Skull, Cranial Bones (1) PDF

SKULL, CRANIAL BONES  Skull The skull is a bony structure that supports the face and forms a protective cavity for the brain. It is comprised of many bones, which are joined by sutures (fibrous joints). Consists of approximately 30 bones anatomica...

SKULL, CRANIAL BONES  Skull The skull is a bony structure that supports the face and forms a protective cavity for the brain. It is comprised of many bones, which are joined by sutures (fibrous joints). Consists of approximately 30 bones anatomically divided into the cranial bones (neurocranium) the facial bones (viscerocranium)  Cranium The superior aspect of the skull forms the cranium (also known as the neurocranium). It encloses and protects the brain, meninges, and cerebral vasculature. Anatomically, the cranium can be subdivided into a roof and a base: Cranial roof – comprised of the frontal, occipital and two parietal bones. It is also known as the calvarium. Cranial base – comprised of the frontal, sphenoid, ethmoid, occipital, parietal, and temporal bones. These bones articulate with the 1st cervical vertebra (atlas), the facial bones, and the mandible (jaw).  skull Sutures of the Skull Sutures are a type of fibrous joint that are unique to the skull. They are immovable and fuse completely around the age of 20. The main sutures in the adult skull are: Coronal suture – fuses the frontal bone with the two parietal bones. Sagittal suture – fuses both parietal bones to each other. Lambdoid suture – fuses the occipital bone to the two parietal bones.  skull infant's neurocranium Consists of five separate bones (two frontal bones, two parietal bones, and one occipital bone) held together by connective tissue sutures. This allows for stretching and deformation of the skull to facilitate birth and accommodate the brain's growth at a faster rate than the surrounding bone.  skull In neonates, the incompletely fused suture joints give rise to membranous gaps between the bones, known as fontanelles. The two major fontanelles are: Frontal (Anterior) fontanelle – located at the junction of the coronal and sagittal sutures Occipital (Posterior) fontanelle – located at the junction of the sagittal and lambdoid sutures  Frontal bone The frontal bone is one of the eight bones that make up the cranium – the superior aspect of the skull that encloses and protects the brain. Its name is derived from the Latin ‘frons’, meaning ‘forehead’. The frontal bone is an unpaired, ‘shell-shaped’ bone that forms the anterosuperior aspect of the cranium. It consists of four parts: squamous orbital (x2) nasal.  Frontal bone Squamous Part The squamous part is the largest component of the frontal bone, with its external convex surface forming the shape of the forehead. The features of its internal surface include: Sagittal sulcus – vertical groove in the midline of the frontal bone. It contains the superior sagittal sinus. Frontal crest – an extension of the sagittal sulcus edges.  Frontal bone Squamous Part The squamous part also houses the frontal sinuses. They sit above the orbits either side of the midline of the face. It is separated from the orbital part by the supraorbital margin (The supraorbital margin, or superior margin of each orbit). This area contains the supraorbital foramen, where the supraorbital vessels and nerve pass through.  Frontal bone Orbital Part The two orbital parts of the frontal bone are comprised of a triangular-shaped flat plate that extends posteriorly from the supraorbital margin – forming the roof of the orbit. anterolateral portion consists of the lacrimal fossa (where the lacrimal gland sits).  Frontal bone Nasal Part The nasal part of the frontal bone is a small area that projects inferiorly between the supraorbital margins. It serves as an area of articulation for the nasal bones and frontal processes of the maxilla.  Frontal bone Articulations The frontal bone articulates with twelve other bones of the skull: Unpaired: ethmoid, sphenoid Paired: nasal, maxilla, lacrimal, parietal, zygomatic  Parietal bone The parietal bone has four borders which articulate with the surrounding skull bones via various sutures: Sagittal (superiorly) squamosal (inferiorly) frontal (anteriorly) occipital (posteriorly)  Parietal bone Sagittal border, located superiorly. The two parietal bones articulate at the sagittal borders with each other to form the sagittal suture. Squamosal border, situated inferiorly. The squamosal border comes in contact with three bony structures. From anterior to posterior these are the greater wing of the sphenoid bone and the squamous and petrous parts of temporal bone. These articulations form the sphenoparietal and parietomastoid sutures. Frontal border, located anteriorly. coronal suture. Occipital border - forms the of the lambdoid suture by articulating with the occipital bone.  Sphenoid bone ‘butterfly-shaped‘. It consists of: body, greater wings lesser wings two pterygoid processes.  Sphenoid bone Body The body lies at the centre of the sphenoid bone, and is almost completely cubical in shape. It contains the sphenoidal sinuses, which are separated by a septum – meaning that the sphenoid body is essentially hollow. The body articulates with the ethmoid bone anteriorly, and it is here that the sinuses open up into the nasal cavity.  Sphenoid bone Body The superior surface of the sphenoid body contains some important bony landmarks: Sella turcica – a saddle-shaped depression. Chiasmatic groove – a sulcus formed by the optic chiasm (where the optic nerves partially cross).  Sphenoid bone The sella turcica is surrounded by the anterior and posterior clinoid processes. The anterior clinoid processes arise from the sphenoidal lesser wings, while the posterior clinoid processes are the superolateral projections of the dorsum sellae.  Sphenoid bone Greater Wing The greater wing extends from the sphenoid body in a lateral There are three foramina present in the greater wing – the foramen rotundum, foramen ovale and foramen spinosum.  Sphenoid bone Foramen rotundum : passageway for the maxillary division of cranial nerve V Foramen ovale : passageway for the mandibular division of cranial nerve V Foramen spinosum : passageway for the middle meningeal artery  Sphenoid bone Lesser Wing The lesser wing arises from the anterior aspect of the sphenoid body in a superolateral direction. It separates the anterior cranial fossa from the middle cranial fossa. It also forms the lateral border of the optic canal – through which the optic nerve and ophthalmic artery travel to reach the eye. The medial border of the optic canal is formed by the body of the sphenoid. There is a ‘slit-like’ gap between the lesser and greater wings of the sphenoid – the superior orbital fissure. Numerous structures pass through here to reach the bony orbit.  Sphenoid bone Pterygoid Process The pterygoid process descends inferiorly from the point of junction between the sphenoid body and the greater wing. It consists of two parts: Medial pterygoid plate – supports the posterior opening of the nasal cavity. Lateral pterygoid plate – site of origin of the medial and lateral pterygoid muscles  Sphenoid bone Articulations Unpaired bones – Occipital, vomer, ethmoid and frontal bones. Paired bones – Temporal, parietal, zygomatic and palatine bones.  Ethmoid bone The ethmoid bone is a small unpaired bone, located in the midline of the anterior cranium Has spongy structure.  Ethmoid bone is situated at the roof of the nasal cavity, and between the two orbital cavities. It contributes to the medial wall of the orbit and forms part of the anterior cranial fossa, where it separates the nasal cavity (inferiorly) from the cranial cavity (superiorly). The olfactory nerve (CN I) has a close anatomical relationship with the ethmoid bone. Its numerous nerve fibres pass through the cribriform plate of the ethmoid bone to innervate the nasal cavity with the sense of smell.  Ethmoid bone The ethmoid bone is made up of three parts – the cribriform plate, the perpendicular plate, and the ethmoidal labyrinth. The cribriform plate forms the roof of the nasal cavity. It is pierced by numerous olfactory nerve fibers, which gives it a sieve-like structure. Another projection of bone descends from the cribriform plate – the perpendicular plate. It forms the superior two-thirds of the nasal septum.  Ethmoid bone the ethmoid bone contains two ethmoidal labyrinths. These are large masses located at either side of the perpendicular plate, which contain the ethmoidal air cells (sinuses). Two sheets of bone form each labyrinth: Orbital plate – the lateral sheet of bone, which also forms the medial wall of the orbit Medial sheet – forms the upper lateral wall of the nasal cavity, from which the superior and middle conchae extend into the nasal cavity.  Ethmoid bone Articulations The ethmoid bone articulates with 13 others: Paired – nasal bones, maxillae, lacrimal bones, palatine bones, inferior conchae. Unpaired – frontal, vomer and sphenoid bones.  Temporal bone The temporal bone contributes to the lower lateral walls of the skull. It contains the middle and inner portions of the ear, and is crossed by the majority of the cranial nerves. The lower portion of the bone articulates with the mandible, forming the temporomandibular joint of the jaw.  Temporal bone Anatomical Structure The temporal bone itself is comprised of five constituent parts. The squamous, tympanic, and petromastoid parts make up the majority of the bone, with the zygomatic and styloid processes projecting outwards.  Temporal bone Squamous Also known as the squama temporalis, this is the largest part of the temporal bone. It is flat and plate- like, located superiorly. The outer-facing surface of the squamous bone is convex in shape, forming part of the temporal fossa.  Temporal bone Zygomatic Process The zygomatic process arises from the lower part of the squama temporalis. It projects anteriorly, articulating with the temporal process of the zygomatic bone. These two structures form the zygomatic arch (palpable as ‘cheekbones’).  Temporal bone Tympanic The tympanic part of the temporal bone lies inferiorly to the squamous, and anteriorly to the petromastoid part. It surrounds the external auditory opening, which leads into the external auditory meatus of the external ear.  Temporal bone Styloid Process The styloid process located immediately underneath the opening to the auditory meatus. It acts as an attachment point for muscles and ligaments, such as the stylomandibular ligament of the TMJ.  Temporal bone Petromastoid Located posteriorly. mastoid part mastoid process, an inferior projection of bone, palpable just behind the ear. mastoid air cells. - hollowed out areas within the temporal bone. They act as a reservoir of air, equalizing the pressure within the middle ear in the case of auditory tube dysfunction. Petrous part The petrous part is pyramidal shaped, and lies at the base of temporal bone. It contains the inner ear. The mastoid air cells can also become infected, known as mastoiditis. mastoidi tis  Temporal bone Articulations A major articulation of the temporal bone is with the mandible (i.e. jaw bone) to form the temporomandibular joint. The squamous part of the temporal bone also articulates with the sphenoid bone and the parietal bone. The zygomatic process of the temporal bone also articulates with the zygomatic bone to form the zygomatic arch (i.e. cheekbones).  Occipital bone Unpaired bone Covers the back of the head (occiput). Makes up a large portion of the basilar part of the neurocranium  Occipital bone There are three parts to the occipital bone squamous condylar basilar.  Occipital bone Squamous The squamous part of the occipital bone is the largest and most posterior portion. It borders the parietal and temporal bones, and forms the posterior part of the foramen magnum.  Occipital bone Internal Surface The internal surface of the squamous part of the occipital bone is marked by grooves produced by the dural venous sinuses. The grooves are arranged like a cross and are known as the cruciform eminence. At the center of the cross is the internal occipital protuberance. Groove for transverse sinus – located either side of the internal occipital protuberance. Groove for superior sagittal sinus – extends from the internal occipital protuberance superiorly Internal occipital crest (occipital sinus) – extends from the internal occipital protuberance inferiorly.  Occipital bone External Surface The external surface is marked by four bony ridges, known as the nuchal lines. They represent the site of attachment for numerous muscles and ligaments. Supreme nuchal line Superior nuchal line Median nuchal line Inferior nuchal line  Occipital bone Condylar The condylar part of the occipital bone forms the lateral walls of the foramen magnum. It is comprised of two occipital condyles and serves as an articular surface with the atlas (at the atlanto-occipital joint). Condylar Occipital bone The inner side of the condyle base contains the hypoglossal canal, via which the hypoglossal nerve exits Projecting from the occipital condyles on either side are the jugular processes. A jugular notch lies anterior to these processes, forming the posterior part of the jugular foramen. The jugular foramen lies lateral to the hypoglossal canal, located at the junction of the occipital bone and the petrous portion of the temporal bone.  Occipital bone Basilar The basilar region of the occipital bone forms the anterior part of the foramen magnum. It articulates with the temporal and sphenoid bones.  Occipital bone Foramen Magnum The foramen magnum is a large opening in the occipital bone. It allows passage of structures to and from the cranial cavity. The medulla oblongata (part of the brainstem), accessory nerves, vertebral arteries pass through the foramen magnum.  Occipital bone Articulations The occipital bone articulates with two paired bones (parietal and temporal) and two unpaired bones (sphenoid bone and atlas): Parietal bones on either side superiorly to form the lambdoid suture. Mastoid portion of the temporal bone on each side inferolaterally to form the occipitomastoid structure. Sphenoid bone anteriorly. Atlas (C1) inferiorly at the atlanto-occipital joint.