skull(redirected from Cranial bones)
Also found in: Dictionary, Thesaurus, Medical.
skull,the skeletal structure of the head, composed of the facial and cranial bones. The skull houses and protects the brain and most of the chief sense organs; i.e., the eyes, ears, nose, and tongue. Among humans, some 14 bones shape the face, most occurring in symmetrical pairs. They are the lacrimals at the inner sides of the eyes, the nasals and nasal conchae of the nose, the palatines (palate), the zygomatics, or malars at the cheeks, the vomer (nasal septum), and the maxillae, or upper jaw. The mandible, or lower jaw, is not technically part of the skull. The adult human cranium, or braincase, is formed of fused skull bones: the parietals, temporals, ethmoid, sphenoid, frontal, and occipital. These are separate plates of bone in the fetus, but by birth they have generally grown sufficiently for most of their edges to meet. The remaining separations are known as fontanels, the most prominent being the soft spot atop a newborn's head. By the age of two years, all of these fontanels have been closed over by the growing cranial bones. However, the seams, or sutures, between the bones do not completely knit until the age of 20. The occipital bone at the base of the skull forms a complex joint with the first vertebra of the neck, known as the atlas, permitting rotation and bending of the head (see spinal columnspinal column,
bony column forming the main structural support of the skeleton of humans and other vertebrates, also known as the vertebral column or backbone. It consists of segments known as vertebrae linked by intervertebral disks and held together by ligaments.
..... Click the link for more information. ). Study of the fossil skulls of humans and their precursors has made important contributions to evolutionary theory, and to the science of physical anthropology. Earlier skulls of human ancestors, for instance, have been shown to have markedly smaller cranial capacities, as well as more powerful jaws, than do the Homo sapiens species which exist today.
(also cranium), the skeleton of the head of vertebrates, including man. A distinction is made between the axial cranium and the visceral cranium. The axial, or cerebral, cranium is the anterior continuation of the axial skeleton of the trunk that grows around the brain, olfactory organs, and inner ear. The visceral, or facial, cranium is the skeleton of the anterior part of the intestine (pharynx), which originally consisted of branchial, or visceral, arches separating gills.
In animals. The cranial changes that evolved in organisms were caused by progressive development of the brain and sense organs, replacement of gill respiration by pulmonary respiration, and change in feeding habits resulting from the emergence of organisms onto land from water. The cerebral cranium consists of a brain case, nasal capsules (surrounding the olfactory organs), and auditory capsules (enclosing the inner ear). The brain case is divided into anterior (prechordal) and posterior (chordal) portions. The former embraces the orbital and nasal regions of the skull, and the latter embraces the occipital and aural portions. The anterior portion develops from trabecular cartilages and orbital cartilages situated above them. In embryos the posterior portion develops around the fore end of the chord from the parachordal cartilages and auditory capsules. The parachordal cartilages correspond to the fused neural arches of the most anterior vertebrae, hence the “vertebral” origin of the posterior portion. Both portions remain independent in crossopterygians but grow together in the embryos of other vertebrates. The anterior portion usually contains only the forebrain (the cerebral hemispheres), with the posterior portion containing the greater part of the brain. In the adult cranium the boundary between the portions passes through the hypophysis and the opening by which the trigeminal nerve emerges from the cranium. The nasal capsules fuse with the anterior portion. Crossopterygians and terrestrial vertebrates have, in addition to nostrils, internal nares (choanae) that open into the anterior part of the oral cavity.
Platybasic and tropibasic types of brain case structure are distinguished according to the structure of the orbital portion. In a platybasic skull the prechordal portion has a wide base, and the brain extends into the orbital region. This type of cranium characterizes cyclostomes, sharks, lungfishes, recent amphibians, and mammals. All other vertebrates have a tropibasic skull, whose anterior portion has a narrow base and whose cerebral hemispheres are situated behind the orbital region.
The visceral cranium serves as a support for the anterior part of the digestive tract. It consists of metamerically arranged cartilaginous arches that encircle the initial portion of the digestive tract like hoops. The anterior visceral arches are transformed into jaws in all vertebrates except the most primitive organisms—the Agnatha. The cartilaginous base of the upper jaw, the palatoquadrate cartilage, was formed by the fusion of the upper portions of the two anterior arches, the premaxillary and the maxillary proper. The cartilaginous base of the lower jaw—Meckel’s cartilage— corresponds to the lower portion of the second visceral, or maxillary, arch. The upper portion of the third visceral arch, or hyoid arch, was transformed in fish into an appendage that usually attaches the jaws to the axial cranium. In most fish the maxillary arch is joined to the axial cranium only by an appendage (hyostylic). The palatoquadrate cartilage in chimeras, lungfishes, and terrestrial vertebrates is directly joined to the axial cranium without an appendage (autostylic). In extinct shelled and primitive bony fishes and some sharks the maxillary arch is joined to the axial cranium both directly and by means of an appendage (amphistylic).
In terrestrial vertebrates, the appendage is transformed into an auditory ossicle that conducts sounds from the tympanic membrane to the inner ear; the posterior visceral arches (gill arches proper) are transformed into the hyoid bone and laryngeal cartilages. Mammals have additional auditory ossicles, the incus and the malleus, which arose from detached posterior elements of the palatoquadrate and Meckel’s cartilages—the quadrate and articular bones.
In higher fishes and terrestrial vertebrates, the embryonic cartilaginous cranium more or less ossifies and becomes invested with dermal bones formed in the deep layers of the skin. On the outside the dermal bones form the calvaría, which is made up of nasal, frontal, parietal, squamous, and other bones. Originally the calvaría had openings only for the eyes and nostrils (anapsid skull), but most terrestrial vertebrates now have openings behind the orbits—temporal windows separated by the temporal, or zygomatic, arches. The function of the upper jaws was transferred to the premaxillary and maxillary dermal bones. The palatal and pterygoid dermal bones of the palatoquadrate cartilage form the palatal surface of the bony cranium. In some reptiles and mammals these bones form a secondary palate, which separates the choanal region from the main part of the oral cavity.
L. P. TATARINOV
In man. The human cerebral cranium is made up of six flat curved bones: the unpaired frontal and temporal bones, the paired parietal and temporal bones, and the sphenoid and ethmoid bones (situated at the base of the skull and possessing air spaces). The bones are connected by sutures. The upper part of the skull is called the skullcap, and the lower part is the base. The latter is divided from within into three fossae: anterior, middle, and posterior. The posterior cranial fossa contains the cerebellum. Nerves and blood vessels pass through the base of the skull by way of numerous canals and openings. The ear is housed in the temporal bone.
The shape and size of the skull vary: the circumference ranges from 52 to 64 cm, the length from 15 to 18 cm, and the width from 12 to 15 cm. Three basic shapes have been determined according to the cranial index. The shape of the skull changes with growth. An infant is born with incompletely developed bones, between which are remnants of the membranous cranium in the form of fontanelles and wide sutures.
The facial cranium consists of 14 comparatively small bones and the hyoid bone. The largest of these bones are the upper and lower jaws, which bound part of the oral cavity. The bones of the facial cranium as a whole determine the shape of the face. The lacrimal, zygomatic, and maxillary bones contribute to the formation of the orbit. The nasal, palatine, maxillary, vomer, and inferior turbinate bones form the nasal cavity, which communicates with the frontal, maxillary, and sphenoidal air sinuses. Other important topographical formations of the cranium include the infratemporal and pterygoid fossae, through which blood vessels and nerves pass.
V. V. KUPRIANOV
REFERENCESShmal’gauzen, I. I. Osnovy sravnitel’noi anatomii pozvonochnykh zhivotnykh, 4th ed. Moscow, 1947.
Prives, M. G., N. K. Lysenkov, and V. I. Bushkovich. Anatomiia cheloveka, 8th ed. Leningrad, 1974.
De Beer, G. R. The Development of the Vertebrate Skull. Oxford, 1937.
Jarvik, E. Théories de l’évolution des vertébrés. Paris, 1960.
Anthropological significance. Study of variations in the size and shape of the cranium and of the cranium’s individual constituent bones is important in all branches of anthropology. Certain cranial indexes can be determined by measurement (craniometry) or from descriptive characteristics (cranioscopy). Human morphology investigates normal sex- and age-related cranial variations in different population groups and explains the relationship between such variations and types of body build, hormonal status, social and environmental living conditions, and heredity. It has been found that men have larger skulls and cranial bones than women; male skulls are also marked by more expressed bone configuration (superciliary arch, the lines of attachment of muscles to the temporal, occipital, and lower jaw bones). Important age-related variations are changes in the correlation of the cerebral and facial portions; changes in the development, succession, and structure of teeth; and changes in the overgrowth of sutures.
Particularly important is the study of cranial transformations in the course of anthropogenesis, during which the skull gradually lost its “simian” features and acquired the structure characteristic of modern man. The cerebral portion of the skull enlarged, thereby dominating the maxillary portion. The highly developed supraorbital ridges in fossil man diminished and converted into superciliary arches, and the longitudinal crest of the cranium disappeared. The skullcap enlarged markedly: its frontal portion became wider and higher, and the occiput became round and lost the ridge and other bone growths (pronounced in ancient people) that served as sites of attachment of powerful neck muscles. The rearrangement of the cerebral portion was closely related to the enlargement and complication of the structure of the brain in the course of anthropogenesis.
The size of the brain averages 500–600 cc in the extinct higher primate Australopithecus, 1,000 cc in the oldest human beings (Archantropinae), 1,350 in ancient peoples (Palaeoanthropus), and 1,500 cc in modern man. These evolutionary changes in the calvaría, particularly the increased size of the fornix and the development of a high and almost straight forehead, were caused by active enlargement of the frontal and parietotemporal regions of the brain, where the specific cortical zones associated with man’s work and speech are located.
In the course of anthropogenesis, the facial portion of the skull became less protrusive, and paired nasal bones whose lower part protrudes beyond the vertical plane of the face developed. The mandibular teeth were arranged in a horseshoe fashion rather than in the form of a U, as in anthropoid apes. The mental protuberance lacking in apes and some fossil humans developed in front, and the shape of the upper jaw and hard palate changed.
The structural characteristics of the cerebral and, to a greater extent, facial portions of the skull are used in raciology to identify racial types in modern and, especially, ancient populations. The vertical profile of the face, the degree of flatness of the upper and middle parts of the face, the shape of the nostrils and orbital cavities, and the protrusion of the nasal bones are among the features of the major races. Characteristics of the cerebral portion of the skull, for example, the cephalic and cerebral indexes, are used to distinguish smaller units of anthropological classification within major races. Craniological data are an important historical source for study of the formation of ethnic communities (nations, nationalities). The data are particularly useful in the study of peoples lacking a written history. The results of anthropometry of ancient peoples in a given region, together with archeological, historical, linguistic, ethnographic, and anthropological data on modern peoples, are used to explain the ethnogeny of large and small groups of people in the USSR.
REFERENCESBunak, V. V. Cherep cheloveka i stadii ego formirovaniia u iskopaexsmykh liudei i sovremennykh ras. Moscow, 1959. (Tr. In-ta etnografii im. N. N. Miklukho-Maklaia: Novaia seriia, vol. 49.)
Zhedenov, V. N. Sravnitel’naia anatomiia primatov (vkliuchaia cheloveka). Moscow, 1962.
Roginskii, Ia. Ia., and M. G. Levin. Antropologiia, 2nd ed. Moscow, 1963.
A closed craniocerebral injury is associated with brief or prolonged unconsciousness, headache, nausea, and vertigo. If severe, there may also be mental, respiratory, and circulatory disorders combined with a variety of neurological symptoms. Such disorders are not pronounced in brain concussion. With cerebral hermorrhaging, there are signs of compression (two-stage loss of consciousness with a “lucid” interval, state of inhibition, prolonged headaches). If the base of the skull is fractured, signs of irritation of the meninges appear against a background of severe general brain symptoms. Treatment may be conservative or surgical. Conservative treatment is possible in cases not associated with compression of the brain. Bed rest and pain relievers are prescribed for skull fractures with no signs of brain injury and for mild brain concussions and contusions. More severe cases call for spinal taps and drugs that reduce intracranial pressure. Surgery is required for skull fractures with bone depression, brain compression by an intracranial hematoma, and increasing brain edema. Surgery involving opening of the cranial cavity is performed under local or general anesthesia.
Open craniocerebral trauma occurs less often than closed trauma in peacetime. It may be nonpenetrating (without injury to the dura mater) or penetrating (with injury to the dura mater). The clinical signs of general cerebral disorders are less pronounced than in closed trauma, but there is a much greater danger of infection or development of meningitis, encephalitis, or other complications. Treatment is only by surgery (surgical treatment of a wound, trephination, and plastic surgery for large defects in bone).
The sequelae of craniocerebral trauma may be diseases of the brain, including arachnoencephalitis, psychological disorders (for example, twilight state), and, in the long term, asthenia, encephalopathy, traumatic epilepsy, and mental deficiency.
REFERENCESTiazhelaia zakrytaia travma cherepa i golovnogo mozga. Edited by V. M. Ugriumov. Leningrad, 1974.
Wertheimer, P., and G. Descotes. Traumatologie crânienne. Paris, 1961.
A. K. VLADIMIROVA
What does it mean when you dream about a skull?
Danger and death are symbolized by the skull and crossbones, which is often found on labels that warn of poisonous contents. The skull and crossbones was also adopted by pirate ships and the Hell’s Angels. Each Halloween the skull—with or without the crossbones—is still used to warn of danger and death.