including the bones of the middle ear, the head contains 28 bones.
List of bones of the human skeleton | Skull
The body is elongated. The posterior margin of the third tergite of the abdomen shows three small wide teeth.
Trichrysis | Description
The part of the skeleton that is "tucked under" (the doublure) has no sutures crossing it to form a rostral plate. The tailshield (or pygidium) is always smaller than the headshield (or cephalon), a situation called micropygous. In the Phacopidae a merger of the anterior and the two pairs of neighbouring lobes of the glabella forms a frontal lobe that expands forward and can be inflated and overhanging the frontal border. To the back of the glabella two furrows (or sometimes one) cross the glabella forming two rings ("intercalating/pre-occipital ring" and "occipital ring"). The cephalon does not end in genal spines.
Viaphacops | Description
albisetosus" measure . The body is moderately stout and there is a slight constriction at the first and second abdominal somites (body segments containing the same internal structures). Spiracles are small and no spinules are on the integument. Body hairs are short and sparse; hairs on the head are sparse and also short. The antennae are small with three sensilla (sensory receptors), and the labrum (a flap-like structure that lies immediately in front of the mouth) is short. The mandibles are sclerotized, and the apex forms a long slender tooth that is medially curved. The maxillae are small with a spinulose apex.
Novomessor albisetosus | Description
A fully grown adult features 26 bones in the spine, whereas a child can have 34.
List of bones of the human skeleton | Spine (vertebral column)
The styloglossus, the shortest and smallest of the three styloid muscles, arises from the anterior and lateral surfaces of the styloid process near its apex, and from the stylomandibular ligament. Passing inferiorly and anteriorly between the internal and external carotid arteries, it divides upon the side of the tongue near its dorsal surface, blending with the fibers of the longitudinalis inferior in front of the hyoglossus; the other, oblique, overlaps the Hyoglossus and decussates with its fibers.
Styloglossus | Introduction
Males have a body about 13 mm long, females about 19 mm long.
Zorodictyna oswaldi | Introduction
The only preserved bone of the pectoral girdle is the left scapula (shoulder blade). This bone has a hatchet-like shape, expanded near the shoulder as in other therizinosauroids which preserve a scapula and measures about long. The pelvis (hip) preserves fragments of the pubis and part of the ischium, which is very flattened (from the side) near the hip socket. Three vertebral fragments are preserved. These include a strip of bone from the upper portion of a cervical (neck) vertebra, the centrum (main body) of a dorsal (back) vertebra, and the lower portion of a vertebra believed to be a distal caudal (i.e.
Martharaptor | Other bones
The femur was lightly-built and twisted, similar to mycterosaurines. The rest of the leg and foot was present but incomplete. The larger specimen may have been 20 to 30 centimeters (7.9-12 inches) long from the tip of the snout to the base of the tail, and its full tail length is unknown but likely elongated. The small skeleton's poorly-preserved skull was triangular and pointed like those of "Archaeovenator", "Pyozia", and "Heleosaurus". It had thin, curved teeth on the maxilla (without a canine region) and smaller teeth on the palate, which also possessed a varanopid-like pterygoid and cultriform process. Isolated hyoid bones were also identifiable.
Dendromaia | Description
Furculae have only rarely been recovered from early theropod fossils; other examples include those of "Segisaurus" and "Coelophysis". The lower arm bones, the ulna and the radius, had a length of about seven centimetres. Hand elements are present but a formula of the phalanges could not determined. In the pelvis, the pubic bone had a length of 212 millimetres. It points obliquely to the front. The pubic foot is moderately broadened in side view, bot at the front and at the rear. The shaft of the ischium is with a length of 129 millimetres markedly shorter than the pubic shaft.
Dracoraptor | Skeleton
The length of the skull can range from 84mm to 104mm for midline length. to Through the analysis of the middle ear region of "Dendrerpeton acadianum", it could be determined that this taxa has a tympanic ear that is not there as a means of support for the palatoquadrate, but rather the positioning of the stapes indicates that it is used for the movement of sound. This morphology demonstrates "Dendrerpeton" having a hearing system that mimics that of anurans. This was contrary to what was previously thought, which was that it had a "squamosal embayment" rather than an otic notch.
Dendrerpeton | Skull and Skeleton
Little is known of the precise anatomical features of the Denisovans since the only physical remains discovered so far are a finger bone, three teeth, long bone fragments, a partial jawbone, and a parietal bone skull fragment. The finger bone is within the modern human range of variation for women, which is in contrast to the large, robust molars which are more similar to those of Middle to Late Pleistocene archaic humans. The third molar is outside of the range of any "Homo" species except "H. habilis" and "H. rudolfensis", and is more like those of australopithecines. The second molar is larger than those of modern humans and Neanderthals, and is more similar to those of "H.
Denisovan | Anatomy