Basic Skull Anatomy
- 1. Basic Skull Anatomy DR. HADI MUNIB ORAL AND MAXILLOFACIAL SURGERY RESIDENT
- 2. Outline • Introduction • Bones of the Skull • Composition • External Views of The Skull • Cranial Cavity • Vault of the skull • Base of the skull • Neonatal Skull • References
- 3. Introduction • The head is formed mainly by the skull with the brain and its covering meninges enclosed in the cranial cavity. • The brain gives rise to 12 pairs of cranial nerves, all the cranial nerves are distributed to structures in the head and neck, except the 10th, which supplies structures in the chest and abdomen.
- 4. Bones of the Skull • Several separate bones united at immobile joints called Sutures. • Exception • The Connective tissue between the bones is called a sutural ligament. • Cranium Bones – Vault and Base - 8 • Facial Bones – 14 • Skull bones are made of external and internal compact bone separated by spongy bone layer called the Diploë. • Spongy bone layer is thinner and more Brittle
- 6. Cranial Bones • Frontal bone: 1 • Parietal bones: 2 • Occipital bone: 1 • Temporal bones: 2 • Sphenoid bone: 1 • Ethmoid bone: 1
- 7. Facial Bones • Zygomatic bones: 2 • Maxillae: 2 • Nasal bones: 2 • Lacrimal bones: 2 • Vomer: 1 • Palatine bones: 2 • Inferior conchae: 2 • Mandible: 1
- 8. External views of the skull • Anterior View • Lateral View • Posterior View • Superior View • Inferior View
- 9. Anterior view of the skull • Frontal/ Forehead bone; Medial: Frontal Processes of the Maxilla and the nasal bone and Lateral: Zygomatic Bone • The Super-ciliary arches can be seen, and the supraorbital notch/ foramen.
- 10. Anterior view of the skull - 2 • The orbital margins; • Frontal bone superiorly • Zygomatic bone laterally • Maxilla inferiorly • Processes of the maxilla and frontal bone medially. • Above the orbital margins, are two hollow spaces lined with mucous membrane called the frontal air sinuses. • Air sinuses communicate with the nose and serve as voice resonators
- 11. Anterior view of the skull - 3 • The two nasal bones form the bridge of the nose. • Anterior nasal aperture; Lower borders of nasal bones and the Maxillae. • The nasal cavity is divided into two by the bony nasal septum, which is largely formed by the vomer. • The superior and middle conchae are shelves of bone that project into the nasal cavity from the ethmoid on each side • The inferior conchae are separate bones.
- 12. Anterior view of the skull - 4 • The two maxillae form the upper jaw, the anterior part of the hard palate, part of the lateral walls of the nasal cavities, and part of the floors of the orbital cavities. • The two bones meet in the midline at the inter-maxillary suture and form the lower margin of the nasal aperture. • The alveolar processes project downward to create the Alveolar Arch which carries the upper teeth. • Maxillary Sinus; a large, pyramid-shaped cavity lined with mucous membrane.
- 13. Anterior view of the Skull - 5 • Zygomatic Bone; the prominence of the cheek and part of the lateral wall and floor of the orbital cavity. • Medially; the maxilla and laterally; the Zygomatic process of the temporal bone to form the zygomatic arch. • The zygomatic bone is perforated by two foramina for the zygomatico- facial and zygomaticotemporal nerves. • The mandible; consists of a horizontal body and two vertical rami
- 15. Lateral View of The Skull • The frontal bone forms the anterior part of the side of the skull and articulates with the parietal bone at the coronal suture. • The parietal bones form the sides and roof of the cranium and articulate with each other in the midline at the sagittal suture, they articulate with the occipital bone behind, at the lambdoid suture. • The skull is completed at the side by the squamous part of the occipital bone; parts of the temporal bone, namely, the squamous, tympanic, mastoid process, styloid process, and zygomatic process; and the greater wing of the sphenoid. • The ramus and body of the mandible lie inferiorly. • The thinnest part of the lateral wall of the skull is where the antero-inferior corner of the parietal bone articulates with the greater wing of the sphenoid; this point is referred to as the pterion. • Clinically, the pterion overlies the anterior division of the middle meningeal artery and vein. • The superior and inferior temporal lines, which begin as a single line from the posterior margin of the zygomatic process of the frontal bone and diverge as they arch backward.
- 16. • The temporal fossa lies below the inferior temporal line. • The infratemporal fossa lies below the infratemporal crest on the greater wing of the sphenoid. • The pterygo-maxillary fissure is a vertical fissure that lies within the fossa between the pterygoid process of the sphenoid bone and back of the maxilla. It leads medially into the pterygopalatine fossa. • The inferior orbital fissure is a horizontal fissure between the greater wing of the sphenoid bone and the maxilla. It leads forward into the orbit. • The pterygopalatine fossa is a small space behind and below the orbital cavity, it communicates laterally with the infratemporal fossa through the pterygo- maxillary fissure, medially with the nasal cavity through the sphenopalatine foramen, superiorly with the skull through the foramen rotundum, and anteriorly with the orbit through the inferior orbital fissure. Lateral View of The Skull
- 18. Posterior View of the Skull • The posterior parts of the two parietal bones with the intervening sagittal suture. • Below, the parietal bones articulate with the squamous part of the occipital bone at the lambdoid suture. • On each side the occipital bone articulates with the temporal bone. • External Occipital Protuberance ; a roughened elevation in the midline of the occipital bone, which gives attachment to muscles and the ligamentum nuchae. • On either side of the protuberance the superior nuchal lines extend laterally toward the temporal bone.
- 19. Superior View of the Skull • The frontal bone articulates with the two parietal bones at the coronal suture. • Occasionally, the two halves of the frontal bone fail to fuse, leaving a midline metopic suture. • Behind, the two parietal bones articulate in the midline at the sagittal suture.
- 21. Inferior View of the Skull • If the mandible is discarded, the anterior part of this aspect of the skull is seen to be formed by the hard palate. • Incisive fossa and foramen; Anterior. Posterolaterally are the greater and lesser palatine foramina. • Above the posterior edge of the hard palate are the choanae (posterior nasal apertures). • These are separated from each other by the posterior margin of the vomer and are bounded laterally by the medial pterygoid plates of the sphenoid bone. • The inferior end of the medial pterygoid plate is prolonged as a curved spike of bone, the pterygoid Hamulus. • Posterolateral to the lateral pterygoid plate, the greater wing of the sphenoid is pierced by the large foramen ovale and the small foramen spinosum.
- 22. • Posterolateral to the foramen spinosum is the spine of the sphenoid. • Behind the spine of the sphenoid, in the interval between the greater wing of the sphenoid and the petrous part of the temporal bone, is a groove for the cartilaginous part of the auditory tube. • The mandibular fossa of the temporal bone and the articular tubercle form the upper articular surfaces for the temporomandibular joint. • Separating the mandibular fossa from the tympanic plate posteriorly is the squamotympanic fissure, through the medial end of which the chorda tympani nerve exits from the tympanic cavity. Inferior View of the Skull
- 23. • The styloid process of the temporal bone projects downward and forward from its inferior aspect. • The opening of the carotid canal can be seen on the inferior surface of the petrous part of the temporal bone. • The medial end of the petrous part of the temporal bone is irregular and, together with the basilar part of the occipital bone and the greater wing of the sphenoid, forms the foramen lacerum. • During life, the foramen lacerum is closed with fibrous tissue, and only a few small vessels pass through this foramen from the cavity of the skull to the exterior. • The tympanic plate, which forms part of the temporal bone, is C shaped on section and forms the bony part of the external auditory meatus. Inferior View of the Skull
- 24. • Suprameatal crest on the lateral surface of the squamous part of the temporal bone, the suprameatal triangle, and the suprameatal spine. • In the interval between the styloid and mastoid processes, the stylomastoid foramen can be seen. • Medial to the styloid process, the petrous part of the temporal bone has a deep notch together with a shallower notch on the occipital bone, forms the jugular foramen. • Behind the posterior apertures of the nose and in front of the foramen magnum are the sphenoid bone and the basilar part of the occipital bone. • The pharyngeal tubercle is a small prominence on the undersurface of the basilar part of the occipital bone in the midline. Inferior View of the Skull
- 25. • The occipital condyles should be identified; they articulate with the superior aspect of the lateral mass of the first cervical vertebra, the atlas. • Superior to the occipital condyle is the hypoglossal canal for transmission of the hypoglossal nerve. • Posterior to the foramen magnum in the midline is the external occipital protuberance. • The superior nuchal lines should be identified as they curve laterally on each side. Inferior View of the Skull
- 27. Cranial Cavity • Brain • Surroundings - Meninges • Portions of cranial nerves, arteries and veins • Venous Sinuses.
- 28. Vault of the Skull • The internal surface shows the coronal, sagittal, and lambdoid sutures. • In the midline a shallow sagittal groove that lodges the superior sagittal sinus is seen. • On each side of the groove are several small pits, called granular pits, which lodge the lateral lacunae and arachnoid granulations. • Several narrow grooves are present for the anterior and posterior divisions of the middle meningeal vessels as they pass up the side of the skull to the vault.
- 29. Base of the Skull • The interior of the base of the skull is divided into three cranial fossae: anterior, middle, and posterior. • The anterior cranial fossa is separated from the middle cranial fossa by the lesser wing of the sphenoid. • The middle cranial fossa is separated from the posterior cranial fossa by the petrous part of the temporal bone.
- 31. Anterior Cranial Fossa • Lodges the frontal lobes of the cerebral hemisphere. • Anterior: Inner surface of frontal bone • Middle: Crest of attachment of falx cerebri • Posterior: Lesser wing of sphenoid • Lesser wing of sphenoid articulates laterally with the frontal bone and meets the anteroinferior border of the parietal bone. • The medial end the lesser wing of the sphenoid forms the anterior clinoid process on each side, which gives attachment to the tentorium cerebelli. • The median part of the anterior cranial fossa is limited posteriorly by the groove for the optic chiasma.
- 32. Anterior Cranial Fossa • Floor: the ridged orbital plates of the frontal bone laterally and by the cribriform plate of the ethmoid medially • The crista galli is a sharp upward projection of the ethmoid bone in the midline for the attachment of the falx cerebri. • Alongside the crista galli is a narrow slit in the cribriform plate for the passage of the anterior ethmoidal nerve into the nasal cavity. • The upper surface of the cribriform plate supports the olfactory bulbs and the small perforations in the cribriform plate are for the olfactory nerves.
- 34. Middle Cranial Fossa • A small median part and expanded lateral parts. • The median raised part is formed by the body of the sphenoid • The expanded lateral parts form concavities on either side, which lodge the temporal lobes of the cerebral hemispheres. • Anterior: Lesser wings of the sphenoid • Posterior: Superior borders of the petrous parts of the temporal bones. • Lateral: Squamous parts of the temporal bones, the greater wings of the sphenoid, and the parietal bones. • Floor of the lateral part of the middle cranial fossa: Greater wing of the sphenoid and the squamous and petrous parts of the temporal bone.
- 35. Middle Cranial Fossa • Anteriorly, the optic canal transmits the optic nerve and the ophthalmic artery. • The superior orbital fissure- slitlike opening between the lesser and the greater wings of the sphenoid- transmits the lacrimal, frontal, trochlear, oculomotor, nasociliary, and abducent nerves, together with the superior ophthalmic vein. • The spheno-parietal venous sinus runs medially along the posterior border of the lesser wing of the sphenoid and drains into the cavernous sinus.
- 36. Middle Cranial Fossa • The foramen rotundum is behind the medial end of the superior orbital fissure, perforates the greater wing of the sphenoid and transmits the maxillary nerve from the trigeminal ganglion to the pterygopalatine fossa. • The foramen ovale lies posterolateral to the foramen rotundum; it perforates the greater wing of the sphenoid and transmits the large sensory root and small motor root of the mandibular nerve to the infratemporal fossa; the lesser petrosal nerve also passes through it. • The small foramen spinosum lies posterolateral to the foramen ovale and also perforates the greater wing of the sphenoid. The foramen transmits the middle meningeal artery from the infratemporal fossa into the cranial cavity
- 37. Middle Cranial Fossa • The artery then runs forward and laterally in a groove on the upper surface of the squamous part of the temporal bone and the greater wing of the sphenoid. • The anterior branch passes forward and upward to the anteroinferior angle of the parietal bone, deep grooving before it runs backward and upward which is where the artery may be damaged after a blow to the side of the head • The posterior branch passes backward and upward across the squamous part of the temporal bone to reach the parietal bone. • The foramen lacerum lies between the apex of the petrous part of the temporal bone and the sphenoid bone; the inferior opening of the foramen lacerum in life is filled by cartilage and fibrous tissue.
- 38. Middle Cranial Fossa • The cavernous sinus is directly related to the side of the body of the sphenoid. • It carries in its lateral wall the 3rd and 4th cranial nerves and the ophthalmic and maxillary divisions of the 5th cranial nerve. • The internal carotid artery and the 6th cranial nerve pass forward through the sinus.
- 44. Posterior Cranial Fossa • Deep and lodges the parts of the hindbrain, the cerebellum, pons, and medulla oblongata. • Anteriorly; Superior border of the petrous part of the temporal bone. • Posteriorly; Internal surface of the squamous part of the occipital bone • The floor; Basilar, condylar, and squamous parts of the occipital bone and the mastoid part of the temporal bone. • The roof; fold of dura, the tentorium cerebelli, which intervenes between the cerebellum below and the occipital lobes of the cerebral hemispheres above.
- 45. Posterior Cranial Fossa • The foramen magnum occupies the central area of the floor and transmits the medulla oblongata and its surrounding meninges, the ascending spinal parts of the accessory nerves, and the two vertebral arteries. • The hypoglossal canal is situated above the anterolateral boundary of the foramen magnum and transmits the hypoglossal nerve. • The jugular foramen lies between the lower border of the petrous part of the temporal bone and the condylar part of the occipital bone. It transmits the following structures from before backward: • The inferior petrosal sinus; the 9th, 10th, and 11th cranial nerves; and the large sigmoid sinus. • The inferior petrosal sinus descends in the groove on the lower border of the petrous part of the temporal bone to reach the foramen. The sigmoid sinus turns down through the foramen to become the internal jugular vein.
- 46. Posterior Cranial Fossa • The internal acoustic meatus pierces the posterior surface of the petrous part of the temporal bone and transmits the vestibulocochlear nerve and the motor and sensory roots of the facial nerve. • The internal occipital crest runs upward in the midline posteriorly from the foramen magnum to the internal occipital protuberance; to it is attached the small falx cerebelli over the occipital sinus. • On each side of the internal occipital protuberance is a wide groove for the transverse sinus that sweeps around on either side, on the internal surface of the occipital bone, to reach the posteroinferior angle or corner of the parietal bone. • The groove now passes onto the mastoid part of the temporal bone, and the transverse sinus becomes the sigmoid sinus.
- 47. Posterior Cranial Fossa • The superior petrosal sinus runs backward along the upper border of the petrous bone in a narrow groove and drains into the sigmoid sinus. • As the sigmoid sinus descends to the jugular foramen, it deeply grooves the back of the petrous bone and the mastoid part of the temporal bone, it lies directly posterior to the mastoid antrum.
- 50. Neonatal Skull • Disproportionate large cranium relative to the face. • The growth of the mandible, the maxillary sinuses, and the alveolar processes of the maxillae results in a great increase in length of the face. • The bones of the skull are smooth and unilaminar, there being no diploë present. • Most of the skull bones are ossified at birth, but the process is incomplete, and the bones are mobile on each other, being connected by fibrous tissue or cartilage. • The bones of the vault are ossified in membrane; the bones of the base are ossified in cartilage. • The bones of the vault are not closely knit at sutures, as in the adult, but are separated by unossified membranous intervals called fontanelles.
- 51. Neonatal Skull • Clinically, the anterior and posterior fontanelles are most important and are easily examined in the midline of the vault. • The anterior fontanelle is diamond shaped and lies between the two halves of the frontal bone in front and the two parietal bones behind. • The fibrous membrane forming the floor of the anterior fontanelle is replaced by bone and is closed by 18 months of age. • The posterior fontanelle is triangular and lies between the two parietal bones in front and the occipital bone behind. • By the end of the first year the fontanelle is mostly non- palpable
- 52. Neonatal Skull • The tympanic part of the temporal bone is merely a C-shaped ring at birth, compared with a C-shaped curved plate in the adult. • The external auditory meatus is almost entirely cartilaginous in the newborn, and the tympanic membrane is nearer the surface. • Although the tympanic membrane is nearly as large as in the adult, it faces more inferiorly. • The tympanic plate grows laterally, forming the bony part of the meatus, and the tympanic membrane comes to face more directly laterally. • The mastoid process is not present at birth and develops later in response to the pull of the sternocleidomastoid muscle when the child moves his or her head. • At birth, the mastoid antrum lies about 3 mm deep to the floor of the suprameatal triangle. As growth of the skull continues, the lateral bony wall thickens so that at puberty the antrum may lie as much as 15 mm from the surface.
- 53. Neonatal Skull • The mandible has right and left halves at birth, united in the midline with fibrous tissue. • The two halves fuse at the symphysis menti by the end of the 1st year. • The angle of the mandible at birth is obtuse. • The head being placed level with the upper margin of the body and the coronoid process lying at a superior level to the head. • It is only after eruption of the permanent teeth that the angle of the mandible assumes the adult shape and the head and neck grow so that the head comes to lie higher than the coronoid process. • As the alveolar part of the bone becomes smaller, the ramus becomes oblique in position so that the head is bent posteriorly.
- 55. References • Chapter 11: The Head and Neck
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