skull skull the skull is a bone protective cavity for the brain. the skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. however two parts are more prominent: the cranium and the mandible. in humans, these two parts are the neurocranium and the viscerocranium (facial skeleton) that includes the mandible as its largest bone. the skull forms the anterior-most portion of the skeleton and is a product of cephalisation—housing the brain, and several sensory structures such as the eyes, ears, nose, and mouth. in humans these sensory structures are part of the facial skeleton. skullvolume rendering of a mouse skulldetailssystemskeletal systemidentifiersmeshd012886ta98a02.1.00.001ta2406fma54964anatomical terminology functions of the skull include protection of the brain, fixing the distance between the eyes to allow stereoscopic vision, and fixing the position of the ears to enable sound localisation of the direction and distance of sounds. in some animals, such as horned ungulates (mammals with hooves), the skull also has a defensive function by providing the mount (on the frontal bone) for the horns. the english word skull is probably derived from old norse skulle, while the latin word cranium comes from the greek root κρανίον (kranion). the human skull fully develops two years after birth.the junctions of the skull bones are joined by structures called sutures. the skull is made up of a number of fused flat bones, and contains many foramina, fossae, processes, and several cavities or sinuses. in zoology there are openings in the skull called fenestrae. structure humans for details and the constituent bones, see neurocranium and facial skeleton. skull in situ anatomy of a flat bone – the periosteum of the neurocranium is known as the pericranium human skull from the front side bones of skull the human skull is the bone structure that forms the head in the human skeleton. it supports the structures of the face and forms a cavity for the brain. like the skulls of other vertebrates, it protects the brain from injury. the skull consists of three parts, of different embryological origin—the neurocranium, the sutures, and the facial skeleton (also called the membraneous viscerocranium). the neurocranium (or braincase) forms the protective cranial cavity that surrounds and houses the brain and brainstem. the upper areas of the cranial bones form the calvaria (skullcap). the membranous viscerocranium includes the mandible. the sutures are fairly rigid joints between bones of the neurocranium. the facial skeleton is formed by the bones supporting the face. bones except for the mandible, all of the bones of the skull are joined by sutures—synarthrodial (immovable) joints formed by bony ossification, with sharpey's fibres permitting some flexibility. sometimes there can be extra bone pieces within the suture known as wormian bones or sutural bones. most commonly these are found in the course of the lambdoid suture. the human skull is generally considered to consist of twenty-two bones—eight cranial bones and fourteen facial skeleton bones. in the neurocranium these are the occipital bone, two temporal bones, two parietal bones, the sphenoid, ethmoid and frontal bones. the bones of the facial skeleton (14) are the vomer, two inferior nasal conchae, two nasal bones, two maxilla, the mandible, two palatine bones, two zygomatic bones, and two lacrimal bones. some sources count a paired bone as one, or the maxilla as having two bones (as its parts); some sources include the hyoid bone or the three ossicles of the middle ear but the overall general consensus of the number of bones in the human skull is the stated twenty-two. some of these bones—the occipital, parietal, frontal, in the neurocranium, and the nasal, lacrimal, and vomer, in the facial skeleton are flat bones. cavities and foramina ct scan of a human skull in 3d the skull also contains sinuses, air-filled cavities known as paranasal sinuses, and numerous foramina. the sinuses are lined with respiratory epithelium. their known functions are the lessening of the weight of the skull, the aiding of resonance to the voice and the warming and moistening of the air drawn into the nasal cavity. the foramina are openings in the skull. the largest of these is the foramen magnum that allows the passage of the spinal cord as well as nerves and blood vessels. processes the many processes of the skull include the mastoid process and the zygomatic processes. fenestrae a centrosaurus skull scheme of spinosaurus skull the fenestrae (from latin, meaning windows) are openings in the skull. antorbital fenestra mandibular fenestra quadratojugal fenestra subsquamosal fenestra, an opening between two parts of the squamosal bone in some rodents temporal fenestra chimpanzee skull goat skull bones the jugal is a skull bone found in most reptiles, amphibians, and birds. in mammals, the jugal is often called the zygomatic bone or malar bone. the prefrontal bone is a bone separating the lacrimal and frontal bones in many tetrapod skulls. fish fish head parts, 1889, fauna of british india, sir francis day skull of a swordfish the skull of fishes is formed from a series of only loosely connected bones. lampreys and sharks only possess a cartilaginous endocranium, with both the upper and lower jaws being separate elements. bony fishes have additional dermal bone, forming a more or less coherent skull roof in lungfish and holost fish. the lower jaw defines a chin. the simpler structure is found in jawless fish, in which the cranium is normally represented by a trough-like basket of cartilaginous elements only partially enclosing the brain, and associated with the capsules for the inner ears and the single nostril. distinctively, these fish have no jaws. cartilaginous fish, such as sharks and rays, have also simple, and presumably primitive, skull structures. the cranium is a single structure forming a case around the brain, enclosing the lower surface and the sides, but always at least partially open at the top as a large fontanelle. the most anterior part of the cranium includes a forward plate of cartilage, the rostrum, and capsules to enclose the olfactory organs. behind these are the orbits, and then an additional pair of capsules enclosing the structure of the inner ear. finally, the skull tapers towards the rear, where the foramen magnum lies immediately above a single condyle, articulating with the first vertebra. there are, in addition, at various points throughout the cranium, smaller foramina for the cranial nerves. the jaws consist of separate hoops of cartilage, almost always distinct from the cranium proper. in ray-finned fish, there has also been considerable modification from the primitive pattern. the roof of the skull is generally well formed, and although the exact relationship of its bones to those of tetrapods is unclear, they are usually given similar names for convenience. other elements of the skull, however, may be reduced; there is little cheek region behind the enlarged orbits, and little, if any bone in between them. the upper jaw is often formed largely from the premaxilla, with the maxilla itself located further back, and an additional bone, the symplectic, linking the jaw to the rest of the cranium. although the skulls of fossil lobe-finned fish resemble those of the early tetrapods, the same cannot be said of those of the living lungfishes. the skull roof is not fully formed, and consists of multiple, somewhat irregularly shaped bones with no direct relationship to those of tetrapods. the upper jaw is formed from the pterygoids and vomers alone, all of which bear teeth. much of the skull is formed from cartilage, and its overall structure is reduced. tetrapods skull of tiktaalik, an extinct genus transitional between lobe-finned fish and early tetrapods the skulls of the earliest tetrapods closely resembled those of their ancestors amongst the lobe-finned fishes. the skull roof is formed of a series of plate-like bones, including the maxilla, frontals, parietals, and lacrimals, among others. it is overlaying the endocranium, corresponding to the cartilaginous skull in sharks and rays. the various separate bones that compose the temporal bone of humans are also part of the skull roof series. a further plate composed of four pairs of bones forms the roof of the mouth; these include the vomer and palatine bones. the base of the cranium is formed from a ring of bones surrounding the foramen magnum and a median bone lying further forward; these are homologous with the occipital bone and parts of the sphenoid in mammals. finally, the lower jaw is composed of multiple bones, only the most anterior of which (the dentary) is homologous with the mammalian mandible. in living tetrapods, a great many of the original bones have either disappeared or fused into one another in various arrangements. birds cuckoo skull birds have a diapsid skull, as in reptiles, with a prelacrimal fossa (present in some reptiles). the skull has a single occipital condyle. the skull consists of five major bones: the frontal (top of head), parietal (back of head), premaxillary and nasal (top beak), and the mandible (bottom beak). the skull of a normal bird usually weighs about 1% of the bird's total bodyweight. the eye occupies a considerable amount of the skull and is surrounded by a sclerotic eye-ring, a ring of tiny bones. this characteristic is also seen in reptiles. amphibians amphibians' skulls, hans gadow, 1909 amphibia and reptiles living amphibians typically have greatly reduced skulls, with many of the bones either absent or wholly or partly replaced by cartilage. in mammals and birds, in particular, modifications of the skull occurred to allow for the expansion of the brain. the fusion between the various bones is especially notable in birds, in which the individual structures may be difficult to identify. development skull of a new-born child from the side the skull is a complex structure; its bones are formed both by intramembranous and endochondral ossification. the skull roof bones, comprising the bones of the facial skeleton and the sides and roof of the neurocranium, are dermal bones formed by intramembranous ossification, though the temporal bones are formed by endochondral ossification. the endocranium, the bones supporting the brain (the occipital, sphenoid, and ethmoid) are largely formed by endochondral ossification. thus frontal and parietal bones are purely membranous. the geometry of the skull base and its fossae, the anterior, middle and posterior cranial fossae changes rapidly. the anterior cranial fossa changes especially during the first trimester of pregnancy and skull defects can often develop during this time. at birth, the human skull is made up of 44 separate bony elements. during development, many of these bony elements gradually fuse together into solid bone (for example, the frontal bone). the bones of the roof of the skull are initially separated by regions of dense connective tissue called fontanelles. there are six fontanelles: one anterior (or frontal), one posterior (or occipital), two sphenoid (or anterolateral), and two mastoid (or posterolateral). at birth, these regions are fibrous and moveable, necessary for birth and later growth. this growth can put a large amount of tension on the "obstetrical hinge", which is where the squamous and lateral parts of the occipital bone meet. a possible complication of this tension is rupture of the great cerebral vein. as growth and ossification progress, the connective tissue of the fontanelles is invaded and replaced by bone creating sutures. the five sutures are the two squamous sutures, one coronal, one lambdoid, and one sagittal suture. the posterior fontanelle usually closes by eight weeks, but the anterior fontanel can remain open up to eighteen months. the anterior fontanelle is located at the junction of the frontal and parietal bones; it is a "soft spot" on a baby's forehead. careful observation will show that you can count a baby's heart rate by observing the pulse pulsing softly through the anterior fontanelle. the skull in the neonate is large in proportion to other parts of the body. the facial skeleton is one seventh of the size of the calvaria. (in the adult it is half the size). the base of the skull is short and narrow, though the inner ear is almost adult size. clinical significance craniosynostosis is a condition in which one or more of the fibrous sutures in an infant skull prematurely fuses, and changes the growth pattern of the skull. because the skull cannot expand perpendicular to the fused suture, it grows more in the parallel direction. sometimes the resulting growth