Head Anatomy: Cranium, Cranial Base, Scalp, Meninges, and Venous Sinuses
Structure and Landmarks of the Cranium
Features of the Neurocranium and Calvaria (Skullcap):
The calvaria forms the dome-like roof of the neurocranium.
Frontal bone: Positioned anteriorly.
Parietal bones: Right and left parietal bones form the lateral walls; features the parietal eminence (lateral expansion of parietal bones).
Occipital bone: Positioned posteriorly.
Major Cranial Sutures:
Coronal suture: Unites the frontal bone anteriorly with the right and left parietal bones posteriorly.
Sagittal suture: Unites the right and left parietal bones along the midline of the calvaria.
Lambdoid suture: Unites the occipital bone posteriorly with the right and left parietal and temporal bones.
Craniometric Landmarks:
Bregma: The anatomical landmark formed by the intersection of the sagittal suture and coronal suture.
Lambda: The anatomical landmark formed by the junction of the sagittal suture and lambdoid suture; can sometimes be felt as a subtle depression.
Vertex: The superiormost point of the cranium, located near the midpoint of the sagittal suture.
External occipital protuberance: A palpable elevation in the median line on the external surface of the occipital bone.
Superior nuchal line: Extends laterally from each side of the external occipital protuberance; serves as a muscular attachment boundary and is less distinct than the protuberance.
Inferior nuchal line: Lies inferior to the superior nuchal line on the posterior aspect of the occipital bone.
Mastoid process: A prominent, palpable projection of the temporal bone located posterior to the external acoustic meatus, providing muscle attachment sites.
Occipital condyles: Large articular protuberances located on each side of the foramen magnum.
Pterion: Junction region on the lateral aspect of the skull uniting frontal, parietal, temporal, and sphenoid bones.
External Aspect of the Cranial Base
Anterior and Palatal Features of the Basicranium:
Alveolar arch of the maxillae: Free border of the alveolar processes surrounding and supporting the maxillary teeth.
Hard palate (bony palate): Formed anteriorly by the palatine processes of the maxillae and posteriorly by the horizontal plates of the palatine bones.
Incisive fossa: A median depression located immediately posterior to the central incisor teeth.
Greater and lesser palatine foramina: Located posterolaterally on the hard palate.
Choanae (posterior nasal apertures): Paired openings bounded inferiorly by the posterior edge of the hard palate; separated from each other along the midline by the vomer.
Vomer: A thin, flat bone that forms a major part of the bony nasal septum.
Sphenoid Bone:
Wedged centrally between the frontal, temporal, and occipital bones.
Consists of a central body and three pairs of processes: greater wings, lesser wings, and pterygoid processes.
Pterygoid processes: Composed of medial and lateral pterygoid plates extending inferiorly on each side of the sphenoid from the junction of the body and greater wings.
Pharyngotympanic (auditory) tube opening and sulcus: The opening of the bony part and the sulcus (groove) for the cartilaginous part of the tube lie medial to the spine of the sphenoid, inferior to the junction of the greater wing of the sphenoid and the petrous part of the temporal bone.
Temporal and Occipital Features:
Mandibular fossae: Smooth depressions in the squamous part of the temporal bone that accommodate the heads (condyles) of the mandible when the mouth is closed.
Occipital bone: Forms the posterior portion of the cranial base and articulates anteriorly with the sphenoid bone.
Foramen magnum: Large central opening surrounded by parts of the occipital bone through which the brainstem connects with the spinal cord.
Occipital condyles: Two large protuberances on each side of the foramen magnum by which the cranium articulates with the atlas (C1 vertebra) of the vertebral column.
Jugular foramen: A large fissure located between the occipital bone and the petrous part of the temporal bone.
Carotid canal: External opening located immediately anterior to the jugular foramen; conveys the internal carotid artery into the cranial cavity.
Stylomastoid foramen: Opening situated between the mastoid process and styloid process of the temporal bone.
Internal Surface of the Cranial Base and Cranial Fossae
Overview of Cranial Fossae:
The internal surface of the cranial base is divided into three distinct steps or depressions situated at different level elevations: anterior cranial fossa, middle cranial fossa, and posterior cranial fossa.
Anterior Cranial Fossa:
Superiormost fossa; accommodates the frontal lobes of the cerebral hemispheres.
Bones involved: Frontal bone, ethmoid bone, and lesser wings/body of sphenoid bone.
Key landmarks and foramina:
Frontal crest
Foramen cecum
Crista galli
Cribriform plate (ethmoid bone)
Anterior ethmoidal foramen
Posterior ethmoidal foramen
Orbital part of frontal bone (orbital plate)
Lesser wing of sphenoid
Sphenoidal crest
Middle Cranial Fossa:
Butterfly-shaped central fossa; accommodates the temporal lobes of the cerebral hemispheres.
Bones involved: Sphenoid bone (body and greater wings) and temporal bones (squamous and petrous parts).
Key landmarks and foramina:
Superior orbital fissure
Greater wing of sphenoid
Foramen rotundum
Foramen ovale
Foramen spinosum
Foramen lacerum
Groove for middle meningeal artery
Sella Turcica Components:
Prechiasmatic sulcus
Optic canal
Anterior clinoid process
Tuberculum sellae
Hypophysial fossa (central deep depression lodging the pituitary gland)
Posterior clinoid process
Dorsum sellae
Posterior Cranial Fossa:
Deepest and largest fossa; accommodates the cerebellum, pons, and medulla oblongata.
Bones involved: Occipital bone, petrous/mastoid parts of temporal bones, and sphenoid bone.
Key landmarks and foramina:
Petrous ridge of temporal bone
Clivus (sloping bony surface leading to foramen magnum)
Internal acoustic meatus
Jugular foramen
Hypoglossal canal
Foramen magnum
Groove for transverse sinus
Groove for sigmoid sinus
Cerebellar fossa
Internal occipital crest
Anatomy and Clinical Correlates of the Scalp
Five Layers of the Scalp (Mnemonic: S-C-A-L-P):
Skin (S): Thin outer layer containing sweat glands, sebaceous glands, and hair follicles; possesses an abundant arterial blood supply and cutaneous lymphatic drainage.
Connective tissue (dense) (C): Thick, dense, highly vascular subcutaneous layer containing blood vessels and cutaneous nerves; firmly binds skin to underlying aponeurosis.
Aponeurosis (epicranial aponeurosis / galea aponeurotica) (A): Strong, broad tendinous sheet covering the calvaria. Serves as intermediate tendon for frontal and occipital bellies of occipitofrontalis muscle and superior auricular muscle (collectively forming musculo-aponeurotic epicranius).
Loose connective tissue (L): Sponge-like layer containing potential spaces that may distend with fluid, pus, or blood from injury or infection; allows free movement of scalp proper (first 3 layers: Skin, Connective tissue, Aponeurosis) over underlying calvaria.
Pericranium (P): Dense layer of connective tissue forming external periosteum of neurocranium; firmly attached but stripped fairly easily from calvaria of living individuals, except where continuous with fibrous tissue uniting cranial sutures.
Temporal Region Layers:
Temporalis fascia overlays temporalis muscle and divides into superficial layer and deep layer.
Contains frontalis, fat pad, temporalis muscle, and occipitalis muscle attachments.
Clinical Aspects of Scalp Injuries and Infections:
Danger Area of the Scalp: Loose connective tissue layer is designated as the danger area because pus or blood spreads easily within it.
Intracranial Spread: Infection in loose connective tissue layer can pass into cranial cavity through emissary veins, which traverse calvarial foramina to reach intracranial structures such as meninges and dural sinuses.
Posterior Limitation: Infection cannot pass into neck because occipital belly of occipitofrontalis muscle attaches firmly to occipital bone and mastoid parts of temporal bones.
Lateral Limitation: Infection cannot spread laterally into face beyond zygomatic arches because epicranial aponeurosis is continuous with temporal fascia attaching to zygomatic arches.
Anterior Extension: Infection or fluid (pus/blood) CAN enter eyelids and root of nose because front of occipitofrontalis muscle (frontalis belly) inserts directly into skin and subcutaneous tissue without attaching to bone.
Black Eyes (Periorbital Ecchymosis): Direct trauma or scalp/forehead injury can produce blood accumulation around eyes; ecchymoses (purple patches) develop as a result of extravasation of blood into subcutaneous tissue and skin of eyelids and surrounding periorbital regions.
Cranial Meninges and Cerebrospinal Fluid
Overview and Functions:
Cranial meninges are protective connective tissue coverings of the brain located immediately internal to the cranium.
Enclose and protect brain within fluid-filled subarachnoid space; provide supporting structural framework for arteries, veins, and dural venous sinuses.
Three Layers of Cranial Meninges:
Dura mater (dura): Tough, thick, fibrous outer layer.
Arachnoid mater (arachnoid): Thin, avascular intermediate layer.
Pia mater (pia): Delicate, highly vascular internal layer adhering closely to brain surface.
Leptomeninges (Leptomeninx):
Combined term for continuous membranes formed by arachnoid mater and pia mater.
Subarachnoid Space and Cerebrospinal Fluid (CSF):
Subarachnoid space lies between arachnoid mater and pia mater.
CSF Composition: Clear liquid similar in constitution to blood plasma, but containing less protein and different ion concentrations.
CSF Function: Cushions, protects, and nourishes brain tissue while pressing arachnoid mater against inner surface of dura mater.
CSF Formation & Path: Formed predominantly by choroid plexuses within four ventricles of brain; exits ventricular system to enter subarachnoid space.
Structural Layers of Dura Mater:
External periosteal layer: Formed by periosteum covering internal surface of calvaria.
Internal meningeal layer: Strong fibrous membrane continuous at foramen magnum with dura covering spinal cord.
Dural Infoldings and Reflections
Overview:
Internal meningeal layer reflects away from external periosteal layer of dura mater to form double-layered dural infoldings (reflections).
Divide cranial cavity into distinct compartments and support regions of brain.
Four Major Dural Infoldings:
Falx cerebri (cerebral falx):
Largest dural infolding; sickle-shaped partition lying in longitudinal cerebral fissure separating right and left cerebral hemispheres.
Attached anteriorly in median plane to frontal crest of frontal bone and crista galli of ethmoid bone.
Attached posteriorly to internal occipital protuberance; becomes continuous posteriorly with upper surface of tentorium cerebelli.
Tentorium cerebelli (cerebellar tentorium):
Wide crescentic roof-like septum separating occipital lobes of cerebral hemispheres from cerebellum.
Attached anteriorly to clinoid processes of sphenoid bone.
Attached anterolaterally to petrous part of temporal bone.
Attached posterolaterally to internal surface of occipital bone and part of parietal bone.
Falx cerebri attaches to upper midline surface, holding it up like a tent.
Tentorial notch: Concave anteromedial free border forming gap through which brainstem extends from posterior cranial fossa into middle cranial fossa.
Divides cranial cavity into supratentorial and infratentorial compartments.
Falx cerebelli (cerebellar falx):
Vertical dural infolding lying inferior to tentorium cerebelli in posterior part of posterior cranial fossa.
Partially separates right and left cerebellar hemispheres.
Diaphragma sellae (sellar diaphragm):
Smallest dural infolding; circular horizontal membrane suspended between clinoid processes.
Forms partial roof over hypophysial fossa, covering pituitary gland.
Contains central aperture allowing passage of infundibulum (pituitary stalk) and hypophysial veins.
Dural Venous Sinuses and Venous Drainage
Structure and Function:
Endothelial-lined channels formed between periosteal and meningeal layers of dura mater.
Positioned along attachments of dural infoldings and centrally along cranial base.
Collect venous blood from surface of brain and diploic veins; drain majority of blood into internal jugular veins (IJVs).
Primary Sinuses and Associated Structures:
Superior sagittal sinus:
Runs in attached convex upper border of falx cerebri.
Begins anteriorly at crista galli and ends posteriorly near internal occipital protuberance at confluence of sinuses.
Receives superior cerebral veins; communicates through slit-like openings with lateral venous lacunae (lateral expansions of superior sagittal sinus).
Arachnoid granulations:
Tufted prolongations of arachnoid mater protruding through meningeal layer of dura mater into dural venous sinuses (especially superior sagittal sinus) and lateral venous lacunae.
Function in transport/reabsorption of CSF from subarachnoid space into venous system.
Inferior sagittal sinus:
Significantly smaller sinus running in free concave lower border of falx cerebri; terminates at straight sinus.
Straight sinus:
Formed by union of inferior sagittal sinus and great cerebral vein.
Runs inferoposteriorly along line of attachment between falx cerebri and tentorium cerebelli to join confluence of sinuses.
Confluence of sinuses: Dilated area near internal occipital protuberance joining superior sagittal, straight, occipital, and transverse sinuses.
Transverse and Sigmoid sinuses: Convey blood laterally and inferiorly from confluence toward jugular foramen to form internal jugular vein.
Base Sinuses & Communications: Includes cavernous sinus, superior and inferior petrosal sinuses, basilar plexus, supra-orbital vein, superior/inferior ophthalmic veins, pterygoid plexus, deep facial veins, facial vein, emissary veins, and internal vertebral venous plexus.