The skull is a bony structure that forms the head in vertebrates. It provides a protective cavity for the brain; the skull is composed of two parts: the mandible. In the human, these two parts are the neurocranium and the viscerocranium or 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, several sensory structures such as the eyes, ears and mouth. In humans these sensory structures are part of the facial skeleton. Functions of the skull include protection of the brain, fixing the distance between the eyes to allow stereoscopic vision, fixing the position of the ears to enable sound localisation of the direction and distance of sounds. In some animals such as horned ungulates, the skull has a defensive function by providing the mount for the horns; the English word "skull" is derived from Old Norse "skulle", while the Latin word cranium comes from the Greek root κρανίον.
The skull is made up of a number of fused flat bones, contains many foramina, fossae and several cavities or sinuses. In zoology there are openings in the skull called fenestrae. For details and the constituent bones, see Neurocranium and Facial skeleton The human skull is the bony structure that forms the head in the human skeleton, it forms a cavity for the brain. Like the skulls of other vertebrates, it protects the brain from injury; the skull consists of two parts, of different embryological origin—the neurocranium and the facial skeleton. The neurocranium forms the protective cranial cavity that surrounds and houses the brain and brainstem; the upper areas of the cranial bones form the calvaria. The membranous viscerocranium includes the mandible; the facial skeleton is formed by the bones supporting the face Except for the mandible, all of the bones of the skull are joined together by sutures—synarthrodial joints formed by bony ossification, with Sharpey's fibres permitting some flexibility.
Sometimes there can be extra bone pieces within the suture known as sutural bones. Most these are found in the course of the lambdoid suture; the human skull is 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 and frontal bones; the bones of the facial skeleton are the vomer, two inferior nasal conchae, two nasal bones, two maxilla, the mandible, two palatine bones, two zygomatic bones, two lacrimal bones. Some sources count the maxilla as having two bones; some of these bones—the occipital, frontal, in the neurocranium, the nasal and vomer, in the facial skeleton are flat bones. The skull contains sinuses, air-filled cavities known as paranasal sinuses, 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; the many processes of the skull include the zygomatic processes. The skull is a complex structure; 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 are formed by endochondral ossification, thus frontal and parietal bones are purely membranous. The geometry of the skull base and its fossae, the anterior and posterior cranial fossae changes rapidly; the anterior cranial fossa changes during the first trimester of pregnancy and skull defects can develop during this time. At birth, the human skull is made up of 44 separate bony elements. During development, many of these bony elements fuse together into solid bone.
The bones of the roof of the skull are separated by regions of dense connective tissue called fontanelles. There are six fontanelles: one anterior, one posterior, two sphenoid, two mastoid. At birth these regions are fibrous and moveable, necessary for birth and growth; this growth can put a large amount of tension on the "obstetrical hinge", 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, one sagittal suture; the posterior fontanelle 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 parietal bones. Careful observation will show that you can count a baby's heart
Middle nasal concha
The medial surface of the labyrinth of ethmoid consists of a thin lamella, which descends from the under surface of the cribriform plate, ends below in a free, convoluted margin, the middle nasal concha. It is rough, marked above by numerous grooves, directed nearly vertically downward from the cribriform plate. Nasal concha This article incorporates text in the public domain from page 156 of the 20th edition of Gray's Anatomy Anatomy figure: 33:01-01 at Human Anatomy Online, SUNY Downstate Medical Center Anatomy figure: 22:02-10 at Human Anatomy Online, SUNY Downstate Medical Center upstate.edu - Frontal Atlas image: rsa1p9 at the University of Michigan Health System - lateral Atlas image: rsa1p6 at the University of Michigan Health System - coronal "Anatomy diagram: 34256.000-1". Roche Lexicon - illustrated navigator. Elsevier. Archived from the original on 2014-01-01
The alveolar process is the thickened ridge of bone that contains the tooth sockets on the jaw bones that hold teeth. In humans, the tooth-bearing bones are the mandible; the curved part of each alveolar process on the jaw is called the alveolar arch. On the maxilla, the alveolar process is a ridge on the inferior surface, on the mandible it is a ridge on the superior surface, it makes up the thickest part of the maxillae. The alveolar process contains a region of compact bone adjacent to the periodontal ligament, called the lamina dura when viewed on radiographs, it is this part, attached to the cementum of the roots by the periodontal ligament. It is uniformly radiopaque. Integrity of the lamina dura is important; the alveolar process has a supporting bone, both of which have the same components: fibers, intercellular substances, blood vessels, lymphatics. The alveolar process is the lining of alveolus. Although the alveolar process is composed of compact bone, it may be called the cribriform plate because it contains numerous holes where Volkmann canals pass from the alveolar bone into the PDL.
The alveolar bone proper is called bundle bone because Sharpey fibers, a part of the fibers of the PDL, are inserted here. Similar to those of the cemental surface, Sharpey fibers in alveolar bone proper are each inserted at 90 degrees, or at a right angle, but are fewer in number, although thicker in diameter than those present in cementum; as in cellular cementum, Sharpey fibers in bone are mineralized only at their periphery. The alveolar crest is the most cervical rim of the alveolar bone proper. In a healthy situation, the alveolar crest is apical to the cementoenamel junction by 1.5 to 2 mm. The alveolar crests of neighboring teeth are uniform in height along the jaw in healthy situation; the supporting alveolar bone consists of both cortical trabecular bone. The cortical bone, or cortical plates, consists of plates of compact bone on the facial and lingual surfaces of the alveolar bone; these cortical plates are about 1.5 to 3 mm thick over posterior teeth, but the thickness is variable around anterior teeth.
The trabecular bone consists of cancellous bone, located between the alveolar bone proper and the plates of cortical bone. The alveolar bone between two neighboring teeth is the interdental septum. Inorganic matrix Alveolar bone is 67% inorganic material by weight; the inorganic material is composed of the mineral’s calcium and phosphate. The mineral content is in the form of calcium hydroxyapatite crystals. Organic matrix; the organic material consists of non-collagenous material. The cellular component of bone consists of osteoblasts and osteoclasts. Osteoblasts are cuboidal and elongated in shape, they synthesise. These cells have a high level of alkaline phosphatase on the outer surface of their plasma membrane; the functions of osteoblasts are bone formation by synthesising the organic matrix of bone, cell to cell communication and maintenance of bone matrix. Osteocytes are modified osteoblasts which become entrapped in lacunae during the secretion of bone matrix; the osteocytes have processes called canaliculi.
These canaliculi bring oxygen and nutrients to the osteocytes through blood and remove metabolic waste products. Osteoclasts are multinucleated giant cells, they are found in Howship’s lacunae. Bone is lost through the process of resorption which involves osteoclasts breaking down the hard tissue of bone. A key indication of resorption is; this is known as Howship’s lacuna. The resorption phase lasts as long as the lifespan of the osteoclast, around 8 to 10 days. After this resorption phase, the osteoclast can continue resorbing surfaces in another cycle or carry out apoptosis. A repair phase follows the resorption phase. In patients with periodontal disease, inflammation lasts longer and during the repair phase, resorption may override any bone formation; this results in a net loss of alveolar bone. Alveolar bone loss is associated with periodontal disease. Periodontal disease is the inflammation of the gums. Studies in osteoimmunology have proposed 2 models for alveolar bone loss. One model states that inflammation is triggered by a periodontal pathogen and which activates the acquired immune system to inhibit bone coupling by limiting new bone formation after resorption.
Another model states that cytokinesis that may inhibit the differentiation of osteoblasts from their precursors therefore limiting bone formation. This results in a net loss of alveolar bone; the developmental disturbance of anodontia, in which tooth germs are congenitally absent, may affect the development of the alveolar processes. This occurrence can prevent the alveolar processes of either the maxillae or the mandible from developing. Proper development is impossible because the alveolar unit of each dental arch must form in response to the tooth germs in the area. After extraction of a tooth, the clot in the alveolus fills in with immature bone, remodeled into mature secondary bone. However, with the partial or total loss of teeth, the alveolar process undergoes resorption; the underlying basal bone of the body of the maxilla or mandible remains less affected, because it does not need the presence of teeth to remain viable. The loss of alveolar bone, coupled with a
Frontal process of maxilla
The frontal process of maxilla is a strong plate, which projects upward and backward from the maxilla, forming part of the lateral boundary of the nose. Its lateral surface is smooth, continuous with the anterior surface of the body, gives attachment to the quadratus labii superioris, the orbicularis oculi, the medial palpebral ligament, its medial surface forms part of the lateral wall of the nasal cavity. The upper border articulates with the anterior with the nasal; the lateral margin of the groove is named the anterior lacrimal crest, is continuous below with the orbital margin. Anatomy photo:22:os-1903 at the SUNY Downstate Medical Center - "Osteology of the Skull: The Maxilla" "Anatomy diagram: 34256.000-1". Roche Lexicon - illustrated navigator. Elsevier. Archived from the original on 2014-01-01
Anatomical terms of bone
Many anatomical terms descriptive of bone are defined in anatomical terminology, are derived from Greek and Latin. A long bone is one, cylindrical in shape, being longer than it is wide. However, the term describes the shape of a bone, not its size, relative. Long bones are found in the legs, as well as in the fingers and toes. Long bones function as levers, they are responsible for the body's height. A short bone is one, cube-like in shape, being equal in length 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 support as well as some limited motion; the term “flat bone” is something of a misnomer because, although a flat bone is thin, it is often curved. Examples include the cranial bones, the scapulae, the sternum, the ribs. Flat bones serve as points of attachment for muscles and protect internal organs. Flat bones do not have a medullary cavity. An irregular bone is one that does not have an classified shape and defies description.
These bones tend to have more complex shapes, like the vertebrae that support the spinal cord and protect it from compressive forces. Many facial bones the ones containing sinuses, are classified as irregular bones. A sesamoid bone is a round bone that, as the name suggests, is shaped like a sesame seed; these bones form in tendons. The sesamoid bones protect tendons by helping them overcome compressive forces. Sesamoid bones vary in number and placement from person to person but are found in tendons associated with the feet and knees; the only type of sesamoid bone, common to everybody is the kneecap, the largest of the sesamoid bones. A condyle is the round prominence at the end of a bone, most part of a joint – an articulation with another bone; the epicondyle refers to a projection near a condyle the medial epicondyle of the humerus. These terms derive from Greek. An eminence refers to a small projection or bump of bone, such as the medial eminence. A process refers to a large projection or prominent bump, as does a promontory such as the sacral promontory.
Both tubercle and tuberosity refer to a projection or bump with a roughened surface, with a "tubercle" smaller than a "tuberosity". These terms are derived from Tuber. A ramus refers to an extension of bone, such as the ramus of the mandible in the jaw or Superior pubic ramus. Ramus may be used to refer to nerves, such as the ramus communicans. A facet refers to a flattened articular surface. A line refers to a long, thin projection with a rough surface. Ridge and crest refer to a narrow line. Unlike many words used to describe anatomical terms, the word ridge is derived from Old English. A spine, as well as referring to the spinal cord, may be used to describe a long, thin projection or bump; these terms are used to describe bony protuberances in specific parts of the body. The Malleolus is the bony prominence on each side of the ankle; these are known as the lateral malleolus. Each leg is supported by two bones, the tibia on the inner side of the leg and the fibula on the outer side of the leg; the medial malleolus is the prominence on the inner side of the ankle, formed by the lower end of the tibia.
The lateral malleolus is the prominence on the outer side of the ankle, formed by the lower end of the fibula. The trochanters are parts of the femur, it may refer to the greater, lesser, or third trochanter The following terms are used to describe cavities that connect to other areas: A foramen is any opening referring to those in bone. Foramina inside the body of humans and other animals allow muscles, arteries, veins, or other structures to connect one part of the body with another. A canal is a long, tunnel-like foramen a passage for notable nerves or blood vessels; the following terms are used to describe cavities that do not connect to other areas: A fossa is a depression or hollow in a bone, such as the hypophyseal fossa, the depression in the sphenoid bone. A meatus is a short canal. A fovea is a small pit on the head of a bone. An example of a fovea is the fovea capitis of the head of the femur; the following terms are used to describe the walls of a cavity: A labyrinth refers to the bony labyrinth and membranous labyrinth, components of the inner ear, due to their fine and complex structure.
A sinus refers to a bony cavity within the skull. A joint, or articulation is the region where adjacent bones contact each other, for example the elbow, shoulder, or costovertebral joint. Terms that refer to joints include: articular process, referring to a projection that contacts an adjacent bone. Suture, referring to an articulation between cranial bones. Bones are described with the terms head, shaft and base The head of a bone refers to the proximal end of the bone; the shaft refers to the elongated sections of long bone, the neck the segment between the head and shaft. The end of the long bone opposite to the head is known as the base; the cortex of a bone is used to refer to its outer layers, medulla used to
The zygomatico-orbital foramina are two canals in the skull, that allow nerves to pass through. The orifices are seen on the orbital process of the zygomatic bone. One of these canals opens into the other on the malar surface of the bone; the former transmits the latter the zygomaticofacial nerve. This article incorporates text in the public domain from page 165 of the 20th edition of Gray's Anatomy
In human anatomy, the facial skeleton of the skull the external surface of the mandible is marked in the median line by a faint ridge, indicating the mandibular symphysis or line of junction where the two lateral halves of the mandible fuse at an early period of life. It is not a true symphysis; this ridge divides below and encloses a triangular eminence, the mental protuberance, the base of, depressed in the center but raised on either side to form the mental tubercle. The lowest end of the mandibular symphysis — the point of the chin — is called the "menton", it serves as the origin for the genioglossus muscles. Solitary mammalian carnivores that rely on a powerful canine bite to subdue their prey have a strong mandibular symphysis, while pack hunters delivering shallow bites have a weaker one; when filter feeding, the baleen whales, of the suborder Mysticeti, can dynamically expand their oral cavity in order to accommodate enormous volumes of sea water. This is made possible thanks to its mandibular skull joints the elastic mandibular symphysis which permits both dentaries to be rotated independently in two planes.
This flexible jaw, which made the titanic body sizes of baleen whales possible, is not present in early whales and most evolved within Mysticeti. This article incorporates text in the public domain from page 172 of the 20th edition of Gray's Anatomy