Cranium Health Dictionary

This is the collection of 22 ?at and irregularly shaped bones which protect the brain and form the face (see BONE).

Arrangement of the bones In childhood, the bones are independent, gradually fusing together by sutures, and in old age fusing completely so that the cranium forms a solid bony case. At the time of birth the growth of several bones of the infant’s head has not been quite completed, so that six soft spots, or fontanellas, present; here the brain is covered only by skin and membranes, and the pulsations of its blood vessels may be seen. One of these spots, the anterior fontanelle, does not close completely until the child is 18 months to 2••• years old.

Parts of the skull The cranium, enclosing the brain, consists of eight bones, while the face, which forms a bony framework for the eyes, nose and mouth, consists of 14 bones. These two parts can be detached.

Shape of the skull The development of large central hemispheres of the brain in humans has in?uenced the skull shape. Unlike in other mammals, the cranium extends above as well as behind the face which therefore looks forwards. The skull’s proportions change with age: the cranium in children is larger in comparison with the face – one-eighth of the whole head – than is the case in adults, where sizes are about the same. Old age reduces the size of the face because of the loss of teeth and absorption of their bony sockets. Women’s skulls tend to be lighter and smoother with less obvious protuberances than those in men.... skull

The brain and spinal cord together form the central nervous sytem (CNS). Twelve cranial nerves leave each side of the brain (see NERVES, below) and 31 spinal nerves from each side of the cord: together these nerves form the peripheral nervous system. Complex chains of nerves lying within the chest and abdomen, and acting largely independently of the peripheral system, though linked with it, comprise the AUTONOMIC NERVOUS SYSTEM and govern the activities of the VISCERA.

The control centre of the whole nervous system is the brain, which is located in the skull or cranium. As well as controlling the nervous system it is the organ of thought, speech and emotion. The central nervous system controls the body’s essential functions such as breathing, body temperature (see HOMEOSTASIS) and the heartbeat. The body’s various sensations, including sight, hearing, touch, pain, positioning and taste, are communicated to the CNS by nerves distributed throughout the relevant tissues. The information is then sorted and interpreted by specialised areas in the brain. In response these initiate and coordinate the motor output, triggering such ‘voluntary’ activities as movement, speech, eating and swallowing. Other activities – for example, breathing, digestion, heart contractions, maintenance of BLOOD PRESSURE, and ?ltration of waste products from blood passing through the kidneys – are subject to involuntary control via the autonomic system. There is, however, some overlap between voluntary and involuntary controls.... brain

Either one of a pair of bones that form the top and sides of the cranium of the SKULL.... parietal bone

A slow-growing, benign tumour in the auditory canal arising from the Schwann cells of the acoustic cranial nerve. The neuroma, which accounts for about 7 per cent of all tumours inside the CRANIUM, may cause facial numbness, hearing loss, unsteady balance, headache, and TINNITUS. It can usually be removed surgically, sometimes with microsurgical techniques that preserve the facial nerve.... acoustic neuroma

An abnormal accumulation of CEREBROSPINAL FLUID, or CSF, within the skull, as a result of one or more of three main causes: (i) excessive CSF production; (ii) defective CSF absorption;

(iii) blockage of the circulation of CSF. Such disturbances in the circulation of the ?uid may be due to congenital reasons (most commonly associated with SPINA BIFIDA), to MENINGITIS, or to a tumour.

Symptoms In children, the chief symptoms observed are the gradual increase in size of the upper part of the head, out of all proportion to the face or the rest of the body. The head is globular, with a wide anterior FONTANELLE and separation of the bones at the sutures. The veins in the scalp are prominent, and there is a ‘crackpot’ note on percussion. The normal infant’s head should not grow more than 2·5 cm (1 inch) in each of the ?rst two months of life, and much more slowly subsequently; growth beyond this rate should arouse suspicions of hydrocephalus, so medical professionals caring for infants use centile charts for this purpose.

The cerebral ventricles are widely distended, and the convolutions of the brain ?attened, while occasionally the ?uid escapes into the cavity of the cranium, which it ?lls, pressing down the brain to the base of the skull. As a consequence of such changes, the functions of the brain are interfered with, and in general the mental condition of the patient is impaired. Untreated, the child is dull and listless, irritable and sometimes suffers from severe mental subnormality. The special senses become affected as the disease advances, especially vision, and sight is often lost, as is also hearing. Towards the end, paralysis is apt to occur.

Treatment Numerous ingenious operations have been devised for the treatment of hydrocephalus. The most satisfactory of these utilise unidirectional valves and shunts (tubes), whereby the cerebrospinal ?uid is bypassed from the brain into the right atrium of the heart or the peritoneal cavity. The shunt may have to be left in position inde?nitely.... hydrocephalus

An extensive, soft swelling on the scalp of a newborn infant, which is caused by bleeding into the space between the cranium and its fibrous covering due to pressure on the baby’s head during delivery.

The swelling is not serious and gradually subsides.... cephalhaematoma

A natural opening in a bone or other body structure, usually to allow the passage of nerves or blood vessels. For example, the foramen magnum is a hole in the base of the cranium through which the spinal cord passes.... foramen

(cleido-, clid-, clido-) combining form denoting the clavicle (collar bone). Example: cleidocranial (of the clavicle and cranium).... cleid

n. 1. surgical removal of a portion of the skull (cranium), performed to expose the brain and *meninges for inspection or biopsy or to relieve excessive intracranial pressure (as in a subdural *haematoma). 2. surgical perforation of the skull of a dead fetus during difficult labour, so that delivery may continue. For both operations the instrument used is called a craniotome.... craniotomy

The framework upon which the rest of the body is built up. The bones are generally called the skeleton, though this term also includes the cartilages which join the ribs to the breastbone, protect the larynx, etc.

Structure of bone Bone is composed partly of ?brous tissue, partly of bone matrix comprising phosphate and carbonate of lime, intimately mixed together. The bones of a child are about two-thirds ?brous tissue, whilst those of the aged contain one-third; the toughness of the former and the brittleness of the latter are therefore evident.

The shafts of the limb bones are composed of dense bone, the bone being a hard tube surrounded by a membrane (the periosteum) and enclosing a fatty substance (the BONE MARROW); and of cancellous bone, which forms the short bones and the ends of long bones, in which a ?ne lace-work of bone ?lls up the whole interior, enclosing marrow in its meshes. The marrow of the smaller bones is of great importance. It is red in colour, and in it red blood corpuscles are formed. Even the densest bone is tunnelled by ?ne canals (Haversian canals) in which run small blood vessels, nerves and lymphatics, for the maintenance and repair of the bone. Around these Haversian canals the bone is arranged in circular plates called lamellae, the lamellae being separated from one another by clefts, known as lacunae, in which single bone-cells are contained. Even the lamellae are pierced by ?ne tubes known as canaliculi lodging processes of these cells. Each lamella is composed of very ?ne interlacing ?bres.

GROWTH OF BONES Bones grow in thickness from the ?brous tissue and lime salts laid down by cells in their substance. The long bones grow in length from a plate of cartilage (epiphyseal cartilage) which runs across the bone about 1·5 cm or more from its ends, and which on one surface is also constantly forming bone until the bone ceases to lengthen at about the age of 16 or 18. Epiphyseal injury in children may lead to diminished growth of the limb.

REPAIR OF BONE is e?ected by cells of microscopic size, some called osteoblasts, elaborating the materials brought by the blood and laying down strands of ?brous tissue, between which bone earth is later deposited; while other cells, known as osteoclasts, dissolve and break up dead or damaged bone. When a fracture has occurred, and the broken ends have been brought into contact, these are surrounded by a mass of blood at ?rst; this is partly absorbed and partly organised by these cells, ?rst into ?brous tissue and later into bone. The mass surrounding the fractured ends is called the callus, and for some months it forms a distinct thickening which is gradually smoothed away, leaving the bone as before the fracture. If the ends have not been brought accurately into contact, a permanent thickening results.

VARIETIES OF BONES Apart from the structural varieties, bones fall into four classes: (a) long bones like those of the limbs; (b) short bones composed of cancellous tissue, like those of the wrist and the ankle; (c) ?at bones like those of the skull; (d) irregular bones like those of the face or the vertebrae of the spinal column (backbone).

The skeleton consists of more than 200 bones. It is divided into an axial part, comprising the skull, the vertebral column, the ribs with their cartilages, and the breastbone; and an appendicular portion comprising the four limbs. The hyoid bone in the neck, together with the cartilages protecting the larynx and windpipe, may be described as the visceral skeleton.

AXIAL SKELETON The skull consists of the cranium, which has eight bones, viz. occipital, two parietal, two temporal, one frontal, ethmoid, and sphenoid; and of the face, which has 14 bones, viz. two maxillae or upper jaw-bones, one mandible or lower jaw-bone, two malar or cheek bones, two nasal, two lacrimal, two turbinal, two palate bones, and one vomer bone. (For further details, see SKULL.) The vertebral column consists of seven vertebrae in the cervical or neck region, 12 dorsal vertebrae, ?ve vertebrae in the lumbar or loin region, the sacrum or sacral bone (a mass formed of ?ve vertebrae fused together and forming the back part of the pelvis, which is closed at the sides by the haunch-bones), and ?nally the coccyx (four small vertebrae representing the tail of lower animals). The vertebral column has four curves: the ?rst forwards in the neck, the second backwards in the dorsal region, the third forwards in the loins, and the lowest, involving the sacrum and coccyx, backwards. These are associated with the erect attitude, develop after a child learns to walk, and have the e?ect of diminishing jars and shocks before these reach internal organs. This is aided still further by discs of cartilage placed between each pair of vertebrae. Each vertebra has a solid part, the body in front, and behind this a ring of bone, the series of rings one above another forming a bony canal up which runs the spinal cord to pass through an opening in the skull at the upper end of the canal and there join the brain. (For further details, see SPINAL COLUMN.) The ribs – 12 in number, on each side – are attached behind to the 12 dorsal vertebrae, while in front they end a few inches away from the breastbone, but are continued forwards by cartilages. Of these the upper seven reach the breastbone, these ribs being called true ribs; the next three are joined each to the cartilage above it, while the last two have their ends free and are called ?oating ribs. The breastbone, or sternum, is shaped something like a short sword, about 15 cm (6 inches) long, and rather over 2·5 cm (1 inch) wide.

APPENDICULAR SKELETON The upper limb consists of the shoulder region and three segments – the upper arm, the forearm, and the wrist with the hand, separated from each other by joints. In the shoulder lie the clavicle or collar-bone (which is immediately beneath the skin, and forms a prominent object on the front of the neck), and the scapula or shoulder-blade behind the chest. In the upper arm is a single bone, the humerus. In the forearm are two bones, the radius and ulna; the radius, in the movements of alternately turning the hand palm up and back up (called supination and pronation respectively), rotating around the ulna, which remains ?xed. In the carpus or wrist are eight small bones: the scaphoid, lunate, triquetral, pisiform, trapezium, trapezoid, capitate and hamate. In the hand proper are ?ve bones called metacarpals, upon which are set the four ?ngers, each containing the three bones known as phalanges, and the thumb with two phalanges.

The lower limb consists similarly of the region of the hip-bone and three segments – the thigh, the leg and the foot. The hip-bone is a large ?at bone made up of three – the ilium, the ischium and the pubis – fused together, and forms the side of the pelvis or basin which encloses some of the abdominal organs. The thigh contains the femur, and the leg contains two bones – the tibia and ?bula. In the tarsus are seven bones: the talus (which forms part of the ankle joint); the calcaneus or heel-bone; the navicular; the lateral, intermediate and medial cuneiforms; and the cuboid. These bones are so shaped as to form a distinct arch in the foot both from before back and from side to side. Finally, as in the hand, there are ?ve metatarsals and 14 phalanges, of which the great toe has two, the other toes three each.

Besides these named bones there are others sometimes found in sinews, called sesamoid bones, while the numbers of the regular bones may be increased by extra ribs or diminished by the fusion together of two or more bones.... bone

CEREBRUM This forms nearly 70 per cent of the brain and consists of two cerebral hemispheres which occupy the entire vault of the cranium and are incompletely separated from one another by a deep mid-line cleft, the longitudinal cerebral ?ssure. At the bottom of this cleft the two hemispheres are united by a thick band of some 200 million crossing nerve ?bres

– the corpus callosum. Other clefts or ?ssures (sulci) make deep impressions, dividing the cerebrum into lobes. The lobes of the cerebrum are the frontal lobe in the forehead region, the parietal lobe on the side and upper part of the brain, the occipital lobe to the back, and the temporal lobe lying just above the region of the ear. The outer 3 mm of the cerebrum is called the cortex, which consists of grey matter with the nerve cells arranged in six layers. This region is concerned with conscious thought, sensation and movement, operating in a similar manner to the more primitive areas of the brain except that incoming information is subject to much greater analysis.

Numbers of shallower infoldings of the surface, called furrows or sulci, separate raised areas called convolutions or gyri. In the deeper part, the white matter consists of nerve ?bres connecting di?erent parts of the surface and passing down to the lower parts of the brain. Among the white matter lie several rounded masses of grey matter, the lentiform and caudate nuclei. In the centre of each cerebral hemisphere is an irregular cavity, the lateral ventricle, each of which communicates with that on the other side and behind with the third ventricle through a small opening, the inter-ventricular foramen, or foramen of Monro.

BASAL NUCLEI Two large masses of grey matter embedded in the base of the cerebral hemispheres in humans, but forming the chief part of the brain in many animals. Between these masses lies the third ventricle, from which the infundibulum, a funnel-shaped process, projects downwards into the pituitary body, and above lies the PINEAL GLAND. This region includes the important HYPOTHALAMUS.

MID-BRAIN or mesencephalon: a stalk about 20 mm long connecting the cerebrum with the hind-brain. Down its centre lies a tube, the cerebral aqueduct, or aqueduct of Sylvius, connecting the third and fourth ventricles. Above this aqueduct lie the corpora quadrigemina, and beneath it are the crura cerebri, strong bands of white matter in which important nerve ?bres pass downwards from the cerebrum. The pineal gland is sited on the upper part of the midbrain.

PONS A mass of nerve ?bres, some of which run crosswise and others are the continuation of the crura cerebri downwards.

CEREBELLUM This lies towards the back, underneath the occipital lobes of the cerebrum.

MEDULLA OBLONGATA The lowest part of the brain, in structure resembling the spinal cord, with white matter on the surface and grey matter in its interior. This is continuous through the large opening in the skull, the foramen magnum, with the spinal cord. Between the medulla, pons, and cerebellum lies the fourth ventricle of the brain.

Structure The grey matter consists mainly of billions of neurones (see NEURON(E)) in which all the activities of the brain begin. These cells vary considerably in size and shape in di?erent parts of the brain, though all give o? a number of processes, some of which form nerve ?bres. The cells in the cortex of the cerebral hemispheres, for example, are very numerous, being set in layers ?ve or six deep. In shape these cells are pyramidal, giving o? processes from the apex, from the centre of the base, and from various projections elsewhere on the cell. The grey matter is everywhere penetrated by a rich supply of blood vessels, and the nerve cells and blood vessels are supported in a ?ne network of ?bres known as neuroglia.

The white matter consists of nerve ?bres, each of which is attached, at one end, to a cell in the grey matter, while at the other end it splits up into a tree-like structure around another cell in another part of the grey matter in the brain or spinal cord. The ?bres have insulating sheaths of a fatty material which, in the mass, gives the white matter its colour; they convey messages from one part of the brain to the other (association ?bres), or, grouped into bundles, leave the brain as nerves, or pass down into the spinal cord where they end near, and exert a control upon, cells from which in turn spring the nerves to the body.

Both grey and white matter are bound together by a network of cells called GLIA which make up 60 per cent of the brain’s weight. These have traditionally been seen as simple structures whose main function was to glue the constituents of the brain together. Recent research, however, suggests that glia are vital for growing synapses between the neurons as they trigger these cells to communicate with each other. So they probably participate in the task of laying down memories, for which synapses are an essential key. The research points to the likelihood that glial cells are as complex as neurons, functioning biochemically in a similar way. Glial cells also absorb potassium pumped out by active neurons and prevent levels of GLUTAMATE – the most common chemical messenger in the brain – from becoming too high.

The general arrangement of ?bres can be best understood by describing the course of a motor nerve-?bre. Arising in a cell on the surface in front of the central sulcus, such a ?bre passes inwards towards the centre of the cerebral hemisphere, the collected mass of ?bres as they lie between the lentiform nucleus and optic thalamus being known as the internal capsule. Hence the ?bre passes down through the crus cerebri, giving o? various small connecting ?bres as it passes downwards. After passing through the pons it reaches the medulla, and at this point crosses to the opposite side (decussation of the pyramids). Entering the spinal cord, it passes downwards to end ?nally in a series of branches (arborisation) which meet and touch (synapse) similar branches from one or more of the cells in the grey matter of the cord (see SPINAL CORD).

BLOOD VESSELS Four vessels carry blood to the brain: two internal carotid arteries in front, and two vertebral arteries behind. These communicate to form a circle (circle of Willis) inside the skull, so that if one is blocked, the others, by dilating, take its place. The chief branch of the internal carotid artery on each side is the middle cerebral, and this gives o? a small but very important branch which pierces the base of the brain and supplies the region of the internal capsule with blood. The chief importance of this vessel lies in the fact that the blood in it is under especially high pressure, owing to its close connection with the carotid artery, so that haemorrhage from it is liable to occur and thus give rise to stroke. Two veins, the internal cerebral veins, bring the blood away from the interior of the brain, but most of the small veins come to the surface and open into large venous sinuses, which run in grooves in the skull, and ?nally pass their blood into the internal jugular vein that accompanies the carotid artery on each side of the neck.

MEMBRANES The brain is separated from the skull by three membranes: the dura mater, a thick ?brous membrane; the arachnoid mater, a more delicate structure; and the pia mater, adhering to the surface of the brain and containing the blood vessels which nourish it. Between each pair is a space containing ?uid on which the brain ?oats as on a water-bed. The ?uid beneath the arachnoid membrane mixes with that inside the ventricles through a small opening in the fourth ventricle, called the median aperture, or foramen of Magendie.

These ?uid arrangements have a great in?uence in preserving the brain from injury.... divisions

This is one of the most common of the congenital (present at birth) malformations. It is one of the three types of neural-tube anomaly, the other two being ANENCEPHALY and cranium bi?dum. It takes two main forms – spina bi?da occulta being much the commoner. There is a de?cit in the posterior part of the SPINAL COLUMN, usually in the LUMBAR region, and it is generally asymptomatic unless the underlying spinal cord is affected. Occasionally it is associated with a hairy patch or birthmark on the back, and a few children develop a mild spastic gait or bladder problems.

Much more serious is spina bi?da cystica, in which the spinal-wall defect is accompanied by a protrusion of the spinal cord. This may take two forms: a meningocele, in which the MENINGES, containing CEREBROSPINAL FLUID, protrude through the defect; and a meningomyelocele, in which the protrusion contains spinal cord and nerves.

Meningocele is less common and has a good prognosis. HYDROCEPHALUS and neurological problems affecting the legs are rare, although the bladder may be affected. Treatment consists of surgery which may be in the ?rst few days of life or much later depending upon the precise situation; long-term follow-up is necessary to pick up any neurological problems that may develop during subsequent growth of the spine.

Meningomyelocele is much more serious and more common, accounting for 90 per cent of all cases. Usually affecting the lumbo-sacral region, the range of severity may vary considerably and, while early surgery with careful attention in a minor case may achieve good mobility, normal bladder function and intellect, a more extensive protrusion may cause complete ANAESTHESIA of the skin, with increased risk of trauma; extensive paralysis of the trunk and limbs, with severe deformities; and paralysis and insensitivity of the bladder and bowel. Involuntary movements may be present, and hydrocephalus occurs in 80 per cent of cases. The decision to operate can only be made after a full examination of the infant to determine the extent of the defect and any co-existent congenital abnormalities. The child’s potential can then be estimated, and appropriate treatment discussed with the parents. Carefully selected patients should receive long-term treatment in a special centre, where full attention can be paid to all their various problems.

There is growing evidence of the value of vitamin supplements before and during pregnancy in reducing the incidence of spina bi?da. Parents of affected infants may obtain help, advice, and encouragement from the Association for Spina Bi?da and Hydrocephalus which has branches throughout the country, or the Scottish Spina Bi?da Association.... spina bifida

An operation in which a portion of the CRANIUM is removed. Originally the operation was performed with an instrument resembling a carpenter’s brace and known as the trephine or trepan, which removes a small circle of bone; but now this instrument is only used, as a rule, for making small openings, whilst, for wider operations, gouge forceps, circular saws driven by electric motor, or wire saws are used.

Trepanning is used in cases of fracture, with splintering of the skull; the operation is performed to remove fragments of bone and any foreign bodies, like a bullet, which may have entered. In compression of the brain with unconsciousness following an injury, the skull is trephined and any blood clots removed, or torn vessels ligatured. The operation may also be done for an ABSCESS within the skull and for other conditions where operative access to the brain is required.... trepanning

Twelve pairs of nerves that emerge directly from the underside of the brain. Each of the nerves has a number as well as a name. The numbers indicate the sequence in which the nerves emerge from the brain.

The main function of some cranial nerves is to deliver sensory information from the ears, nose, and eyes to the brain.

These are the vestibulocochlear nerve (hearing and balance), olfactory nerve (smell), and optic nerve (vision). Other cranial nerves carry impulses that move muscles of the head and neck area. These are the oculomotor, trochlear, and abducent nerves (movements of the eye), spinal accessory nerve (head and shoulder movements), and hypoglossal nerve (tongue movements).

Some cranial nerves have both sensory and motor functions. These are the facial nerve (facial expressions, taste, and the secretion of saliva and tears) trigeminal nerve (facial sensation and jaw movements) and glossopharyngeal nerve (taste and swallowing movements). The vagus nerve has branches to all the main digestive organs, the heart, and the lungs, and is a major component of the parasympathetic nervous system, which is concerned with maintaining the body’s automatic functions.

All but 2 of the cranial nerve pairs connect with nuclei in the brainstem (the olfactory and optic nerves link directly with parts of the cerebrum). The nerves emerge through openings in the cranium; many then soon divide into branches.... cranial nerves

n. the structures that cover the cranium, i.e. all layers of the scalp.... epicranium

a bone in the floor of the cranium that contributes to the nasal cavity and orbits. The part of the ethmoid forming the roof of the nasal cavity – the cribriform plate – is pierced with many small holes through which the olfactory nerves pass. See also nasal concha; skull.... ethmoid bone

n. a bilateral symmetrical hypertrophy of the bones of the face and cranium, said to resemble the appearance of a lion’s head. It is a rare feature of untreated *Paget’s disease; the cause is unknown. Medical name: leontiasis ossea.... leontiasis

n. the lower jawbone. It consists of a horseshoe-shaped body, the upper surface of which bears the lower teeth (see alveolus), and two vertical parts (rami). Each ramus divides into a condyle and a *coronoid process. The condyle articulates with the temporal bone of the cranium to form the temporomandibular joint (a hinge joint). See also maxilla; skull. —mandibular adj.... mandible

the space inside the nose that lies between the floor of the cranium and the roof of the mouth. It is divided into two halves by a septum: each half communicates with the outside via the nostrils and with the nasopharynx through the posterior nares.... nasal cavity

n. 1. (in surgery) a strip of tissue dissected away from the underlying structures but left attached at one end so that it retains its blood and nerve supply in a *pedicle. The flap is then used to repair a defect in another part of the body by suturing its free end into the area. When the flap has ‘healed into’ its new site the other end can be detached and the remainder of the flap can be sewn in, depending on the type of flap being used. Flaps are commonly used by plastic surgeons in treating patients who have suffered severe skin and tissue loss after mutilating operations (e.g. mastectomy; see TRAM flap) or after burns or injuries not amenable to repair by split skin grafting (see skin graft). Skin flaps may also be used to cover the end of a bone in an amputated limb. In neurosurgery combined skin and bone (osteoplastic) flaps are commonly raised to provide access to the cranium. 2. (in dentistry) a piece of mucous membrane and periosteum attached by a broad base. It is lifted back to expose the underlying bone and enable a procedure such as surgical *extraction or *apicectomy to be performed. It is subsequently replaced and stabilized using sutures during the healing period.... flap

a group of congenital abnormalities caused by failure of the *neural tube to form normally. In *spina bifida the bony arches of the spine, which protect the spinal cord and its coverings (the meninges), fail to close. More severe defects of fusion of these bones will result in increasingly serious neurological conditions. A meningocele is the protrusion of the meninges through the gap in the spine, the skin covering being vestigial. There is a constant risk of damage to the meninges, with resulting infection. Urgent surgical treatment to protect the meninges is therefore required. In a meningomyelocele (myelomeningocele, myelocele) the spinal cord and the nerve roots are exposed, often adhering to the fine membrane that overlies them. There is a constant risk of infection and this condition is accompanied by paralysis and numbness of the legs and urinary incontinence. *Hydrocephalus and an *Arnold–Chiari malformation are usually present. A failure of fusion at the cranial end of the neural tube (cranium bifidum) gives rise to comparable disorders. The bone defect is most often in the occipital region of the skull but it may occur in the frontal or basal regions. A protrusion of the meninges alone is known as a cranial meningocele. The terms meningoencephalocele, encephalocele, and cephalocele are used for the protrusion of brain tissue through the skull defect. This is accompanied by severe mental and physical disorders.... neural tube defects

a saucer-shaped bone of the *skull that forms the back and part of the base of the cranium. At the base of the occipital are two occipital condyles: rounded surfaces that articulate with the first (atlas) vertebra of the backbone. Between the condyles is the foramen magnum, the cavity through which the spinal cord passes.... occipital bone

a bone forming the base of the cranium behind the eyes. It consists of a body, containing air spaces continuous with the nasal cavity (see paranasal sinuses); two wings that form part of the orbits; and two pterygoid processes projecting down from the point where the two wings join the body. See skull.... sphenoid bone

either of a pair of bones of the cranium. The squamous portion forms part of the side of the cranium. The petrous part contributes to the base of the skull and contains the middle and inner ears. Below it are the *mastoid process, *styloid process, and zygomatic process (see zygomatic arch). See also skull.... temporal bone