The base of the skull is a complex structure that forms the floor of the cranial cavity, housing critical neurovascular pathways and supporting the brain’s weight. This anatomical illustration provides both external and internal views, highlighting the intricate arrangement of bones like the sphenoid, temporal, and occipital, along with key foramina and fossae. These views reveal the skull base’s role in separating the cranial fossae and facilitating communication between the brain and the body. This article explores each labeled component, offering a detailed examination of their anatomical significance and physiological contributions to human health.
Labeled Parts of the Base of Skull
Vomer
The vomer is a thin, midline bone forming the inferior part of the nasal septum. It separates the nasal cavities and supports the nasal cartilage, contributing to the structure of the nasal passage.
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Maxilla: Palatine Process
The maxilla: palatine process forms the anterior portion of the hard palate, creating the roof of the mouth. It supports the upper teeth and facilitates speech and chewing by providing a stable surface for the oral cavity.
Zygomatic Bone (Horizontal Portion)
The zygomatic bone (horizontal portion) extends laterally to form part of the zygomatic arch and the floor of the orbit. It contributes to the cheek’s prominence and supports the structure of the eye socket.
Medial and Lateral Pterygoid Plates
The medial and lateral pterygoid plates are extensions of the sphenoid bone, located in the infratemporal fossa. They serve as attachment points for the pterygoid muscles, which are crucial for jaw movement during mastication.
Articular Tubercle
The articular tubercle is a small projection on the temporal bone, anterior to the mandibular fossa. It guides the mandible’s movement during jaw opening and closing, ensuring smooth articulation at the temporomandibular joint.
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Foramen Ovale
The foramen ovale is an opening in the sphenoid bone that transmits the mandibular branch of the trigeminal nerve (cranial nerve V3). This nerve innervates muscles of mastication and provides sensory input from the lower face.
Mandibular Fossa
The mandibular fossa is a depression on the temporal bone that articulates with the mandible to form the temporomandibular joint. It facilitates jaw movements like chewing and speaking, supported by surrounding ligaments.
External Auditory Meatus
The external auditory meatus is a canal in the temporal bone that leads to the eardrum. It transmits sound waves to the middle ear, playing a vital role in the auditory system.
Mastoid Process
The mastoid process is a bony prominence on the temporal bone, located behind the ear. It serves as an attachment point for neck muscles, such as the sternocleidomastoid, which aids in head rotation.
Styloid Process
The styloid process is a slender projection of the temporal bone, extending downward. It anchors ligaments and muscles involved in tongue movement and swallowing, such as the stylohyoid muscle.
Stylomastoid Foramen
The stylomastoid foramen is an opening between the styloid and mastoid processes, transmitting the facial nerve (cranial nerve VII). This nerve controls facial expressions and taste sensation from the anterior two-thirds of the tongue.
Entrance to Carotid Canal
The entrance to carotid canal is an opening in the temporal bone that allows the internal carotid artery to enter the skull. This artery supplies blood to the brain, making the canal a critical neurovascular pathway.
External Occipital Protuberance
The external occipital protuberance is a midline projection on the occipital bone, palpable at the back of the head. It serves as an attachment point for the nuchal ligament, which supports the head’s weight.
Zygomatic Arch
The zygomatic arch is formed by the zygomatic bone and the zygomatic process of the temporal bone. It provides structural support to the cheek and serves as an attachment for the masseter muscle, essential for chewing.
Sphenoid Bone
The sphenoid bone is a central bone of the skull base, contributing to the cranial floor and orbits. It houses the pituitary gland in the sella turcica and contains foramina for cranial nerves and blood vessels.
Foramen Spinosum
The foramen spinosum is a small opening in the sphenoid bone that transmits the middle meningeal artery. This artery supplies blood to the meninges, and its location makes it clinically significant in skull fractures.
Foramen Lacerum
The foramen lacerum is an irregular opening at the base of the skull, partially filled with cartilage in life. It allows the passage of small emissary veins and the internal carotid artery as it transitions into the cranial cavity.
Jugular Foramen
The jugular foramen is a large opening between the temporal and occipital bones, transmitting the internal jugular vein and cranial nerves IX, X, and XI. It plays a key role in venous drainage and neural communication.
Occipital Condyle
The occipital condyle is a rounded projection on the occipital bone that articulates with the atlas (C1 vertebra). It facilitates head movement, such as nodding, by forming part of the atlanto-occipital joint.
Temporal Bone
The temporal bone forms the sides of the skull base, housing auditory structures and the temporomandibular joint. It includes the petrous portion, which separates the middle and posterior cranial fossae and protects the inner ear.
Foramen Magnum
The foramen magnum is a large opening in the occipital bone, through which the spinal cord passes to connect with the brainstem. It also transmits the vertebral arteries and cranial nerve XI, supporting neural and vascular continuity.
Occipital Bone
The occipital bone forms the posterior and inferior part of the cranial base, encasing the cerebellum and brainstem. It provides structural support and contains the foramen magnum for spinal cord passage.
Superior Nuchal Line
The superior nuchal line is a ridge on the occipital bone, marking the attachment of neck muscles like the trapezius. It contributes to head stability and movement by anchoring these muscles.
Frontal Bone
The frontal bone forms the forehead and the anterior cranial fossa, supporting the frontal lobes of the brain. It also contributes to the superior orbital fissure, through which cranial nerves pass to the orbit.
Ethmoid Bone: Crista Galli
The ethmoid bone: crista galli is a midline projection in the anterior cranial fossa, serving as an attachment for the falx cerebri. This dural fold separates the cerebral hemispheres, stabilizing the brain.
Ethmoid Bone: Cribriform Plate
The ethmoid bone: cribriform plate is a perforated structure in the anterior cranial fossa, allowing the passage of olfactory nerve fibers. It connects the nasal cavity to the brain, facilitating the sense of smell.
Sphenoid Bone: Lesser Wing
The sphenoid bone: lesser wing forms the boundary between the anterior and middle cranial fossae. It contributes to the optic canal, through which the optic nerve (cranial nerve II) passes to the eye.
Sphenoid Bone: Hypophyseal Fossa
The sphenoid bone: hypophyseal fossa (within the sella turcica) houses the pituitary gland, a key endocrine structure. The pituitary regulates hormones like thyroid-stimulating hormone (TSH), which controls T3 and T4 production for metabolism.
Sphenoid Bone: Sella Turcica
The sphenoid bone: sella turcica is a saddle-shaped depression that encases the pituitary gland. It protects this gland and positions it for optimal hormonal regulation via the hypothalamus.
Foramen Lacerum and Exit of Carotid Canal
The foramen lacerum and exit of carotid canal marks the region where the internal carotid artery exits the carotid canal. This pathway ensures the brain receives oxygenated blood, critical for its function.
Temporal Bone Petrous Portion (Petrous Ridge)
The temporal bone petrous portion (petrous ridge) is a dense, pyramid-shaped region that separates the middle and posterior cranial fossae. It houses the inner ear structures, including the cochlea and semicircular canals, for hearing and balance.
Jugular Foramen
The jugular foramen (also labeled in the inferior view) facilitates the passage of the internal jugular vein and cranial nerves IX, X, and XI. It ensures venous drainage from the brain and neural control of visceral functions.
Parietal Bone
The parietal bone forms the superior and lateral walls of the cranial vault, contributing to the skull base’s periphery. It protects the brain and provides attachment points for muscles involved in head movement.
Superior Orbital Fissure
The superior orbital fissure is a slit-like opening between the sphenoid’s lesser and greater wings, transmitting cranial nerves III, IV, V1, and VI to the orbit. It facilitates eye movement and sensation.
Foramen Rotundum
The foramen rotundum is an opening in the sphenoid bone that transmits the maxillary branch of the trigeminal nerve (cranial nerve V2). This nerve provides sensory innervation to the midface, including the upper teeth and palate.
Foramen Ovale
The foramen ovale (also labeled in the inferior view) allows the passage of the mandibular branch of the trigeminal nerve (cranial nerve V3). It supports motor and sensory functions of the lower face.
Internal Acoustic Meatus
The internal acoustic meatus is an opening in the temporal bone’s petrous portion, transmitting the facial (VII) and vestibulocochlear (VIII) nerves. It supports facial movement, taste, hearing, and balance.
Hypoglossal Canal
The hypoglossal canal is a passage in the occipital bone that transmits the hypoglossal nerve (cranial nerve XII). This nerve innervates the tongue muscles, enabling speech and swallowing.
Anatomical Overview of the Base of Skull
The base of the skull is a complex structure formed by the frontal, ethmoid, sphenoid, temporal, and occipital bones, creating a foundation for the cranial cavity. Its external and internal views reveal the intricate organization of fossae, foramina, and bony landmarks that support the brain and facilitate neurovascular communication.
- Cranial Fossae Division: The lesser wing of the sphenoid bone separates the anterior and middle cranial fossae, while the petrous ridge of the temporal bone divides the middle and posterior fossae. This tiered arrangement accommodates the brain’s natural curvature and protects its distinct regions.
- Neurovascular Pathways: Foramina like the foramen ovale, foramen spinosum, and jugular foramen allow the passage of cranial nerves and blood vessels, such as the internal carotid artery through the carotid canal. These pathways ensure neural control and blood supply to the brain and face.
- Bone Integration: The sphenoid bone acts as a central keystone, connecting with the frontal, ethmoid, temporal, and occipital bones to form a stable base. The occipital bone anchors the skull to the vertebral column via the occipital condyles, facilitating head movement.
- Protective Features: The base of the skull encases the brain, with the sella turcica protecting the pituitary gland and the petrous portion safeguarding the inner ear. The robust structure of the occipital bone and temporal bone provides additional protection to the brainstem and cerebellum.
Physical Functions of the Base of Skull Structures
The base of the skull supports critical physiological functions, from protecting the brain and cranial nerves to facilitating sensory and motor activities. Its design ensures both structural integrity and functional connectivity between the brain and the body.
- Brain and Neural Support: The foramen magnum allows the spinal cord to connect with the brainstem, while the internal acoustic meatus transmits nerves for hearing and balance. The anterior cranial fossa, supported by the cribriform plate, facilitates the sense of smell via olfactory nerves.
- Endocrine Regulation: The hypophyseal fossa within the sella turcica houses the pituitary gland, which regulates hormones like adrenocorticotropic hormone (ACTH), influencing cortisol production for stress response. The sphenoid bone’s central position ensures the pituitary’s protection and connectivity with the hypothalamus.
- Vascular Supply: The entrance to carotid canal and jugular foramen ensure the brain’s blood supply and venous drainage, with the internal carotid artery delivering oxygenated blood. The foramen spinosum supplies the meninges, supporting their role in brain protection.
- Head and Jaw Movement: The occipital condyle and mandibular fossa enable head nodding and jaw movement, respectively, supporting functions like chewing and speaking. The zygomatic arch and articular tubercle further facilitate mastication by anchoring muscles and guiding the mandible.
The base of the skull is a remarkable example of anatomical engineering, balancing the need for protection, support, and connectivity in the human body. Its intricate design underscores the importance of each bone and foramen in maintaining neurological and physiological health, making it a critical area of study for understanding human anatomy.
