Cranium and Contents Anatomy
The part of the skull that contains the brain and its immediate relations is called the neurocranium. Although the detailed anatomy of the central nervous system is outside the scope of this book, there are some important surface features of the brain (Figs 7.73 & 7.74) to which reference should be made when considering the bony features of the interior of the cranium.
The vault of the skull consists of four flat bones. Anteriorly is the frontal bone, posteriorly, the occipital bone and on each side is a parietal bone (Fig. 7.72). The frontal and parietal bones meet along the coronal suture, and the two parietal bones meet along the midline sagittal suture. Posteriorly, the parietal bones meet the occipital bone at the lambdoid suture. The undersurface of the vault bears a long shallow midline groove for the superior sagittal venous sinus (see below), which terminates at a prominence, the internal occipital protuberance (Fig. 7.75). Below the level of each parietal bone, the greater wing of the sphenoid bone and the squamous part of the temporal bone complete the lateral wall of the vault.
The cranial base is characterized by the anterior, middle and posterior fossae, arranged in step-like sequence (Fig. 7.75), the anterior being superior. The anterior fossa, which accommodates the frontal lobes of the brain, is formed by the frontal bone laterally, the cribriform plates and crista galli of the ethmoid bone medially and is completed posteriorly by the lesser wings of the sphenoid bone. The numerous small foramina in the cribriform plates communicate with the nasal cavity (p. 356).
The middle fossa, occupied by the temporal lobes of the brain, is formed by the greater wings and body of the sphenoid bone. Posteriorly, the fossa is bounded by the petrous part of each temporal bone, while anteriorly the fossa is completed by the lesser wings of the sphenoid. Each of the lesser wings is perforated by the optic canal, which continues forwards into the orbit. Between the lesser and greater wings is the superior orbital fissure, which also provides access to the orbit. The greater wing is traversed by the foramen rotundum (opening into the pterygopalatine fossa; p. 353) and the foramina ovale and spinosum (both opening into the infratemporal fossa; Fig. 7.82). Lying in the angle between the greater wing and body of the sphenoid and the apex of the petrous part of the temporal bone is the foramen lacerum, which forms part of the roof of the infratemporal fossa. In the midline the body of the sphenoid is hollowed, forming the sella turcica. This is limited by the paired anterior and posterior clinoid processes and accommodates the pituitary gland.
The posterior cranial fossa contains the cerebellum, the pons and the medulla oblongata which is in continuity through the foramen magnum with the spinal cord. The floor of this fossa is formed mostly by the occipital bone, supplemented anteriorly by the body of the sphenoid and the posterior surface of the petrous part of each temporal bone. Anterior to the foramen magnum, the occipital and sphenoid bones fuse to form a smooth incline, the clivus, on which the brainstem lies. The internal acoustic meatus opens onto the posterior surface of the petrous part of the temporal bone while in the suture between this bone and the occipital bone is the jugular foramen. Running horizontally from the internal occipital protuberance is a groove for the transverse venous sinus. An S-shaped groove for the sigmoid sinus (Fig. 7.75) links the groove for the transverse sinus with the jugular foramen. The hypoglossal canal traverses the occipi- tal bone anterolateral to the foramen magnum (Fig. 7.82).
Surrounding the brain are three membranes, the meninges, which comprise the dura mater, arachnoid mater and pia mater.
The dura mater consists of outer periosteal and inner meningeal layers. The periosteal dura attaches to and closely follows the bony contours of the cranial cavity and is continuous through the sutures and foramina with the periosteum (pericra- nium) on the outer surface of the skull. Although generally bound to the periosteal dura, the meningeal dura is raised in places as a double layer, forming the dural folds.
The largest of these folds is the midline falx cerebri (Figs 7.76 & 7.77), which projects downwards between the two cerebral hemispheres.
Anteriorly, the falx cerebri is attached to the crista galli of the ethmoid bone. It arches over the corpus callosum and gains further attachment to the frontal, parietal and occipital bones. Posteriorly, the falx ends by attaching to another dural fold, the tentorium cerebelli (Figs 7.76–7.78). The tentorium forms an incomplete roof over the posterior cranial fossa and separates the occipital lobes of the cerebrum from the cerebellum. Each side of the tentorium slopes upwards towards its midline attach- ment to the falx cerebri. Anteriorly, there is a large aperture in the tentorium through which the brainstem passes. The thickened free edge of the tentorium surrounding this aperture continues forwards to reach the anterior clinoid process. The attached border of the tentorium runs laterally from the internal occipital protuberance along a horizontal groove on the inner surface of the occipital bone, then continues medially along the superior border of the petrous part of the temporal bone to reach the posterior clinoid process. Posteriorly, the small falx cerebelli descends vertically from the tentorium and partially separates the two cerebellar hemispheres. The sella turcica in the middle fossa has a roof of dura called the diaphragma sellae, which is attached to the four clinoid processes and has a central aperture for the pituitary stalk.
Dural venous sinuses
Lying between the two layers of dura are endothelium-lined venous channels (Fig. 7.77). These dural venous sinuses, which often groove the adjacent bones, collect blood from the brain and meninges. They also drain cerebrospinal fluid that has been secreted into the subarachnoid space by the choroid plexuses of the brain. A characteristic of these sinuses is the absence of valves.
The superior sagittal sinus lies in the attached margin of the falx cerebri (Figs 7.76 & 7.77). The sinus drains posteriorly and has along its length several dilatations called lacunae. These lacunae possess arachnoid granulations through which the reabsorption of cerebrospinal fluid takes place. The sinus also receives numerous cerebral veins. At the internal occipital protuberance the superior sagittal sinus turns laterally, usually to the right, and continues as the transverse sinus in the attached margin of the tentorium cerebelli (Fig. 7.78). Just before reaching the petrous part of the temporal bone, the sinus turns inferiorly to continue as the sigmoid sinus (Fig. 7.79). This follows an S-shaped course to reach the jugular foramen, through which it is continuous with the internal jugular vein.
The smaller inferior sagittal sinus lies in the free border of the falx cerebri (Fig. 7.76). The sinus runs posteriorly and at the tentorium cerebelli is joined by the great cerebral vein, which drains the deeper structures of the cerebral hemispheres. The union of these vessels forms the straight sinus (Figs 7.76–7.78), which continues posteriorly in the attachment of the falx cerebri to the tentorium as far as the internal occipital protuberance. Here, the straight sinus usually turns to the left to form the transverse sinus, whose course mirrors that on the opposite side. This region is known as the confluence of sinuses.
The cavernous venous sinuses (Figs 7.79 & 7.80) lie on either side of the pituitary gland and the body of the sphenoid bone. They contain numerous interconnected venous spaces, producing a spongy appearance. The two sinuses communicate with each other and receive blood from vessels that pass through the superior and inferior orbital fissures from the ophthalmic veins and pterygoid venous plexuses. Posteriorly each cavernous sinus drains via the superior and inferior petrosal sinuses. The superior petrosal sinus runs along the superior border of the petrous part of the temporal bone to terminate in the junction of the transverse and sigmoid sinuses. The inferior petrosal sinus descends into the posterior cranial fossa and unites with the sigmoid sinus in the jugular foramen to form the internal jugular vein.
The arachnoid mater, the middle of the meningeal layers, is loosely attached to the dura mater, generally following its folds. The arachnoid is separated from the deeper pia mater by the subarachnoid space, which contains cerebrospinal fluid and is traversed by the arteries of the brain and the cranial nerves. Delicate fibres from the arachnoid mater cross the subarachnoid space and attach to the pia mater.
The pia mater is the innermost of the meninges and clings to the surface of the brain, dipping into its numerous grooves or sulci.
Of the many arteries entering the cranium to supply the meninges, one of particular importance is the middle meningeal artery, which arises from the maxillary artery (p. 346) and enters through the foramen spinosum. This vessel runs laterally across the floor of the middle cranial fossa, grooving the bone, and divides on the squamous part of the temporal bone into frontal (anterior) and parietal (posterior) branches (Fig. 7.79). These branches arch superiorly on the inner surface of the lateral part of the skull and supply the meninges lining most of the vault. Meningeal veins follow the arteries and communicate with the dural venous sinuses and with veins lying outside the skull.
Bleeding from veins or arteries between the meningeal layers can raise intracranial pressure. An extradural (epidural) haematoma results from extravasation between the dura and the skull. A subdural haematoma is produced by bleeding between the dura and arachnoid layers, where normally no space exists. Blood leaking from the vessels that cross the subarachnoid space will give rise to a subarachnoid haemorrhage, the blood intermin- gling with cerebrospinal fluid.
Arterial supply to the brain
The brain receives arterial blood from the vertebral and internal carotid arteries (Fig. 7.80). The vertebral arteries (p. 330) enter the posterior cranial fossa through the foramen magnum. Passing upwards and forwards they unite in the midline on the clivus to form the basilar artery. Branches to the brainstem and cerebellum arise from the vertebral and basilar arteries before the latter divides at the upper border of the pons to form the left and right posterior cerebral arteries. Before supplying the posterior part of the cerebral hemisphere, each of these vessels gives rise to a posterior communicating artery, which passes forwards to form part of the cerebral arterial circle (circle of Willis) by anastomosing with the internal carotid artery.
The internal carotid artery traverses the carotid canal (Fig. 7.82) to enter the middle cranial fossa, emerging from the upper part of the foramen lacerum. The artery turns anteriorly to enter the cavernous sinus, then continues superiorly to leave the sinus through its roof. Here, near the anterior clinoid process, the ophthalmic artery arises and accompanies the optic nerve through the optic canal into the orbit. The internal carotid artery terminates as the anterior and middle cerebral arteries. The middle cerebral artery supplies the lateral portion of the cerebral hemisphere, while the anterior cerebral artery ascends between the frontal lobes to supply the medial surface of the hemisphere.
The cerebral arterial circle is formed between the branches of the internal carotid and vertebral arteries (Fig. 7.80). The two anterior cerebral arteries are joined by the anterior communicating artery. On each side, an anastomosis, via the posterior communicating artery, between the posterior cerebral branch of the basilar artery and the internal carotid artery completes the cerebral arterial circle. Aneurisms of the cerebral arterial circle are subject to rupture and subsequent subarachnoid haemorrhage.
The 12 pairs of cranial nerves enter or leave the skull through various foramina (Figs 7.79 & 7.81). The olfactory (I) nerves emerge from the nasal cavity as a number of short branches, which traverse the cribriform plates and terminate in the olfactory bulbs.
The optic (II) nerve leaves the orbit via the optic canal and joins the optic chiasma immediately anterior to the pituitary stalk. From the chiasma the optic tracts pass backwards to enter the brain.
Three cranial nerves enter the orbit through the superior orbital fissure. To reach the fissure the oculomotor (III) and trochlear (IV) nerves run forwards in the lateral wall of the cavernous sinus, while the abducens (VI) nerve passes through the cavity of the sinus. Infection from the face can spread through veins to the cavernous sinus, which may thrombose, causing damage to the abducens nerve and double vision.
The ganglion of the sensory part of the trigeminal (V) nerve lies covered in dura in a small depression on the apex of the petrous part of the temporal bone. The three divisions of the nerve converge on the anterior surface of the ganglion. From the orbit the branches of the ophthalmic (V1) division traverse the superior orbital fissure, coalesce and continues backwards, embedded in the lateral wall of the cavernous sinus, to reach the ganglion. The maxillary (V2) division leaves the pterygopalatine fossa via the foramen rotundum and passes backwards along the lower edge of the sinus to the ganglion. The sensory part of the mandibular (V3) division, accompanied by the motor root of the trigeminal nerve, ascends from the infratemporal fossa through the foramen ovale. The motor root passes beneath and not through the ganglion to traverse the foramen ovale.
The facial (VII) nerve enters, and the vestibulocochlear (VIII) nerve emerges from, the internal acoustic meatus in the petrous part of the temporal bone.
Three nerves leave via the jugular foramen to enter the carotid sheath, namely the glossopharyngeal (IX), vagus (X) and acces- sory (XI) nerves.
Finally, the hypoglossal (XII) nerve traverses the hypoglossal canal.