Cranial cavity
The floor of the cranial cavity is divided into three distinct depressions. They are known as the anterior
cranial fossa, middle cranial fossa and posterior cranial fossa. Each fossa accommodates a different part
of the brain.
The anterior cranial fossa is the most shallow and superior of the three cranial fossae. It lies superiorly
over the nasal and orbital cavities.Thefossaa anteroinferior portions of the frontal lobes of the brain.
In this article, we shall look at the borders, contents and clinical correlations of the anterior cranial fossa.
Borders
The anterior cranial fossa consists of three bones: the frontal bone, ethmoid bone and sphenoid bone.
It is bounded as follows:
Anteriorly and laterally it is bounded by the inner surface of the frontal bone.
Posteriorly and medially it is bounded by the limbus of the sphenoid bone. The limbus is a bony ridge that
forms the anterior border of the prechiasmatic sulcus (a groove running between the right and left optic
canals).
Posteriorly and laterally it is bounded by the lesser wings of the sphenoid bone (these are two triangular
projections of bone that arise from the central sphenoid body).
The floor consists of the frontal bone, ethmoid bone and the anterior aspects of the body and lesser wings
of the sphenoid bone.
Contents
There are several bony landmarks present in the anterior cranial fossa.
The frontal bone is marked in the midline by a body ridge, known as the frontal crest. It projects upwards,
and acts as a site of attachment for the falx cerebri (a sheet of dura mater that divides the two cerebral
hemispheres).
In the midline of the ethmoid bone, the crista galli (latin for cock’s comb) is situated. This is an upwards
projection of bone, which acts as another point of attachment for the falx cerebri.
On either side of the crista galli is the cribriform plate which supports the olfactory bulb and has
numerous foramina that transmit vessels and nerves.
The anterior aspect of the sphenoid bone lies within the anterior cranial fossa. From the central body, the
lesser wings arise. The rounded ends of the lesser wings are known as the anterior clinoid processes. They
serve as a place of attachment for the tentorium cerebelli (a sheet of dura mater that divides the cerebrum
from the cerebellum). The lesser wings of the sphenoid bone also separate the anterior and middle cranial
fossae
Foramina
The ethmoid bone in particular contains the main foramina (openings that transmit vessels and nerves) of
the anterior cranial fossa. The cribriform plate is a sheet of bone seen either side of the crista galli which
contains numerous small foramina – these transmit olfactory nerve fibres (CN I) into the nasal cavity. It
also contains two larger foramen:
Anterior ethmoidal foramen – transmits the anterior ethmoidal artery, nerve and vein.
Posterior ethmoidal foramen – transmits the posterior ethmoidal artery, nerve and vein.
Bone of cranial cavity:
Cranium
The cranium (also known as the neurocranium) is formed by the superior
aspect of the skull. 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).
Face
The facial skeleton (also known as the viscerocranium) supports the soft
tissues of the face.
It consists of 14 bones, which fuse to house the orbits of the eyes, the
nasal and oral cavities, and the sinuses. The frontal bone, typically a
bone of the calvaria, is sometimes included as part of the facial skeleton.
The facial bones are:
Zygomatic (2) – forms the cheek bones of the face and articulates with
the frontal, sphenoid, temporal and maxilla bones.
Lacrimal (2) – the smallest bones of the face. They form part of the
medial wall of the orbit.
Nasal (2) – two slender bones that are located at the bridge of the nose.
Inferior nasal conchae (2) – located within the nasal cavity, these bones
increase the surface area of the nasal cavity, thus increasing the amount
of inspired air that can come into contact with the cavity walls.
Palatine (2) – situated at the rear of oral cavity and forms part of the hard
palate.
Maxilla (2) – comprises part of the upper jaw and hard palate.
Vomer – forms the posterior aspect of the nasal septum.
Mandible (jaw) – articulates with the base of the cranium at the
temporomandibular joint (TMJ).
The cranial cavity is formed by several bones of the skull. Let's explore the
individual bones that contribute to the anatomy of the cranial cavity:
Frontal Bone: The frontal bone forms the forehead and contributes to the anterior
cranial fossa. It extends backward to the coronal suture, which connects it to the
parietal bones.
Parietal Bones: There are two parietal bones, one on each side of the skull. They
join together at the midline and form the sagittal suture. The parietal bones make
up the majority of the cranial vault.
Temporal Bones: There are two temporal bones, one on each side of the skull.
They contribute to the lateral walls of the cranial cavity. Each temporal bone
consists of several parts, including the squamous part, tympanic part, mastoid part,
and petrous part. The petrous part houses the middle and inner ear structures.
Occipital Bone: The occipital bone forms the posterior part of the cranial cavity. It
has a large opening called the foramen magnum through which the spinal cord
passes. The occipital bone also contains the occipital condyles, which articulate
with the first cervical vertebra (atlas).
Sphenoid Bone: The sphenoid bone is a complex bone situated at the base of the
skull. It contributes to the anterior and middle cranial fossae. Key features of the
sphenoid bone include the greater wings, lesser wings, sella turcica (which houses
the pituitary gland), and several foramina, such as the optic canal and superior
orbital fissure.
Ethmoid Bone: The ethmoid bone is located between the eye sockets and
contributes to the anterior cranial fossa. It forms part of the nasal cavity and
contains the cribriform plate, which has tiny perforations for the passage of
olfactory nerves.
These bones join together at specific points called sutures. The major sutures of the
cranial cavity include the coronal suture (between the frontal and parietal bones),
sagittal suture (between the two parietal bones), lambdoid suture (between the
parietal and occipital bones), and squamous sutures (between the temporal and
parietal bones).
Anatomy of the brain
The brain is a complex organ that controls and coordinates various functions of the
body. It is divided into several distinct regions, each with specific structures and
functions. Here is an overview of the anatomy of the brain:
Cerebrum: The cerebrum is the largest and most prominent part of the brain. It is
divided into two hemispheres, the right and left, which are connected by a bundle
of nerve fibers called the corpus callosum. The outer layer of the cerebrum is
called the cerebral cortex, which is highly convoluted and responsible for higher
cognitive functions such as memory, language, perception, and voluntary
movement.
Cerebellum: The cerebellum is located at the back of the brain, beneath the
cerebrum. It is involved in the coordination and fine-tuning of movement, posture,
and balance. The cerebellum has a highly folded surface, increasing its surface
area.
Brainstem: The brainstem is the lower part of the brain that connects the cerebrum
and cerebellum to the spinal cord. It consists of three main regions:
Medulla Oblongata: The medulla oblongata controls vital functions such as
breathing, heart rate, blood pressure, and swallowing.
Pons: The pons serves as a bridge connecting the cerebrum, cerebellum, and
medulla oblongata. It plays a role in relaying messages between different parts of
the brain.
Midbrain: The midbrain is involved in functions such as visual and auditory
processing, motor control, and regulation of sleep and wake cycles.
Diencephalon: The diencephalon is located between the brainstem and the
cerebrum. It consists of several structures, including:
Thalamus: The thalamus acts as a relay center, receiving sensory information and
sending it to the appropriate areas of the cerebral cortex.
Hypothalamus: The hypothalamus plays a crucial role in maintaining homeostasis
by regulating body temperature, hunger, thirst, hormone production, and
controlling the pituitary gland.
Ventricles: The brain contains a system of interconnected fluid-filled cavities
called ventricles. These ventricles produce and circulate cerebrospinal fluid (CSF),
which provides nourishment and cushioning for the brain. There are four
ventricles: two lateral ventricles (one in each hemisphere), the third ventricle in the
diencephalon, and the fourth ventricle located in the brainstem.
Meninges: The brain is enveloped and protected by three layers of membranes
called meninges. These layers are the dura mater (outer layer), arachnoid mater
(middle layer), and pia mater (inner layer). The meninges help protect the brain
from mechanical damage and provide a supportive framework.
Blood Supply: The brain receives its blood supply from two pairs of arteries: the
internal carotid arteries and the vertebral arteries. These arteries give rise to a
network of blood vessels that supply oxygen and nutrients to the brain.
ANATOMY OF CRANIAL NERVES:
The cranial nerves are a set of twelve pairs of nerves that originate from the brain
and primarily innervate the structures of the head and neck. They play a crucial
role in sensory and motor functions, including vision, hearing, taste, facial
expression, and swallowing. Let's explore the anatomy of the cranial nerves in
more detail:
Olfactory Nerve (I): The olfactory nerve is responsible for the sense of smell. It
originates from the olfactory epithelium in the nasal cavity and projects directly to
the olfactory bulbs located on the ventral surface of the frontal lobe.
Optic Nerve (II): The optic nerve carries visual information from the eyes to the
brain. It arises from the retina of each eye, crosses at the optic chiasm, and
continues to the lateral geniculate nucleus of the thalamus. From there, visual
signals are transmitted to the visual cortex in the occipital lobe for processing.
Oculomotor Nerve (III): The oculomotor nerve controls the movement of most eye
muscles, including those responsible for upward, downward, and inward eye
movements, as well as pupil constriction. It originates from the midbrain and
innervates muscles that control eye movements.
Trochlear Nerve (IV): The trochlear nerve controls the superior oblique muscle,
which primarily rotates the eye downward and outward. It originates from the
midbrain and has the longest intracranial course among the cranial nerves.
Trigeminal Nerve (V): The trigeminal nerve is the largest cranial nerve and has
both sensory and motor functions. It consists of three divisions:
Ophthalmic Division (V1): It provides sensory innervation to the forehead, scalp,
upper eyelids, and the front of the head.
Maxillary Division (V2): It supplies sensory innervation to the lower eyelid, upper
lip, cheek, and the side of the nose.
Mandibular Division (V3): It controls the muscles of mastication (chewing) and
provides sensory innervation to the lower jaw, lower lip, and part of the ear.
Abducens Nerve (VI): The abducens nerve controls the lateral rectus muscle,
which abducts the eye (moves it outward). It originates from the pons and plays a
crucial role in horizontal eye movement.
Facial Nerve (VII): The facial nerve has both sensory and motor functions and is
involved in facial expression, taste sensation, and the functioning of salivary and
tear glands. It arises from the pons and innervates the muscles of facial expression.
Vestibulocochlear Nerve (VIII): The vestibulocochlear nerve, also known as the
auditory nerve, is responsible for hearing and balance. It consists of two divisions:
Vestibular Division: It carries information related to balance and spatial orientation
from the vestibular apparatus in the inner ear.
Cochlear Division: It transmits auditory information from the cochlea in the inner
ear to the brain.
Glossopharyngeal Nerve (IX): The glossopharyngeal nerve is involved in taste
sensation, swallowing, and the functioning of salivary glands. It originates from
the medulla and innervates the posterior third of the tongue, the oropharynx, and
the parotid gland.
Vagus Nerve (X): The vagus nerve is a major cranial nerve that has extensive
functions in the thorax and abdomen. It controls various involuntary functions,
such as heart rate, digestion, respiration, and vocalization. It also provides sensory
innervation to the pharynx, larynx, and parts of the ear.
Accessory Nerve (XI): The accessory nerve controls the muscles involved in head
movement and shoulder elevation. It has two components: the spinal accessory
component, which originates from the spinal cord, and the cranial accessory
component, which arises from the medulla.
Hypoglossal Nerve (XII): The hypoglossal nerve controls the muscles of the
tongue, allowing for speech, swallowing, and tongue movement. It originates from
the medulla and innervates the intrinsic and extrinsic muscles of the tongue.
THE END