The human skull is a bony structure which forms the head of the human skeleton. It supports the structures of the face and forms a cavity for the brain. As is the function of the skull in every other vertebrate, its job is to protect the brain from injury. The skull consists of three parts, of differing embryological origin: the neurocranium, the sutures and the facial skeleton which are also called membraneous viscerocranium. The neurocranium – also known as the braincase – forms the protective cranial cavity that surrounds and houses the brain and the brainstem. The upper reaches of the cranial bones forms the calvaria, informally known as the skullcap. Within the membraneous viscerocranium are included the mandibles, also known as the jawbones. The sutures are fairly rigid joint bones of the neurocranium, whereas the facial skeleton is formed by the bones supporting the face.
Barring the mandibles, all of the bones of the skull are joined together by the sutures – synathroadal joints which are created out of bony ossification, with Sharpey’s fibres providing some amount of flexibility. At times there can be extra bone pieces with the suture that are called the wormian bones or sutural bones. Often, they are to be found in the course of the lambdoid suture. Generally, the human skull is considered to be made up of twenty-two bones, which can be further divided into eight cranial bones and fourteen skeleton bones.
Being s complex structure, the bones of the skull are formed both by intramembranous and endochondral ossification. At birth, the human skull is made up of 44 separate bony elements, many of which eventually fuse together into solid bones. Being such an important part of human existence and the human experience, the human skull has attained cultural significance in various cultures all around the world for many reasons, symbolizing many things; the most common being a motif of mortality.
The neurocranium is usually divided into two broad sections: a membranous part which consist of flat bones and which surround the brain and a cartilaginous part – the chondrocranium – which constitute the bones at the base of the skull. In human beings, the neurocranium is usually made up of eight bones, namely, the ethmoid bone, the frontal bone, the occipital bone, a pair of parietal bones, the sphenoid bone and a pair of temporal bones. Usually the ossicles, which number three on each side, are not considered as bones of the neurocranium. As a variation, there also might be the presence of extra sutural bones. Below the neurocranium is a complex of openings called the foramina and bones, including the foramen magnum that houses the spine. Located in the same region are the auditory bullae that aid in hearing.
As it has been mentioned the function of the neurocranium enables the brain to be housed and prevents from any kind of trauma or injury. Hence, the bones making up the neurcoranium have to be strong and durable that is able to withstand most types of trauma. However, it can be affected by severe physical trauma if enough force is applied on the skull with the intent to damage it.
Apart from physical trauma inflicted by forces external to the skullcap, there a number of diseases which when affecting the skullcap can prove itself to be fatal. Some of these include meningitis which is usually caused due to a fungal, bacterial or viral infection of the meninges located just beneath the neurocranium and within the brain civility that houses the brain. Often, the size of the neurocranium’s capacity is considered to be the determinant of a species’ level of intelligence and understanding the present cranial volume in humans has been the creation of evolutionary processes and environmental adaptations over the course of time.
The facial skeleton constitutes a total of fourteen individual bones. They are vomer, two inferior nasal conchae, a pair of nasal bones, a pair of maxilla, the mandible, two palatine bones, a pair of zygomatic bones and two lacrimal bones. Depending on the source, sometimes a pair of bones are considered to be a single bone or the maxilla being thought of as a bone possessing two bones as its parts. Again, some sources include the hyoid bone – most commonly called the three ossicles of the middle ear, although such considerations seem to be not accepted by a majority of the scientific and medical community around the world. Often, the ethmoid bone and the sphenoid bone as well are sometimes included when classifying the constituents of the facial skeleton although they are usually considered to be a part of the neurocranium. Since the maxillary bones are fused, they are also often classified as a single bone. On the other hand, the mandible are thought to be separate from the rest of the cranium.
The facial skeleton is composed of dermal bone and are ultimately derived from neural crest cells or from the sclerotomem, which in turn is derived from the somite block of the mesoderm. The variation observed in craniofacial features are largely a result of differing patterns of biological inheritance. Research has identified specific genes that control the nature and extent of this craniofacial feature within every human. These craniofacial features are often the building blocks which lead to the creation of an individual’s unique, physical identity and which helps human beings to distinguish each other through visual means. The facial skeleton also helps in the consumption of foodstuffs and beverages by means of the mandibles that help a person to bite their food into digestible morsels. Thus the facial skeleton is singnificant in a number of miscellaneous ways.
In the realm of anatomy, the orbit is the name given to the cavity or socket located in the skull where the eye and its assorted appendages are situated. Although referable to any body socket, it is most popularly used in the medical community to refer to the cavities that fixes the eyes of the position. In an adult human, the volume of the orbit is 30 millilitres, of which the eye occupies only 6.5 millilitres. The orbital contents comprise the eye, the orbital and retrobulbar fascia, extraocular muscles, five cranial nerves (cranial nerves II, III, IV, V and VI), blood vessels, fat, the lacrimal gland with its sac and nasolacrimal duct, the eyelids, medial and lateral palpebral ligaments, check ligaments, the suspensory ligament, septum, ciliary ganglion and short ciliary nerves.
The orbits are conical or quadrangular pyramidal cavities that open into the midline of the face. And point back into the head.. Each orbit consists of a base, an apex and four walls. These bony walls of the orbital canal in human beings do not derive from a single but they are rather a mosaic of seven embryologically different structures: the zygomatic bone laterally, the sphenoid bone with its lesser wing forming the lateral posterior portion of the bony orbital process, the maxillary bone inferiorly and medially which, along with the lacrimal and ethmoid bones, forms the medial wall of the orbital canal. The thmoid air cells are extremely thin, forming a structure known as the lamina papyracea, the most delicate bony structure in the entire skull and which also happens to be one of the most commonly fractured bones in the event of orbital trauma. The lacrimal bone also contains the nasolacrimal duct. The superior body margin of the orbital rim, also known as the orbital process, is formed by the frontal lobe. Thus, these are some of the few physical characteristics of the orbit and their location in the skull.
The foramina are various openings in the skull, their primary function and purpose are to act as openings for nerves and blood vessels in order to connect the brain with the rest of the body via the spinal cord. There are numerous foramen and they fulfil various functions for the contents of the skull. The supraorbital foramen is a bony, elongated opening located above the orbit and under the forehead, lying directly under the eyebrow. Often this foramen can found to be incomplete and is then termed as the supraorbital notch. This foramen is responsible for allowing the supraorbital nerve to access the orbits and to the eyes. The foramina of the cribriform plate allows for the olfactory nerve bundles to have access to the nose. The optical canal is one of the most important foramen in the skull as it allows passage to the optic canal from the eyes to the brain. Of all the foramen, the foramen magnum is the most significant as not only does it enable the anterior and posterior spinal arteries and vertebral arteries to pass through, it also allows to house the lowest part of the medulla oblongata and the passage of the accessory nerve. As far as blood flow to the face and the control of facial muscles to express various emotions are concerned, the stylomastoid foramen is the opening through which not only does the stylomastoid artery pass through, it also lets the facial nerve to pass through and it therefore enables the person to have control over what they are able to express. Therefore, the purpose of foramen is important as far as the anatomy of the skull is concerned for human beings as they allow a person to do a lot of things without encountering any issues and thus survive through the environment with a strong mental and physical bond created through blood vessels and nerves.
A suture is a type of fibrous joint that is only found in the skull, hence the reason that they are also called cranial sutures. These bones are bound together by Sharpey’s fibres. A tiny amount of movement is permitted within sutures, which accounts for the compliance and elasticity of the skull. These joints are called synarthroses. Normally, many of the bones at birth are observed to remain unfused and the process of this unfused bones is known as craniosynostosis. The term fontanelle is used to describe the consequent soft spots. The relative positions of the bones continue to change throughout the life of the adult, even if it is less rapid in nature. This phenomena has found itself to be of particular use in providing crucial information in such fields as forensics and archaeology. In old age, it is possible for these cranial sutures to undergo complete ossification, that it, these sutures may completely become bone. An exception to these cranial sutures are the joints between the gumphoses and the temporomandibular joint, where one may not find any kind of sutures whatsoever.
The various kinds of sutures include the coronal suture, the lambdoid suture, the occipitomastoid suture, the sphenofrontal suture, the sphenoparietal suture, sphenosquamosal suture, sphenozygomatic suture, squamosal suture, zygomatictemporal suture, zygomatic frontal suture, frontal suture, sagittal suture, frontoenthmoidal suture, petrosquamos suture, sphenoethmoidal suture and the sphenopetrosal suture. All of these sutures combined allows a person to move their skull and thereby contribute greatly to head movement in order to enhance the body gestures with which a person is able to express their non-verbal cues and communication. These sutures also further protect the brain and related vital organs nestled in the skull from injury or dislocation by keeping the skull into a compact, fixed skeletal unit that protects the essential organs from inflicting any kind of harm upon itself.
The paranasal sinuses are a group of four air-filled spaces that surround the nasal cavity. The maxillary sinuses are located under the eyes, the frontal sinuses are located above the eyes, the ethmoidal sinuses are between the eyes and the sphenoidal are behind the eyes. These sinuses are named for the facial bones in which they are located. Humans possess four-paired sinuses, divided into subgroups that are, as mentioned, named according to the bones in which they are essentially located. The largest of the sinuses, the maxillary sinuses, are located under the eyes and within the maxillary bones that are open at the back of the semilunar hiatus of the nose. They are innervated by the trigeminal nerve. The frontal sinuses are located above the eyes and within the frontal bone. They are too innervated by the trigeminal nerve. The ethmoidal nerve, which are formed from numerous discrete air-cells within the ethmoid bone between the nose and the eyes. They are innervated by the ethmoidal nerves, which in turn branch from the nasociliary nerve of the trigeminal nerve. Lastly, the sphenoidal sinuses, located in the sphenoid bone, are innervated by the trigeminal nerve as well. All of the sinuses are lined with repertory epithelium cells. The purposes of the sinuses is to provide a measure of balance to the brain, so that it does not crush the vertebrae upon which the skull is supported upon. Thus, sinuses act as an air-filled counter-weight which balances out the weight of the brain occupying the neurocranium.
A fontanelle,, informally known as soft-spots, is an anatomical feature of the infant human skull which comprises any of the membranous gaps between the cranial bones and that make up the calvaria of a foetus or infant. These allow for the rapid stretching and deformation of the neurocranium as the brain expands faster than the surrounding brain can grow. Premature complete ossification of the sutures is called craniosynotosis. After the period of infancy, the anterior fontanelle is known as the bregina.
There are three major forms of fontanelles. The posterior fontanelle is a triangle in shape. It lies at the junction of the sagittal suture and lambdoid suture. At birth, the skull features a small posterior fontanelle with an open area covered by a tough membrane, where the two parietal bones adjoin the occipital bone, at the lambda. The anterior fontanelle is a diamond-shaped membrane-filled space located between the two frontal and two parietal bones of the developing foetal skull. It persist uptil approximately eighteen months after birth. It is at the junction of the coronial suture and sagittal suture. The foetal anterior fontanelle may be palpated until eighteen months. Two smaller fontanelles are located on each side of the head, more anteriorly the sphenodial or anterolateral fontanelle (between the temporal, occipital and parietal bones).
During birth, fontanelles enable the bony plates of the skull to flex, allowing the child’s head to pass through the birth canal. The ossification of the bones of the skull causes the anterior fontanelle to close over by nine to eigjhteen months. The sphenoidal and posterior fontanelles close during the first few months of life. The closures eventually form the sutures of neurocranium. Other than the anterior and posterior fontanelles, the mastoid fontanelle and sphenoidal fontanelle are also significant.
The sign of a normal and usual birth is the pulsating of the fontanelle. It is not known why does this occur but it is thought that it is caused by the heartbeats via perhaps the arterial pulse within the brain vasculature, or in the meninges. This pulsating action is how the fontanelle was given the colloquial name of the soft spot, via the use of an old French word.
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