P. VNormal Atrioventricular septum defect
15.1 The braincase and cranial fossae
The braincase and cranial fossae 115
sinuses, specialized venous channels draining blood from the brain and meningeal blood vessels (see Section 15.1.6 ).
The endosteal layer is the periosteum of the internal surfaces of the braincase and vertebral canal to which it tightly adheres. This layer con- tinues into the foramina, carrying blood vessels and nerves into and out of the cranial cavity and it blends with the periosteum on the exter- nal surface of the braincase. The meningeal layer is tough and fi brous and gives the dura its characteristics. It forms tubular sheaths around the cranial and spinal nerves as they leave the cranial cavity and spinal canal which fuse with the epineurium of the nerves. As illustrated in Figure 15.2 , the meningeal layer is refl ected at certain points to form double-layered folds that project into the cranial cavity between diff er- ent parts of the brain and help stabilize and protect the brain; they are described in Section 15.1.3 .
The pia (mater)
The pia is a delicate layer of richly vascular connective tissue. The pia is represented in black in Figure 15.2 . You can see how it closely covers the surface of the brain and spinal cord at all points; it enters even the deep- est sulci and the walls of the ventricles of the brain to form the choroid plexuses that produce cerebrospinal fl uid.
The arachnoid (mater)
The delicate arachnoid lies against the internal surface of the menin- geal layer of the dura mater and the two are separated by the narrow subdural space that contains a thin fi lm of tissue fl uid. A wider gap separates the arachnoid and the pia. This is the subarachnoid space which is fi lled with cerebrospinal fl uid . As illustrated in Figure 15.2 , fi ne strands of arachnoid tissue extend across the subarachnoid space to the pia and look like a spider’s web; it is this likeness that gives the arachnoid its name (from the Greek = spider). The size of the subarach- noid space varies. It is relatively narrow over convexities of the brain and wider over fi ssures and other depressions. The subarachnoid space is widest on the underside of the brain in areas called the basal cis- terns. If you have the chance to examine a hemisectioned head with the brain still in place, the arachnoid bridging the subarachnoid space
is most clearly seen at the cerebellomedullary cistern occupying the angle between the cerebellum and the posterior surface of the medulla.
The arteries and veins of the brain travel within the subarachnoid space before entering or after leaving the brain tissue. As the cranial nerves cross the subarachnoid space, they pick up sheaths of pia and arachnoid. These sheaths extend as far as the exit of the nerves from the skull where they fuse with the epineurium.
The arachnoid herniates through the meningeal layer of the dura mater to protrude into the venous sinuses and venous lakes in certain areas. Small herniations are called arachnoid villi. The arachnoid granulations are considerably larger and may be seen with the naked eye if the superior sagittal sinus is opened. The villi and granulations are the structures through which cerebrospinal fl uid is absorbed into the bloodstream (see p. 131 )
15.1.3 Dural refl ections
The two largest dural refl ections can be seen in Figure 15.3 in which the right cerebral hemisphere and overlying bone have been removed. The falx cerebri is a double-layered fold of the dura attached along the mid- line inside the cranial vault where it is continuous with the meningeal layer of the dura. Study Figure 15.3 to understand its general shape, its attachments to the skull, and its relationship to venous sinuses. It is attached anteriorly to the crista galli, a short bony process above the nasal cavity, and posteriorly it attaches to the superior surface of the ten- torium cerebelli. The two leaves forming the falx separate and blend on each side with the tentorium cerebelli to enclose the straight sinus . The falx is, therefore, relatively narrow anteriorly and broadens out poste- riorly, resembling the shape of a sickle (hence its name from the Latin falx = sickle). At the convex superior attachment, the two leaves of the falx cerebri are separated to form the superior sagittal venous sinus . The free concave lower border of the falx contains the inferior sagittal venous sinus . The two leaves of the falx are fi rmly united between the superior and inferior sagittal sinuses to form a strong, inelastic membrane that extends into the longitudinal fi ssure between the two cerebral hemi- spheres. The falx, therefore, is ideally positioned to prevent excess move- ment of the brain if the head is subject to violent turning movements.
Endosteal layer of
dura mater Superior sagittal sinus
Venous lake
Superior cerebral vein
Faix cerebri Subarachnoid space
Arachnoid villi and granulations Arachnoid mater
Inferior sagittal sinus Pia mater
Meningeal layer of dura matter
Fig. 15.2 The meningeal layers and formation of venous sinuses.
116 The structure of the central nervous system
As you can see in Figure 15.3 , the tentorium cerebelli (Latin = roof (or tent) over the cerebellum) lies in the horizontal plane at right angles to the falx cerebri. The position and attachments of the tentorium is much more easily appreciated when viewed from directly above as in Figure 15.4 which should be studied as the following description is read. Its two leaves separate on each side to blend with the meningeal dura on the inner aspects of the occipital and parietal bones, enclosing the transverse venous sinus between them. The course of the trans- verse sinuses is marked by prominent grooves in these bones. Anteri- orly, the tentorium is attached to the posterior superior border of the petrous temporal bones; the superior petrosal sinuses are found at these attachments. The anterior attachments extend to just behind the pituitary fossa. Figure 15.4 shows how this arrangement creates short, concave, free inner margins, enclosing an opening occupied by the midbrain. The tentorium projects into the cranial cavity above the cer- ebellum and below the occipital lobes of the cerebral hemispheres. It is well placed to prevent movement of the brain when the head is moved violently forwards and backwards as in nodding the head.
Near the apex of the petrous bone, the lower layer of the tentorium continues forwards to form a small outpouching between the endosteal and meningeal layers of the dura of the middle cranial fossa. This is
the trigeminal (Meckel’s) cave that contains the roots and the sensory ganglion of the trigeminal nerve.
The falx cerebelli is a small curved fold of dura mater which projects into the posterior notch of the cerebellum below the tentorium.
Figure 15.4 illustrates a circular fold of dura, the diaphragma sellae , which forms the roof of the pituitary fossa in the sphenoid bone. The diaphragm of dura completely covers the pituitary gland, apart from a small central opening for the pituitary stalk.
15.1.4 Meninges of the spinal cord
The arrangement of the meninges enclosing the spinal cord is illus- trated in Figure 15.5 . The spinal dura forms the external layer and lies close to the periosteum lining the vertebral canal. The extradural (or epidural ) space between them contains some adipose tissue and a venous plexus. The dura ensheathes the roots of the spinal nerves as they pass through the intervertebral foramina, then fuses with the cov- ering epineurium.
The spinal arachnoid mater lies against the inner surface of the dura. The pia mater closely invests the spinal cord and surfaces of the roots of the spinal nerves. There is a substantial subarachnoid space Fig. 15.3 Dural refl ections and formation of venous sinuses.
Anastamosis with extracranial vein Great cerebral vein Straight sinus Right transverse sinus Tentorium cerebelli
Left sigmoid sinus Left superior petrosal sinus
Right petrosal sinus Falx cerebri Superior sagittal sinus
Inferior sagittal sinus
Fig. 15.4 The tentorium cerebelli and associated venous sinuses from above.
Crista galli Cribriform plate
Foramen ovale Optic nerve
Internal carotid artery Diaphragma sellae
Trigeminal ganglion
Tranverse sinus Straight sinus
Superior petrosal sinus
Middle meningeal artery
Tentorium cerebelli
The braincase and cranial fossae 117
between the pia and the arachnoid containing cerebrospinal fl uid, with strands of arachnoid bridging the gap. The spinal cord is suspended within the subarachnoid space by the denticulate ligaments shown in Figure 15.6 . These are linear structures which run from the upper cervical to the lumbar region on each side of the cord. They extend from the pia to the dura between the exits of successive spinal nerves. Tooth- like projections extend into the interval between the dorsal and ventral roots of the spinal nerves.
The consequences of meningeal infections are described in Box 15.1 .
15.1.5 Nerve and blood supply of the dura mater
The dura of the anterior cranial fossa are supplied by branches of the ophthalmic divisions of the trigeminal nerves ; meningeal branches of the maxillary and mandibular divisions of the trigeminal nerves sup- ply the middle cranial fossa. The dura of the posterior cranial fossa is innervated by ascending branches from the upper cervical spinal nerves entering the cranial cavity through the foramen magnum. The meningeal nerves contain sensory and post-ganglionic sympathetic neurons. The latter may have a vasomotor function and have been implicated in migraine attacks.
The arteries to the dura mater mostly supply the endosteal layer. The principal supply to the dura above the tentorium cerebelli is through the middle meningeal arteries which are branches of the maxillary arteries (see Section 24.5.2 ). They enter the cranial cavity through the foramen spinosum on each side and run laterally across the fl oor of the middle cranial fossa between the endosteal and meningeal layers of the dura and divide into anterior and posterior branches; they groove the bones that they cross. The subtentorial dura is supplied by menin- geal branches of the vertebral arteries (see p. 135 ). The eff ects of blood loss into the meninges are outlined in Box 15.2 .
15.1.6 The venous sinuses
The venous sinuses have been referred to several times as they are formed between the endosteal and meningeal layers of the dura adja- cent to bone and its refl ections. The venous sinuses are wide venous channels draining blood from the brain, the bones of the braincase, and the meninges. The sinus walls are tough sheets of dura with no muscular tissue or valves lined by endothelium like any other blood vessel. Furthermore, they are held patent at all times because of their construction; the sinuses will, therefore, not collapse when blood pres- sure falls, e.g. in shock, meaning that perfusion of the brain should continue.
The arrangement of the venous sinuses can best be pictured by stud- ying Figure 15.7 in conjunction with Figure 15.3 and 15.4.
The superior sagittal sinus receives blood from superior cerebral veins draining blood from the superior, lateral, and medial surfaces of the cerebral hemispheres. Venous blood drains backwards to the back of the skull. Figure 15.3 shows what happens at the point where the straight sinus and the superior sagittal sinus meet at a point marked by a slight bulge of bone, the internal occipital protuberance. The superior sagittal sinus usually turns to the right, but occasionally to the left, to become the transverse sinus of that side.
As illustrated in Figure 15.3 , the inferior sagittal sinus running in the inferior free margin of the falx cerebri receives blood from the falx and ends by opening into the straight sinus formed where the falx attaches to the tentorium cerebella. The straight sinus drains the inferior sagittal sinus and also receives blood from the great cerebral vein which drains the deep parts of the cerebral hemispheres. Figure 15.4 shows that the sinus ends at the internal occipital protuberance by turning to become, in most cases, the left transverse sinus.
The transverse sinuses pass forward until they reach the junction of the petrous and mastoid parts of the temporal bone where they curve downwards to become the sigmoid sinuses. The sigmoid sinuses have Subarachnoid
space Arachnoid mater
Pia mater Dorsal root
Ventral root Dura mater
Denticulate ligament Dorsal root ganglion
Fig. 15.5 A cross section of the spinal cord and covering meninges.
Dorsal median sulcus
Dorsolateral sulcus Gracile fasciculus
Cuneate fasciculus
Dorsal root Ventral root Dura mater Denticulate ligament
Dorsal root ganglion
Fig. 15.6 The denticulate ligaments of the spinal cord. The dura mater dorsal to the cord has been removed.
Box 15.1 Infections of the meninges
Infl ammation of the pia and arachnoid mater is called meningitis and usually involves spinal as well as cerebral meninges. It is most frequently caused by bloodborne infections from microorgan- isms; infective organisms may also spread from a brain abscess or be introduced through a penetrating wound. The outstanding features of meningitis are headache, fever, rash, and the signs of spinal meningeal irritation (neck rigidity). Some of the organisms that cause meningitis are extremely virulent; meningitis can have an extremely rapid onset and may quickly become fatal unless diagnosed and treated urgently.
118 The structure of the central nervous system
a shallow S shape, seen most clearly in Figure 15.7 , as they continue downwards and forwards to the jugular foramina. They deeply groove the endocranial surface of the mastoid part of the temporal bone. The sigmoid sinuses expand to form the jugular bulbs in the posterior part of the jugular foramen; these are continuous with the internal jugular veins below.
15.1.7 The cavernous sinuses
Two important sinuses have not been encountered yet. These are the right and left cavernous sinuses on each side of the body of the sphe- noid shown in Figure 15.7 . If they are viewed in cross section, numerous fi brous partitions cross their cavities, partially dividing them into a series
of small caverns—hence the name for these structures. Like most of the other sinuses, they lie between the endosteal and meningeal layers of the dura. The endosteal layer covers the lateral aspects of the body of the sphenoid as might be expected from the position of the cavern- ous sinuses. The meningeal dura extends laterally from the diaphragma sellae covering the pituitary fossa and then descends to the fl oor of the middle cranial fossa, forming the roof and lateral walls of the sinuses.
Several important structures pass through or along the walls of the sinuses:
• The internal carotid arteries (see Section 15.6.1 );
• The third, fourth, and sixth cranial nerves supplying the extraocular
muscles that move the eyes (see Chapter 18 );
Box 15.2 Extradural and subarachnoid haemorrhage
Intracranial haemorrhage may occur either within or outside the brain. Intracranial haemorrhage occurring outside the brain may occur in the extradural, subdural, or subarachnoid space. The eff ects of haemorrhages inside the brain are described on in Box 15.10 . The extradural space is a potential space, located between the fused endosteal and meningeal layers of the dura mater. Haemor- rhage into this space— extradural haemorrhage —is relatively uncommon and is usually the result of tearing of meningeal vessels or venous sinuses consequent upon a fracture of or heavy blows to the braincase. The meningeal vessels are particularly vulnerable to blows to the temple because the bone is relatively thin.
The subdural space is another potential space between the meningeal layer of the dura and the arachnoid mater. Bleeding into this space ( sub- dural haemorrhage ) is more common, usually due to tearing of one or more of the superior cerebral veins at the point where they enter the superior sagittal sinus. Although the cerebral veins run for most of their extracerebral course in the subarachnoid space, they cross the subdural space to reach the venous sinuses. In blows to the head, the brain may be suddenly moved enough to tear the cerebral veins near where they are fi xed where they pass through the dura mater. Blood then enters the subdural space.
Extradural and subdural haemorrhages result in raised intracranial pressure (ICP). The principal clinical features are a consequence of brain compression caused by the raised ICP. The rate of onset is extremely variable. Essentially, blood forces its way into a very limited space to produce localized, but extremely intense, pressure which raises the ICP quite rapidly. This, in turn, results in ‘coning’ , the compression of the medulla as it is displaced into the foramen magnum; coning compromises the functions of the cardiovascular and respiratory centres of the medulla with life-threatening conse- quences (see Box 3.4 ).
The subarachnoid space is a real space occupied by cerebros- pinal fl uid. Blood has room to spread when vessels haemorrhage into this space ( a subarachnoid haemorrhage ). Subarachnoid haemorrhages may follow the rupture of a congenital aneurysm of one or other of the arteries comprising the arterial circle (see Section 15.5.1 ) or the rupture of cerebral veins by a blow to the head with consequent bleeding into the subarachnoid space.
Although ICP will rise eventually, the principal clinical features of subarachnoid haemorrhage are those of meningeal irritation described above because extravasated blood is a powerful irritant of neuronal tissues.
Fig. 15.7 The venous sinuses seen from above after removal of the tentorium cerebelli.
Ophthalmic veins
Cavernous sinus
Straight sinus
Internal carotid artery in cavernous sinus Inferior petrosal sinus
Superior petrosal sinus
Superior sagittal sinus Transverse sinus Sigmoid sinus Emissary veins
The spinal cord 119
The spinal cord is oval in cross section, being fl attened dorsoventrally as outlined in Figure 15.8 . There is a general reduction in diameter from the cervical to the coccygeal region, but there are two enlarge- ments where the thick spinal nerves supplying the limbs emerge.
The cervical enlargement in the lower cervical and upper thoracic region is where the nerves forming the brachial plexus innervating the upper limb arise. The lumbar enlargement is the area where lumbar and sacral nerves form the lumbar and sacral plexuses to supply the lower limbs.
The external surface of the cord is shown from the dorsal aspect in Figure 15.6 . A shallow dorsal median sulcus marks the midline and there are shallow dorsolateral sulci laterally. Figure 15.8 indicates the corresponding indentations on the ventral aspect; the ventral median fi ssure is obvious, but the ventrolateral sulci are shallow.
Thirty-one bilaterally paired spinal nerves —eight cervical , 12 tho- racic , fi ve lumbar , fi ve sacral , and one coccygeal —are attached to the cord. Each nerve is formed by the fusion of a dorsal and ventral root. Each root is formed from the confl uence of a series of rootlets entering or leaving the corresponding segment of the cord. Dorsal roots exit along the dorsolateral sulcus (see Figure 15.6 ) and ventral roots along the ventrolateral sulcus. The structure of spinal nerves has already been described in Chapter 3 ; their relationship to the spinal cord and vertebrae has also been covered above and illustrated in Figure 3.5 .
15.2.1 Internal structure
As shown in Figure 15.6 and 15.8, there is a clear distinction between the grey and white matter in a transverse section of the spinal cord which can be seen with the naked eye on anatomical preparations of the spi- nal cord. The grey matter is H-shaped in transverse section and forms a continuous mass throughout the length of the spinal cord; grey matter contains neuronal cell bodies, dendrites, and synapses. The crossbar of the H is the central commissure formed by neuronal processes con- necting the right and left halves of the spinal cord. Look carefully at Figure 15.8 and you will see a narrow central canal centrally in the com- missure; the canal is continuous with the ventricular system of the brain.
Box 15.3 The venous sinuses and infection
The venous sinuses drain the cranial cavity whereas the structures in the head and neck lying outside the cranial cavity drain into con- ventional veins. The skin and immediately underlying tissues drain into superfi cial veins whereas deeper structures drain into the deep veins. The superfi cial and deep drainage of the head and neck will be described more fully in Section 23.2.2 .
It is important to realize that the superfi cial, deep, and venous sinus components of the venous drainage are all interconnected. These connections can be a route for the spread of infection which can be particularly serious if it enters the cranial cavity where the meninges or venous sinuses may be infected. The cavernous sinus in particular has numerous connections with extracranial veins. The orbit drains through ophthalmic veins into the cavernous sinuses posteriorly, but also into the venous drainage of the face anteriorly; the two routes
are connected. The ophthalmic veins also communicate with the pterygoid plexus of veins before entering the sinus; the pterygoid plexus receives venous blood from deep structures in and around both jaws, including most of the dental structures (see Section 24.5.3 ). Infection may thus spread from the face or from structures drained by the pterygoid plexuses into the cavernous sinuses. Blood fl ow through the cavernous sinuses is relatively slow because they are partitioned into small interconnected cavities, making it more prone to clot, producing an infected thrombosis. Thrombosis in the cavernous sinuses may produce a retrograde thrombosis into one of the middle cerebral veins that drain deep parts of the brain; retro- grade thrombosis is usually fatal. It will also aff ect the cranial nerves passing through the cavernous sinus, leading to defective eye move- ments (see Box 18.3 ).
• The ophthalmic and maxillary divisions of the trigeminal nerve (see
Chapter 18 ).
The cavernous sinuses receive blood from the lateral and inferior sur- faces of the cerebral hemispheres and the ophthalmic veins from the orbit and drain via the superior and inferior petrosal sinuses that exit from the posterior part of the cavernous sinuses on each side ( Figure
15.7 ). Each superior sinus runs along the superior border of the petrous temporal bone to enter the junction of the transverse and sigmoid sinuses. Each inferior sinus runs more or less directly downwards into the jugular foramen.
The potential spread of infection through venous sinuses and its con- sequences are described in Box 15.3 .
Box 15.4 Lumbar puncture
The cauda equina is surrounded by subarachnoid space. When a sample of cerebrospinal fl uid is required for examination, it is usually taken from this region by means of a lumbar puncture. A needle is inserted between the arches of the third and fourth lum- bar vertebrae and advanced until its tip enters the subarachnoid space. Inserting the needle at this level avoids any possibility of damaging the spinal cord and the nerves forming the cauda equina will fl oat away from the needle. The same site is used for epidural injections that may be used to alleviate pain during childbirth.