A small amount of movement is permitted through these sutures that contributes to the compliance and elasticity of the skull. The joint between the mandible and the cranium, known as the temporomandibular joint, forms the only non-sutured joint in the skull. Most sutures are named for the bones that they articulate. At birth, many of the bones of the skull remain unfused to the soft spots described as fontanelle.
The bones fuse relatively rapidly through a process known as craniosynotosis, although the relative positions of the bones can continue to change through life.
In old age the cranial sutures may ossify completely, reducing the amount of elasticity present in the skull. As such, the degree of ossification can be a useful tool in determining age postmortem. Lateral view of a skull showing sutures : The dotted red lines indicate the location of skull sutures.
Sometimes, they are called soft spots. These spaces are a part of normal development. The cranial bones remain separate for about 12 to 18 months.
They then grow together as part of normal growth. They stay connected throughout adulthood. The sutures and fontanelles are needed for the infant's brain growth and development. During childbirth, the flexibility of the sutures allows the bones to overlap so the baby's head can pass through the birth canal without pressing on and damaging their brain. During infancy and childhood, the sutures are flexible. This allows the brain to grow quickly and protects the brain from minor impacts to the head such as when the infant is learning to hold his head up, roll over, and sit up.
Except for the mandible, all skull bones are joined together by sutures —synarthrodial immovable joints. The skull contains air-filled cavities called sinuses. Their functions are debatable, but may be related to lessening skull weight, contributing to voice resonance, and warming and moistening inspired air. Key Terms viscerocranium : The skeleton that supports facial structure. Cranial Bones The neurocranium is comprised of eight bones: occipital, two temporal bones, two parietal bones, sphenoid, ethmoid, and the frontal bone.
Learning Objectives Differentiate the bones of the neurocranium. Key Takeaways Key Points The eight bones of the neurocranium form major portions of the skull and protect the brain. The neurocranium consists of two temporal bones situated to the base and side of the skull, and two parietal bones that make up the roof of the skull.
A single occipital bone forms the base of the skull, and the frontal bone forms the forehead. The sphenoid and ethmoid bones located to the front of the skull form parts of the orbital sockets and nasal cavity; they also support and protect key organs found in the skull. Key Terms neurocranium : The part of the skull that encloses and protects the brain and brain stem.
Facial Bones The viscerocranium face includes these bones: vomer, 2 inferior nasal conchae, 2 nasals, maxilla, mandible, palatine, 2 zygomatics, and 2 lacrimals. Learning Objectives List the facial bones of the viscerocranium. Key Takeaways Key Points The several bones of the viscerocranium are joined by sutures to each other and the neurocranium, except for the mandible, which articulates with the temporal bones.
The hyoid bone, ethmoid bone, and sphenoid bones are sometimes included in the viscerocranium. Key Terms viscerocranium : The facial skeleton that is formed by the bones of the anterior and lower skull, which are derived from branchial arches. Orbits The orbit is the cavity or socket of the skull in which the eye and its appendages are situated. Learning Objectives Locate the orbits in the skull.
Key Takeaways Key Points The orbits are conical or four-sided pyramidal cavities, which open into the midline of the face and point backwards. To the rear of the orbit the optical foramen opens into the optic canal, which transmits the optic nerve and opthalmic artery. The orbit protects the eye from mechanical injury and provides access for the optic nerve to the brain.
Key Terms optic canal : The canal that transmits the optic nerve and ophthalmic artery into the orbital cavity. Foramina The human skull has numerous holes known as foramina through which cranial nerves, arteries, veins, and other structures pass. Learning Objectives Describe the purpose of foramina in the skull.
Key Takeaways Key Points A foramen plural: foramina is an opening inside the body that allows key structures to connect one part of the body to another. The skull bones that contain foramina include the frontal, ethmoid, sphenoid, maxilla, palatine, temporal, and occipital. There are 21 foramina in the human skull. Key Terms foramina : The openings inside the body that typically allow muscles, nerves, arteries, veins, or other structures to connect one part of the body to another. Sutures A suture is a type of fibrous joint or synarthrosis that only occurs in the skull or cranium.
Learning Objectives List the sutures of the skull. Key Takeaways Key Points It is normal for many of the bones of the skull to be unfused at birth. This allows a tiny amount of movement at the sutures, which contributes to the compliance and elasticity of the skull. Sutures become fused as individuals age; thus, examining sutures can provide an estimate of age postmortem. There are 17 named sutures on the human skull. Key Terms suture : A fairly rigid joint between two or more hard elements, such as the bony plates of the skull.
Paranasal Sinuses The paranasal sinuses four, paired, air-filled spaces surround the nasal cavity, and are located above and between the eyes, and behind the ethmoids. Learning Objectives Describe the structure and function of the paranasal sinuses. Key Takeaways Key Points The sinuses are named for the facial bones that they are located behind.
There are four sinuses; the maxillary, frontal, ethmoid, and sphenoid. Paranasal sinuses form developmentally through excavation of bone by air-filled sacs from the nasal cavity. The biological role of the sinuses is debated, but a number of possible functions have been proposed: decreasing weight of the face; increasing resonance of voice; buffer against blows; insulating sensitive structures from rapid temperature fluctuations; and humidifying and heating of inhaled air.
Key Terms paranasal sinuses : A group of four, paired, air-filled spaces that surround the nasal cavity maxillary sinuses , above the eyes frontal sinuses , between the eyes ethmoid sinuses , and behind the eyes sphenoid sinuses.
Learning Objectives Identify the evolutionary purpose of skull fontanelles. As might be expected all showed simple coronal sutures. The other sutures, however, varied as is shown in the following table:. Two other observations were made. For one point, in these skulls sutural development was by regions more than along the lines of the sutures. All had very simple sutures in the region of bregma, and considerable more complexity in the region of lambda.
It was frequently noted that the sagittal suture would be very simple at bregma but as lambda was approached it would become very complex. The same was true of the lambdoidal suture; it would be simple towards the ends but complex in the region of lambda. It was this condition that usually accounted for the skulls being classified as having equally complex sagittal and lambdoidal sutures. It seems very likely that there is some relationship between this condition and the occipital deformation characteristic of all the skulls, but since we have no undeformed cliff-dweller skulls it is impossible to say whether the complexity around lambda is entirely due to the deformation or whether it is natural.
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