Chapter 15. Scalp, Skull, and Meninges

Big Picture

The scalp consists of five layers of tissue. The five layers, from superficial to deep, are skin, subcutaneous connective tissue, a muscular aponeurotic layer, a loose connective tissue layer, and the pericranium.

Layers of the Scalp

The layers of the scalp can best be remembered by the acronym “SCALP,” with each letter of the word representing the tissue layer associated with it Figure 15-1A.

• Skin. The skin of the scalp contains sweat and sebaceous glands and usually numerous hair follicles.
• Connective tissue. The tissue between the skin and the aponeurotic layers is composed of dense collagenous connective tissue and contains the arteries, veins and nerves supplying the scalp.
• Aponeurosis. The superficial musculoaponeurotic system of the scalp consists of the occipitofrontalis muscle and its investing fascia. This fascia is specialized to form a tendinous epicranial aponeurosis known as the galea aponeurotica. The galea continues into the temples, investing the auricular muscles, and terminates by attaching to the mastoid processes and the zygomatic arch. The frontalis muscle is instrumental in movements of the eyebrows and forehead and is an important muscle of facial expression are innervated by the facial nerve, cranial nerve (CN) VII.
• Loose connective tissue. A sponge-like layer of loose connective tissue forms a subaponeurotic compartment that enables free movement of the top three scalp layers across the pericranium. It also contains the emissary veins.
• Pericranium. The pericranium is the periosteum over the external surface of the skull where the fibrous tissue knits into the sutures.

Innervation of the Scalp

The scalp receives its cutaneous innervation as follows (Figure 15-1B):

• Posterior region of the scalp. Innervated by branches of the cervical plexus, which are principally derived from C2 and C3 (lesser occipital and greater occipital nerves).
• Anterior region of the scalp. Innervated by the supraorbital and supratrochlear nerves, which are derived from the ophthalmic division of the trigeminal nerve (CN V-1).
• Lateral region of the scalp. Innervated by branches of the maxillary (CN V-2) and mandibular (CN V-3) divisions of the trigeminal nerve (the zygomaticotemporal and auriculotemporal nerves, respectively).

Vascular Supply of the Scalp

The scalp is highly vascularized via branches of the external and internal carotid arteries (Figure 15-1C).

• External carotid artery. Branches include the occipital, posterior auricular, and superficial temporal arteries.
• Internal carotid artery. Branches include the supraorbital and supratrochlear arteries.

The supraorbital and supratrochlear veins unite to form the facial vein. The superficial temporal vein joins with the maxillary vein to form the retromandibular vein in the parotid salivary gland. The posterior auricular vein unites with the posterior division of the retromandibular vein to form the external jugular vein.

Scalp veins connect with the diploic and emissary veins.

• Diploic veins. Diploic veins course in the diploe cranial bones of the skull and connect with the dural venous sinuses via emissary veins.
• Emissary veins. Small veins connect veins of the scalp and skull with the dural venous sinuses.

Big Picture

There are 8 cranial bones and 14 facial bones in the skull, all of which serve to protect the brain.

Bones of the Skull

The skull protects the brain and its surrounding meninges (Figure 15-2A). The outer and inner surfaces of the skull, are covered by periosteum, known respectively as the pericranium and the endocranium. The periosteum is continuous at the sutures of the skull. The cranial bones consist of spongy bone “sandwiched” between two layers of compact bone. The bones of the skull are as follows (Figure 15-2B–E):

• Frontal bone. The unpaired frontal bone underlies the forehead, roof of the orbit, and a smooth median prominence called the glabella.
• Parietal bone. The paired parietal bones form the superior and lateral aspects of the skull.
• Temporal bone. The paired temporal bones consist of a squamous part that forms the lateral portion of the skull; the petrous part, which encloses the internal ear (cochlea and semicircular canals) and the middle ear (malleus, incus, and stapes); the mastoid part, which contains the mastoid air cells; and the tympanic part, which houses the external auditory meatus and tympanic cavity.
• Occipital bone. The occipital bone encloses the foramen magnum, which transmits the spinal cord and vertebral arteries.
• Sphenoid bone. The sphenoid bone consists of a body, which houses the sphenoid sinus, and the greater and lesser wings and the pterygoid processes.
• Ethmoid bone. The ethmoid bone is located between the orbits and consists of the cribriform plate, the perpendicular plate, and the ethmoid air cells.

Sutures of the Skull

Sutures are immovable fibrous joints between the bones of the skull. The principal sutures of the skull are as follows:

• Coronal suture. Joins the frontal bone and the two parietal bones. The coronal suture courses in the coronal plane.
• Sagittal suture. Joins the paired parietal bones. The sagittal suture courses in the sagittal plane.
• Squamous suture. Joins the parietal and temporal bones.
• Lambdoid suture. Joins the parietal bones with the occipital bone.
• Pterion. Junction of the frontal, parietal, and temporal bones in the lateral aspect of the skull.

Big Picture

The base of the skull forms the floor on which the brain lies and consists of three large depressions that lie on different levels known as the anterior middle, and posterior cranial fossae (Figure 15-3A).

Anterior Cranial Fossa

The anterior cranial fossa houses the frontal lobes of the cerebrum and has the following bony landmarks (Figure 15-3B and C):

• Cribriform plate. Transmits the olfactory nerves (CN I) from the nasal cavity to the olfactory bulbs.
• Crista galli. Projects superiorly from the cribriform plate and serves as an attachment for the falx cerebri.
• Lesser wing of the sphenoid bone. Forms ridge separating the anterior and the middle cranial fossae.

Middle Cranial Fossa

The middle cranial fossa is deeper than the anterior cranial fossa and is separated from the posterior cranial fossa by the clivus. The middle cranial fossa supports the temporal lobes of the cerebrum and has the following bony landmarks (Figure 15-3B and C):

• Sella turcica. A deep central depression in the sphenoid bone that houses the pituitary gland. It is located superior to the sphenoid sinus.
• Optic canal. A passage that transmits the optic nerve (CN II) and the ophthalmic artery.
• Superior orbital fissure. A longitudinal fissure in the orbit that transmits the oculomotor nerve (CN III), trochlear nerve (CN IV), ophthalmic division of the trigeminal nerve (CN V-1), abducens nerve (CN VI), and the superior ophthalmic veins.
• Carotid canal. Transmits the internal carotid artery from the neck into the cranium. In addition, the carotid plexus of sympathetic nerves accompanies the internal carotid artery to provide innervation to the superior tarsal muscle, the dilator pupil muscle, the sweat glands of the face and scalp, and the blood vessels in the head.
• Foramen rotundum. Located posterior to the medial end of the superior orbital fissure. The foramen rotundum transmits the maxillary nerve (CN V-2) en route to the pterygopalatine fossa. CN V-2 supplies the skin, teeth, and mucosa associated with the maxillary bone.
• Foramen ovale. A large ovale foramen posterolateral to the foramen rotundum. The foramen ovale communicates with the infratemporal fossa and transmits the mandibular nerve (CN V-3).
• Foramen spinosum. Transmits the middle meningeal artery.
• Foramen lacerum. A triangular hole located on the sides of the sphenoid body. In a dried skull (e.g., such as that found in an anatomy laboratory), this foramen is patent; however, in vivo, the foramen is occluded by cartilage. After exiting the carotid canal, the internal carotid artery travels over the roof of the foramen lacerum. The greater petrosal nerve courses through the foramen lacerum en route to the pterygoid canal.

Posterior Cranial Fossa

The posterior cranial fossa is the lowest and deepest of the fossae and houses the cerebellum, pons, and medulla oblongata and has the following bony landmarks (Figure 15-3B and C):

• Internal acoustic (auditory) meatus. Transmits the facial nerve (CN VII) and the vestibulocochlear nerve (CN VIII) along with the labyrinthine artery.
• Jugular foramen. Transmits the glossopharyngeal (CN IX), vagus (CN X), and spinal accessory (CN XI) nerves as well as the internal jugular vein. The temporal and occipital bones form the jugular foramen.
• Hypoglossal canal. Transmits the hypoglossal nerve (CN XII).
• Foramen magnum. The largest foramen of the skull. The medulla oblongata, an extension of the spinal cord, enters and exits the cranial vault, along with the vertebral arteries, through the foramen magnum.
• Mastoid foramen. A branch of the occipital artery to the dura mater and mastoid emissary vein that traverses the mastoid foramen.

Foramina in the Base of the Skull

• Petrotympanic fissure. Located posterior to the mandibular fossa and serves as the opening of the chordae tympani nerve, a branch of CN VII, entering the infratemporal fossa.
• Stylomastoid foramen. An opening between the styloid and mastoid processes of the temporal bone. CN VII exits the foramen en route to innervate muscles of facial expression.
• Incisive canal. Located anteriorly on the hard palate. The nasopalatine nerve traverses through the canal.
• Greater and lesser palatine canals. Located posteriorly on the hard palate. The greater and lesser palatine nerves traverse the canals.

Big Picture

The brain is surrounded and protected by three connective tissue layers called meninges. These meninges, from superficial to deep, are the dura mater, arachnoid mater, and pia mater.

Dura Mater

The dura mater is composed of the following two layers (Figure 15-4A):

• Periosteal layer. The periosteal layer of the dura mater is the periosteum lining the internal surface of the skull and, as such, is intimately attached to the cranial bones and sutures.
• Meningeal layer. The meningeal layer is the dura mater proper and is composed of dense collagenous connective tissue that is continuous with the dura mater of the spinal cord. The dura mater envelopes the cranial nerves like a sleeve, which then fuse with the epineurium of the nerves outside the skull.

The two layers of dura mater are bound together. However, the layers separate at numerous locations to form dural septae or dural venous sinuses.

Dural Septae

Much like a seat belt assists in protecting a passenger from hitting the inside of the vehicle during an accident, four dural septae restrict displacement of the brain during everyday movements (Figure 15-4B).

• Falx cerebri. A sickle-shaped layer of dura mater between the cerebral hemispheres. The falx cerebri is attached anteriorly to the crista galli and posteriorly to the tentorium cerebelli. The superior and inferior sagittal sinuses form the superior and inferior margins. The inferior edge of the falx cerebri courses along the superior surface of the corpus callosum.
• Tentorium cerebelli. Separates the occipital lobes of the cerebrum from the cerebellum. The tentorium cerebelli encloses the transverse sinuses posteriorly and the superior petrosal sinuses anteriorly (Figure 15-4C).
• Falx cerebelli. A small triangular extension of the tentorium cerebelli containing the occipital sinus. The falx cerebelli descends, separating the cerebellar lobes, and terminates at the foramen magnum.
• Diaphragma sellae. A circular horizontal fold of dura that covers the pituitary by forming a roof over the sella turcica.

Dural Venous Sinuses

The dural venous sinuses are venous channels located between the periosteal and the meningeal layers of the dura mater. The dural venous sinuses are lined with endothelium and lack valves. They serve as a receptacle for blood from the cerebral, diploic, and emissary veins. They also receive the cerebrospinal fluid (CSF), drained by the arachnoid granulations. Blood in the dural venous sinuses primarily drains into the internal jugular veins.

• Superior and inferior sagittal sinuses. Course in the midline along the superior and inferior borders of the falx cerebri. The inferior sagittal sinus joins with the great cerebral vein (of Galen) to form the straight sinus.
• Straight sinus. Courses along the line of attachment of the falx cerebri to the tentorium cerebelli.
• Occipital sinus. Courses within the falx cerebelli.
• Confluence of sinuses. Forms the union of the superior sagittal straight and occipital sinuses and is located deep to the internal occipital protuberance.
• Transverse sinus. Courses laterally from the confluence of sinuses within the tentorium cerebelli.
• Sigmoid sinus. A continuation of the transverse sinus and descends in an S-shaped groove to join with the inferior petrosal sinus at the jugular foramen, forming the internal jugular vein.
• Cavernous sinus. Located on each side of the sella turcica. The internal carotid artery and CN VI course through the middle of the sinus. Cranial nerves (CNN) III, IV, V-1, and V-2 course anteriorly through the lateral walls of the sinus. The cavernous sinus communicates with the pterygoid venous plexus via emissary veins and the superior and inferior ophthalmic veins.

• Superior petrosal sinus. Courses along the superior portion of the petrous part of the temporal bone and drains into the transverse sinus.
• Inferior petrosal sinus. Courses along the inferior portion of the petrous part of the temporal bone and drains into the cavernous sinus. The inferior petrosal and sigmoid sinuses unite to become the internal jugular vein.
• Diploic veins. Course within the spongy portion of cranial bones.
• Emissary veins. Course between the scalp and dural venous sinuses.

Arachnoid Mater

The arachnoid mater is a thin, transparent layer that surrounds the brain and spinal cord. It is connected to the pia mater by web-like filaments, hence the name “arachnoid” mater (Figure 15-4D).

• Subarachnoid space. The space between the arachnoid mater and the pia mater in which CSF circulates. Many cerebral vessels course around the surface of the brain within the subarachnoid space.
• Arachnoid villi (granulations). Highly folded arachnoid mater that projects into the superior sagittal sinus and lateral lacunae (lateral extensions of the superior sagittal sinus). Arachnoid villi serve as sites where CSF diffuses into the superior sagittal sinus. Arachnoid villi often produce indentations in the inner surface of the calvarium.

Pia Mater

The pia mater is the most internal and delicate of the meninges surrounding the brain and spinal cord (Figure 15-4D). The pia mater forms a sheath around blood vessels as they course into the fissures and sulci and penetrate the brain. The pia mater joins with the ependymal cells that line the ventricles of the brain to form choroid plexuses that produce CSF.