Chapter 43: The brain, cranial nerves and meninges

Nervous system

The nervous system in general is described in chapter 3. The divisions of the brain are summarized here in table 43-1. This chapter is limited to a brief description of the gross structure of the brain, an account of the ventricles, and some general remarks on the cranial nerves, the meninges, and the blood supply. Further details, including information about the internal structure of the brain, should be sought in books and atlases on neuroanatomy.

Gross structure of brain

The brain consists of (from caudal to rostral) the hindbrain, midbrain, and forebrain.

In their attachment to the brain, the first two cranial nerves are associated with the forebrain, nerves III and IV with the midbrain, and nerves V to XII with the hindbrain.


The hindbrain, or rhombencephalon, consists of the medulla oblongata, the pons, and the cerebellum.

Medulla oblongata (fig. 43-1).

The rostral part of the spinal cord expands, passes through the foramen magnum, and becomes the medulla oblongata. The medulla rests anteriorly on the basilar part of the occipital bone. Posteriorly, the medulla is largely covered by the cerebellum (see fig. 43-3).

The caudal half of the medulla contains the continuation of the central canal of the spinal cord, which, in the rostral half of the medulla, widens to become the fourth ventricle.

The medulla presents an anterior median fissure. The portion of the medulla adjacent to the anterior median fissure on each side is termed the pyramid. It contains the fibers of the pyramidal (corticospinal) tract. Where the caudal part of the medulla meets the cervical spinal cord, the anterior median fissure is interrupted by the decussation (crossing) of the pyramids. About 75 to 90 per cent of the descending pyramidal nerve fibers cross the median plane.

Lateral to each pyramid in the rostral medulla, there is a prominent elevation termed the olive. The hypoglossal (twelfth cranial) nerve emerges from the medulla between the pyramid and the olive, whereas the accessory (eleventh cranial), vagus (tenth cranial), and glossopharyngeal (ninth cranial) nerves emerge posterolateral to the olive.

The dorsal aspect of the medulla presents a posterior median sulcus. On each side, two tracts from the spinal cord (the fasciculus gracilis medially and fasciculus cuneatus laterally) terminate in eminences known as the gracile and cuneate tubercles, respectively. More rostrally, the caudal part of the fourth ventricle is bounded laterally by the inferior cerebellar peduncles, which comprise fibers connecting the medulla and spinal cord with the cerebellum (fig. 43-2).

The medulla contains very important nerve centers associated with functions such as respiration and circulation.

Pons (fig. 43-1).

The pons lies between, and is sharply demarcated from, the medulla and midbrain. It is situated anterior to the cerebellum and appears superficially to bridge (hence its name) the two cerebellar hemispheres. As seen from the anterior side, the transverse fibers of the pons form the middle cerebellar peduncle on each side and enter the cerebellum. The middle peduncle actually comprises fibers that connect one side of the pons to the contralateral cerebellar hemisphere.

The anterior aspect of the pons rests on the basilar part of the occipital bone and on the dorsum sellae. The pons is grooved longitudinally on its anterior side, and this groove is frequently occupied by the basilar artery.

The vestibulocochlear (eighth cranial), facial (seventh cranial), and abducent (sixth cranial) nerves (from lateral to medial) emerge in the groove between the pons and the medulla (see fig. 43-1). More rostrally, the trigeminal (fifth cranial) nerve emerges from the side of the pons by a large sensory and a smaller motor root (fig. 43-3).

The posterior aspect of the pons forms the floor of the rostral fourth ventricle (fig. 43-2), which is bounded laterally by the superior cerebellar peduncles. Each superior peduncle comprises fibers that connect the cerebellum with the midbrain and thalamus.

The trigeminal (fifth cranial) nerve is large and complicated. It is the route of somatic sensation from the face, teeth, mouth, and nasal cavity, and it is motor to the muscles of mastication. It emerges from the side of the pons as a sensory and a motor root, generally with some intermediate fibers. The two portions proceed from the posterior to the middle cranial fossa by passing deep to the attachment of the tentorium cerebelli to the petrous part of the temporal bone and also by passing inferior to the superior petrosal venous sinus. The sensory root expands into a large, flat trigeminal (semilunar) ganglion, which contains the cells of origin of most of the sensory fibers. The ganglion overlies the foramen lacerum, and the roots of the nerve occupy an impression on the anterior surface of the petrous part of the temporal bone near its apex. Most of the ganglion is enclosed in the cavity of the dura known as the trigeminal (Meckle's) cave. The ganglion gives rise to three large divisions: the ophthalmic, maxillary, and mandibular nerves. The motor root, which contains proprioceptive as well as motor fibers, continues inferior to the ganglion and joins the mandibular nerve. The ganglion can be "blocked" by passing a needle through the mandibular notch and the foramen ovale and injecting an anesthetic. The sensory root may be viualized in the middle cranial fossa of patients with trigeminal neuralgia (tic douloureux), where the nerve is often irritated by a closely associated arterial loop.

Cerebellum (figs. 43-3 and 43-16).

The cerebellum is situated on the dorsal aspect of the brain stem, to which it is attached by the three cerebellar peduncles on each side. The inferior peduncles connect the cerebellum with the medulla; the middle connect it with the pons; and the superior connect it with the midbrain. The cerebellum occupies the majority of the posterior cranial fossa. It comprises a median portion, termed the vermis, and two lateral parts known as cerebellar hemispheres. Like the cerebral hemispheres, the cerebellum has a cortex of gray matter. The cerebellar cortex is folded to form transverse folia, which are separated from one another by fissures. The cerebellum is connected by tracts with the cerebral cortex and with the spinal cord. It is important in the coordination of muscular activities and the learning of motor tasks. It may have other roles in cognition, as well.

The archicerebellum (figs. 43-3 and 43-16) is the flocculonodular lobe, which is vestibular in function (i.e., concerned with equilibration). The paleocerebellum comprises most of the vermis and the adjacent parts of the hemispheres and is spinal in function (i.e., concerned with posture and muscle tone). The neocerebellum consists of the lateral parts of the hemispheres and is corticopontine in function (i.e., concerned with the control of voluntary movements).

Midbrain (see figs. 43-1 and 43-16).

The midbrain, or mesencephalon, connects the hindbrain with the forebrain. It is located in the tentorial notch of the dura mater (see fig. 43-15). It consists of a ventral part, the cerebral peduncles, and a dorsal part, the tectum.

The cerebral peduncles (crus cerebri) are two large bundles of nerve fibers that converge as they proceed caudally from the cerebral hemispheres. At their rostral ends they are continuous with a band of white matter termed the internal capsule. These fibers represent the main connections between the cerebral cortex and the brain stem. The rostral part of each peduncle is crossed by the corresponding optic tract. The right and left optic tracts emerge from the optic chiasma, which is formed by the junction of the two optic nerves. The depression posterior to the chiasma, bounded by the optic tracts and the cerebral peduncles is termed the interpeduncular fossa (see fig. 43-7). The interpeduncular fossa contains, from rostral to caudal: (1) the tuber cinereum and the infundibular stem of the hypophysis, (2) the mamillary bodies, and (3) the posterior perforated substance.

The oculomotor (third cranial) nerve emerges from the interpedunculr fossa at the rostral border of the pons and at the medial border of the corresponding cerebral peduncle.

The tegmentum represents most of the core substance of the midbrain while the tectum, or dorsal part of the midbrain, contains four bumps or hillocks. These are known as the superior and inferior colliculi (see fig. 43-2). The superior colliculi are concerned with visual reflex functions, the inferior with auditory functions. The pineal body is attached to the forebrain just superior to the superior colliculi.

The trochlear (fourth cranial) nerve decussates and emerges from the dorsal aspect of the midbrain just caudal to the corresponding inferior colliculus (see fig. 43-2).

The midbrain is traversed by the cerebral aqueduct, which connects the fourth with the third ventricle (see fig. 43-9).

The medulla, pons, midbrain, and (frequently) diencephalon (that part of the forebrain that is adjacent to the midbrain) are collectively known as the brain stem (see fig. 43-1).

Forebrain (figs. 43-4, 43-5 and 43-6)

The forebrain, or prosencephalon, comprises a smaller part, the diencephalon, and a massive portion, the telencephalon.


The diencephalon comprises most of the walls of the third ventricle.

The diencephalon includes the (1) thalami (with the medial and lateral geniculate bodies), (2) pineal body and habenulae, (3) hypothalamus, and (4) subthalamus.

The thalami are two large masses of gray matter situated one on each side of the third ventricle. Each thalamus includes many nuclei and acts as an important processing and relay center for sensory and motor signals enroute to the cerebral cortex.

The medial and lateral geniculate bodies are prominences on the posteroinferior part of the thalmus, just rostral to the colliculi of the midbrain, to which they are connected. They are important auditory and visual relay centers, respectively.

The pineal body, or epiphysis, is located inferior to the splenium of the corpus callosum (see fig. 43-16A) and is frequently visible radiographically because it calcifies.

The term hypothalamus is restricted functionally to the part of the brain comprising the floor and the inferolateral walls of the third ventricle. This region is concerned with autonomic and neuro-endocrinological functions. Anatomically, it is immediately dorsal to the optic chiasma, and the tuber cinereum (to which the infundibular stem of the hypophysis is attached) and the mamillary bodies are portions of the hypothalmus that can be seen on the ventral surface of the brain. The hypophysis is functionally connected to the hypothalmus (see fig. 43-7).


The telencephalon consists of the cerebral hemispheres (figs. 43-4, 43-5 and 43-6) and the deeply-lying basal ganglia (this term is, of course a misnomer, since these are nuclei, not ganglia). The term cerebrum, however, is often used more vaguely to refer either to the brain as a whole or to merely the forebrain or the forebrain and midbrain together. Each hemisphere contains a cavity known as the lateral ventricle.

As seen from the dorsal side, the cerebral hemispheres conceal the other parts of the brain from view. Each hemisphere presents a superolateral, a medial, and an inferior surface. The right and left cerebral hemispheres are partly separated from each other by the longitudinal, interhemispheric fissure, which is occupied by a fold of dura mater, the falx cerebri. The corpus callosum (figs. 43-6 and 43-16A), found in the depths of the longitudinal fissure, is a bundle of fibers connecting the hemispheres. It forms the roof of the frontal horn of the lateral ventricle of each side. It is curved sagittally and consists, from anterior to posterior, of the rostrum, genu, body, and splenium.

Each hemisphere has frontal, occipital, and temporal poles. These poles are located, respectively, in the anterior, posterior, and middle cranial fossae, and they are related to the frontal and occipital bones and the greater wing of the sphenoid bone.

The gray matter of the surface of each hemisphere is termed the cerebral cortex. It is folded or convoluted into gyri, which are separated from each other by sulci. The pattern of folding is variable, and it is necessary to remove the arachnoid to identify individual gyri and sulci.

There are certain gyri and sulci that are relatively consistent and provide landmarks that identify structural and functional subdivisions of the brain. The lateral fissure (sulcus) begins on the inferior surface of the brain. It extends laterally and, on reaching the superolateral surface of the hemisphere, proceeds posteriorward between (1) the frontal and parietal lobes (on its superior side) and (2) the temporal lobe (on the inferior side). A portion of the cerebral cortex termed the insula lies buried in the depths of the lateral fissure.

Language functions are centered in the dominent hemisphere (usually the left hemisphere) in the region adjacent to the lateral fissure. Expressive functions of language (including the ability to write) are located in the inferior part of the frontal lobe of this hemisphere, while receptive functions of language are centered in the posterosuperior part of the temporal lobe, also adjacent to the lateral fissure (fig. 43-6A).

The central sulcus begins on the medial surface of the hemisphere and, on reaching the superolateral surface, descends between the frontal and parietal lobes. The area of cortex immediately anterior to the central sulcus is known as the motor area and is concerned with muscular activity, mostly in the opposite half of the body. The contralateral control may be demonstrated by electrical stimulation of this area (particularly of that part known as the precentral gyrus, or area 4), which will produce movements of the opposite half of the body. Furthermore, the body is represented in an inverted position in the motor area. That is, stimulation of the superior part of the motor area gives rise to movements of the opposite lower limb; stimulation of the middle part, to movements of the upper limb; and stimulation of the inferior part, to movements of the head and neck. The area of cortex immediately posterior to the central sulcus (the postcentral gyrus) is an important primary receptive area for somatic sensation. The various sensory pathways reach this region by means of relays in the thalamus.

The cerebral cortex is arbitrarily divided into frontal, parietal, occipital, and temporal lobes. The frontal lobe is bounded by the central sulcus and lateral fissure. It lies in the anterior cranial fossa and projects superior to the orbits and nasal cavity. The parietal lobe extends posteriorward from the central sulcus to an arbitrary line of separation from the occipital lobe (between the parieto-occipital sulcus on the superior side, and a vague indentation, the pre-occipital notch, on the inferior surface). The occipital lobe lies posterior to this line. The temporal lobe is situated anterior to this line and inferior to the lateral sulcus. It lies in the middle cranial fossa. The gyri and sulci characteristic of these lobes are shown in figures 43-5 and 43-6. The calcarine sulcus, defining the location of primary visual perception, is on the medial surface of the occipital lobe. When the cerebellum is displaced, Portions of each of the four lobes are obscured by the cerebellum (on the inferior side) and in the depths of the interhemispheric fissure (medially).

The olfactory (first cranial) nerves are groups of nerve filaments that, on leaving the nose and passing through the base of the skull (cribriform plate of the ethmoid bone), end in the olfactory bulbs. Each olfactory bulb lies on the inferior aspect of the corresponding frontal lobe and gives rise to an olfactory tract (fig. 43-7) that runs posteriorward and is attached to the brain.

The optic (second cranial) nerves leave the orbits through the optic canals and unite to form the optic chiasm(a) (fig. 43-7). The chiasm gives rise to the right and left optic tracts, which proceed backward and around the cerebral peduncles. The optic chiasm and the interpeduncular fossa are contained within a very important arterial anastomosis known as the arterial circle (circulus arteriosus, of Willis). The infundibular stem of the neurohypophysis emerges from the tuber cinereum in the interpeduncular fossa, anterior to the mamillary bodies. The area immediately anterolateral to each optic tract is pierced by minute branches of the anterior and middle cerebral arteries and is known as the anterior perforated substance.

The basal ganglia (really nuclei) are large masses of gray matter within the white substance of the cerebral hemispheres, especially the corpus striatum (caudate and putamen nuclei), globus pallidus, and claustrum.

The corpus striatum comprises the caudate and lentiform nuclei. The caudate nucleus bulges into the lateral ventricle and presents a head, a body, and a tail. It has an arched form that often results in it being seen twice in a single section through the brain. The head of the caudate lies anteriorly, just posterior to the genu of the corpus callosum; the body extends posteriorward, superior and lateral to the thalamus; and the tail of the nucleus curves inferiorward and anteriorward into the temporal lobe to end when it reaches the amygdaloid body. The lentiform (or lenticular) nucleus, consisting of the antomically contiguous putamen and globus pallidus, lies lateral to the head of the caudate nucleus and to the thalamus. At its anterior extreme, it is connected with the head of the caudate nucleus by bars of gray matter, hence the name corpus striatum for the two nuclei. The lateral part of the lentiform nucleus, known as the putamen, is related laterally to the claustrum and the insula. The two medial parts of the lentiform nucleus are called the globus pallidus.

The amygdala (L., almond), is located deep to the anteromedial part of the temporal lobe and can be seen to raise a prominence of this cortical surface called the uncus.

The internal capsule is a broad band of white matter situated between (1) the lentiform nucleus laterally and (2) the head of the caudate nucleus and the thalamus medially. The internal capsule consists of an anterior limb (between the lentiform and caudate nuclei), a genu, a posterior limb (between the lentiform nucleus and the thalamus), and retrolentiform and sublentiform parts (posterior and inferior to the lentiform nucleus, respectively).

The fibers of the internal capsule, on being traced superiorward, spread out in the hemisphere to form a fan-shaped arrangement termed the corona radiata. The fibers of the corona are intersected by those of the corpus callosum. Inferiorly, the fibers of the internal capsule continue into the crus cerebri of the midbrain.

Craniocerebral topography (fig. 43-8)

Considerable variation occurs in the precise relations of the brain to the skull, so only an approximate localization of the parts of the brain is possible on the surface of the body.

The inferior limit of the cerebral hemisphere lies superior to the eyebrow, zygomatic arch, external acoustic meatus, and external occipital protuberance. Hence the hemispheres lie above the orbitomeatal plane. A considerable portion of the cerebellum, however, lies inferior to that plane.

The central sulcus begins at 1 cm posterior to the vertex, that is, posterior to the midpoint of a line on the head between the nasion and the inion. The sulcus runs inferiorward, anteriorward, and laterally for about 10 cm toward the midpoint of the zygomatic arch. The sulcus makes about three quarters of a right angle with the median plane.

The lateral fissure, on the superolateral surface of the hemisphere, extends from the pterion posteriorward and slightly superiorward and ends just inferior to the parietal eminence.

Ventricles (figs. 43-9, 43-10, 43-11 and 43-16)

The two lateral ventricles (fig. 43-9) communicate with the third ventricle by an interventricular foramen (Monroe) on each side. The third ventricle communicates with the fourth ventricle through the cerebral aqueduct (Sylvius). The fourth ventricle becomes continuous with the central canal of the medulla and spinal cord and opens into the subarachnoid space by means of apertures located just inferior to the cerebellum. There is a midline opening (Majendie) and two lateral openings (Luschka).

The neuroglia that lines the ventricles of the brain and the central canal of the spinal cord is termed ependyma. In the ventricles, vascular fringes of pia mater, known as the tela choroidea, invaginate their covering of modified ependyma and project into the ventricular cavities. This combination of vascular tela and cuboidal ependyma is termed the choroid plexus (see fig. 43-11). The plexuses are invaginated into the cavities of the lateral, third, and fourth ventricles, and they are the site of production of cerebrospinal fluid.

The term blood-cerebrospinal fluid barrier refers to the tissues that intervene between the blood and the cerebrospinal fluid.

Lateral ventricles.

Each lateral ventricle is a cavity in the interior of a cerebral hemisphere, and each communicates with the third ventricle by means of an interventricular foramen. The portion of the lateral ventricle anterior to the foramen is termed its frontal (anterior) horn. Posterior to this foramen is the central part of the ventricle, which divides into occipital (posterior) and temporal (inferior) horns. The horns of the ventricles extend into corresponding lobes of the cerebral hemisphere (fig. 43-9).

The frontal (anterior) horn of the lateral ventricle is bounded inferiorly by the rostrum, anteriorly by the genu, and superiorly by the body, of the corpus callosum. Laterally, it is limited by the bulging head of the caudate nucleus. Medially, it is separated from the lateral ventricle of the opposite side by a thin vertical partition, the septum pellucidum.

The central part of the lateral ventricle lies inferior to the trunk of the corpus callosum and superior to the thalamus and the body of the caudate nucleus. Medially, the two lateral ventricles are separated from each other by the posterior portion of the septum pellucidum and the fornix (which is an arched band of fibers). In the angle between the diverging occipital (posteiror) and temporal (inferior) horns, the floor of the cavity presents a triangular elevation, the collateral trigone, associated with an underlying groove (generally the collateral sulcus).

The variable occipital horn tapers posteriorward into the occipital lobe of the hemisphere. On the superolateral side, each posterior horn is bounded by a sheet of fibers (the tapetum) derived from the body and the splenium of the corpus callosum. Medially, two elevations may project laterally into the occipital horn. The superior elevation (bulb of the occipital horn) is produced by fibers (forceps major) derived from the splenium. The inferior (calcar avis) is associated with a groove (calcarine sulcus) on the exterior of the hemisphere.

The temporal (inferior) horn extends inferiorward and anteriorward as it continues posterior to the thalamus to enter the temporal lobe of the hemisphere. It is bounded laterally by fibers (the tapetum) derived from the corpus callosum. Inferiorly, the most noticeable feature is an elevation known as the hippocampus, which is partly covered by the choroid plexus. Superiorly, the tail of the caudate nucleus runs forward to reach the amygdaloid body, which is at the rostral extent of the temporal horn of the ventricle.

The choroid plexus of each lateral ventricle is invaginated along a curved line known as the choroid fissure. The fissure extends posteriorly from the interventricular foramen, and in an arched manner around the posterior end of the thalamus, as far as the end of the temporal horn. The choroid plexus of the lateral ventricle is practically confined to the central part and the temporal horn. It is best developed at the junction of the central part with the yrmpotsl horn, and it is there known as the glomus choroideum. Calcified areas (corpora amylacea) are frequent in the glomera and can be easily seen on CT scans. The vessels of the plexus are derived from the internal carotid (anterior choroidal artery) and the posterior cerebral (posterior choroidal) arteries. At the interventricular foramina the choroid plexuses of the two lateral ventricles become continuous with each other and with that of the third ventricle.

Third ventricle.

The third ventricle is a narrow cleft between the two thalami. Over a variable area the thalami are frequently adherent to each other, giving rise to the interthalamic adhesion. The floor of the ventricle is formed by the hypothalamus. The anteroinferior extent of the floor of this ventricle is crossed by the optic chiasm. The anterior wall is formed by the lamina terminalis, a delicate sheet that connects the optic chiasma to the corpus callosum. The thin roof  of the ventricle consists of ependyma covered by two layers of pia (known as the velum interpositum).

The third ventricle communicates with the lateral ventricles by means of the interventricular foramina. Each interventricular foramen is situated at the superoanterior portion of the third ventricle, at the anterior limit of the thalamus. This represents the site of outgrowth of the cerebral hemisphere in the embryo. From this foramen, a shallow groove, the hypothalamic sulcus, may be traced posteriorward to the cerebral aqueduct. The sulcus marks the boundary between the thalamus (superior) and the hypothalamus (inferior).

The third ventricle presents several recesses (fig. 43-9): optic, infundibular, pineal, and suprapineal.

The choroid plexuses of the third ventricle invaginates the roof of the ventricle on each side of the median plane (see fig. 43-16B). At the interventricular foramina they become continuous with those of the lateral ventricles. Their vessels (posterior choroidal arteries) are derived from the posterior cerebral.

The aqueduct is the narrow channel in the midbrain that connects the third and fourth ventricles.

Fourth ventricle.

The fourth ventricle is a rhomboid-shaped cavity (see fig. 43-2) dorsal to the pons and medulla and separating these protions of the brain stem from the cerebellum. Rostrally, it narrows to become continuous with the cerebral aqueduct of the midbrain. Caudally, it narrows and leads into the central canal of the medulla, which, in turn, is continuous with the central canal of the spinal cord. Laterally, the widest portion of the ventricle is prolonged on each side as the lateral recess (see fig. 43-1). The superior and inferior cerebellar peduncles form the lateral boundaries of the ventricle.

The ventral aspect or floor of the fourth ventricle (the rhomboid fossa) is formed by the pons (rostral) and by the medulla (caudal) (see fig. 43-2). It is related directly or indirectly to the nuclei of origin of the last eight cranial nerves. A median groove divides the floor into right and left halves. Each half is divided by a longitudinal groove (the sulcus limitans) into medial (basal) and lateral (alar) portions. The medial portion, known as the medial eminence, overlies certain motor nuclei, e.g., those of the abducent and hypoglossal nerves. The area lateral to the sulcus limitans overlies certain sensory nuclei, e.g., that of the vestibular part of the vestibulocochlear nerve.

The caudalmost portion of the floor of the fourth ventricle is shaped like the point of a pen (calamus scriptorius) and contains the important respiratory, cardiac, vasomotor, and deglutition centers.

The posterior boundary or roof of the fourth ventricle is extremely thin and concealed by the cerebellum (see fig. 43-16A). It consists of sheets of white matter (superior and inferior medullary vela), which are lined by ependyma and stretch between the two superior and the two inferior cerebellar peduncles. The caudal portion of the roof presents a deficiency, the median aperture of the fourth ventricle, through which the ventricular cavity communicates with the subarachnoid space. The extremes of the lateral recesses have similar openings, the lateral apertures. The median and lateral apertures are the only means by which cerebrospinal fluid formed in the ventricles enters the subarachnoid space. In the event of occlusion of the apertures (or any obstruction of the free flow of cerebrospinal fluid, for that matter), the ventricles become distended (hydrocephalus).

The choroid plexuses of the fourth ventricle invaginate the roof on each side of the median plane. A prolongation of each plexus protrudes through the corresponding lateral aperture (see fig. 43-1). The vessels to the plexus are derived from cerebellar branches of the vertebral and basilar arteries.

The ventricular system can be examined radiographically in vivo (fig. 43-10).

Cerebrospinal fluid

The cerebrospinal fluid (C.S.F.) is formed by the choroid plexuses. The course of the C.S.F. is shown in figure 43-11. The arachnoid villi and arachnoid granulations are responsible for the drainage of C.S.F. into the venous sinuses of the cranial dura and the spinal veins.

The functions of C.S.F. are several. The liquid acts as a fluid buffer for the protection of the nervous tissue. It also compensates for changes in blood volume within the cranium. It also has some nutritive functions.

The C.S.F. may be examined by means of lumbar puncture (see fig. 41-1).

Hypophysis cerebri (pituitary gland)

The hypophysis cerebri, or pituitary gland (fig. 43-12), is an important endocrine organ, the main portion of which is situated in the hypophysial fossa of the sphenoid bone, where it generally remains after removal of the brain. This main portion is connected to the brain by the infundibulum (figs. 43-7 and 43-12B). The diaphragma sellae (see fig. 43-14A) forms a dural roof for the greater part of the gland and is pierced by the infundibulum. The organ is surrounded by a fibrous capsule fused with the endosteum.

The hypophysis is inferior to the optic chiasm, supeior to an intercavernous venous sinus and the sphenoidal air-sinus (through which it can be approached endonasally), and medial to the cavernous sinus and its associated structures (see fig. 43-22). Hypophysial tumors, by causing pressure on the chiasm, may result in visual defects in the temporal part of the visual fields.


The hypophysisis best divided on embryological grounds into two main portions: the adenohypophysis and the neurohypophysis (fig. 43-12A). The terms anterior lobe and posterior lobe are best avoided because they are defined variously. For example, the pars intermedia is included in the anterior lobe by most anatomists but in the posterior lobe by many physiologists. 

The adenohypophysis includes the pars infundibularis (pars tuberalis), the pars intermedia, and the pars distalis. The neurohypophysis is comprised of the median eminence, the infundibular stem, and the infundibular process (neural lobe). The median eminence is frequently classified also as a part of the tuber cinereum. The term infundibulum (neural stalk) is used for the median eminence and the infundibular stem. The term hypophysial stalk refers to the pars infundibularis and the infundibulum (fig. 43-12B).


The adenohypophysis, which develops as a diverticulum of the buccopharyngeal region, is an endocrine gland, the pars distalis of which secretes a number of hormones, most of which have their influence on functions of other endocrine glands.

The neurohypophysis develops as a diverticulum of the floor of the third ventricle. It is a storehouse for neurosecretions produced by the hypothalamus and carried down axons (the hypothalmohypophysial tract) that arise in certain hypothalmic subnuclei.

Blood supply and innervation.

The hypophysis is supplied by a series of hypophysial arteries from the internal carotid arteries (fig. 43-12C).

The maintenance and regulation of the activity of the adenohypophysis are dependent on the blood supply by way of the hypophysial portal system. Neurons in the hypothalamus liberate releasing factors into the capillary bed in the infundibulum and that these substances are then carried by the portal vessels to the pars distalis of the gland, which they affect.

The neurohypophysis receives its main nerve supply from the hypothalamus by way of fibers known collectively as the hypothalamohypophysial tract (fig. 43-12D). This contains two sets of fibers, arising from distinct hypothalmic nuclei, the supraoptic and paraventricular nuclei.

Cranial nerves

The cranial nerves, that is, the nerves attached to the brain (see fig. 43-1 and tables 43-2 and 43-3) are twelve on each side. They are numbered and named as follows:

1. Olfactory nerve (see fig. 52-5)

2. Optic nerve (see figs. 43-7 and 45-6A)

3. Oculomotor nerve (see fig. 43-15)

4. Trochlear nerve (see fig. 45-6B)

5. Trigeminal nerve (see figs. 43-22 and 48-10)

(a) Ophthalmic nerve (see figs. 45-4 and 45-6B)

(b) Maxillary nerve (see fig. 48-6)

(c) Mandibular nerve (see fig. 48-8)

6. Abducent nerve (see figs. 43-1, 43-22, and 45-5)

7. Facial nerve (see figs. 44-7 and 47-5A)

8. Vestibulocochlear nerve (see fig. 44-9)

9. Glossopharyngeal nerve (see figs. 50-13, 50-14, 50-15 and 50-16)

10. Vagus nerve (see figs. 50-15 and 50-17, 50-18 and 50-19)

11. Accessory nerve (see figs. 50-15 and 50-19)

12. Hypoglossal nerve (see figs. 50-14 and 50-20)

Functional components.

Some of the cranial nerves are exclusively or largely afferent (I, II, and VIII), others are largely efferent (III, IV, VI, XI, and XII), and still others are mixed, that is, contain both afferent and efferent fibers (V, VII, IX, and X). The efferent fibers of the cranial nerves arise within the brain from groups of nerve cells termed motor nuclei. The afferent fibers arise outside the brain from groups of nerve cells, generally in a sensory ganglion along the course of the nerve. The central processes of these nerve cells then enter the brain, where they end in groups of nerve cells termed sensory nuclei.

The four functional types of fibers found in spinal nerves are present also in some of the cranial nerves: somatic afferent, visceral afferent, visceral efferent, and somatic efferent. These four types are termed "general." In certain cranial nerves, however, components that are "special" to the cranial nerves are present. The special afferent fibers comprise visual, auditory, equilibratory, olfactory, taste, and visceral reflex fibers. (The first three are usually classified as somatic, and the last three as visceral.) The special efferent fibers (which are classified as visceral) are those to skeletal muscles either known or thought to be derived from the pharyngeal arches (muscles of mastication, facial muscles, muscles of pharynx and larynx, sternomastoid, and trapezius).

The cranial nerves may be grouped as follows:

Olfactory, optic, and vestibulocochlear nerves (I, II, and VIII) pertain to organs of special sense (special afferent).

Oculomotor, trochlear, abducent, and hypoglossal nerves (III, IV, VI, and XII) supply skeletal muscle of specific regions of the head (eyeballs in the case of III, IV, and VI; tongue in the case of XII). Nerve III also contains parasympathetic fibers to the smooth muscle of the sphincter pupillae and the ciliary muscle (general visceral efferent).

Trigeminal nerve (V) contains motor fibers to the muscles of mastication (special visceral efferent) and sensory fibers from various parts of the head, e.g., face, nasal cavity, tongue, and teeth (general somatic afferent).

Facial, glossopharyngeal, vagus, and accessory nerves (VII, IX, X and XI) contain several components:

(a) Motor fibers to the muscles of facial expression (VII) and the muscles of the pharynx and larynx (IX and X) (special visceral efferent). Many of the fibers to the pharynx and larynx are derived from nerve XI (internal branch) and travel by way of nerve X (hence XI is "accessory" to the vagus).

(b) Parasympathetic secretory fibers to the lacrimal and salivary glands (nervus intermedius of VII), the salivary glands (IX), and certain glands associated with the respiratory and digestive systems (X) (general visceral efferent). Nerve X also supplies most of the smooth muscle of the respiratory and digestive systems, as well as cardiac muscle.

(c) Taste fibers (nervus intermedius of VII; also IX and X) (special visceral afferent).

(d) Fibers from the mucous membrane of the tongue and pharynx (hence the name glossopharyngeal) and of much of the respiratory and digestive systems (general visceral afferent) are contained in nerves IX and X.

(e) The spinal part of nerve XI supplies the sternomastoid and trapezius, two muscles of disputed development.

Parasympathetic ganglia associated with cranial nerves (see fig. 48-11).

The ciliary, pterygopalatine, otic, and submandibular ganglia are associated with certain of the cranial nerves. In these ganglia, parasympathetic fibers synapse, whereas sympathetic and other fibers merely pass through. The chief features of the ganglia are summarized in table 43-3.

Meninges (figs. 43-11, 43-13, 43-14, 43-15 and 43-16)

Dura matter or pachymeninx

The dura mater that surrounds the brain is frequently described as consisting of two layers: an external, or endosteal, and an internal, or meningeal, layer. Because the two layers are indistinguishable except in a few areas, however, it is simpler to consider the dura as one layer, which serves as both endocranium and meninx. * Instead of being considered as separating two layers, the venous sinuses are here described as being situated within the (single) dura.

The dura is particularly adherent at the base and also at the sutures and foramina, where it becomes continuous with the pericranium. Four folds or processes are sent internally from the dura: the falx cerebri, tentorium cerebelli, falx cerebelli, and diaphragma sellae (fig. 43-14).

The falx cerebri, median and sickle shaped, occupies the longitudinal fissure between the two cerebral hemispheres. Anchored anteriorly to the crista galli, its superior and inferior borders enclose the superior and inferior sagittal sinuses, respectively.

The tentorium cerebelli separates the occipital lobes of the cerebral hemispheres from the cerebellum. Its internal, concave, free border contributes to the tentorial notch. (See below.) The external, convex border encloses the transverse sinus where it attaches to the dura over the inside of the occiput. Beyond the "petrous ridge," the tentorium is anchored to the anterior and posterior clinoid processes.

The tentorial notch (fig. 43-15), which contains the midbrain, a part of the cerebellum, and the subarachnoid space, is bounded by the tentorium and the dorsum sellae. Space-occupying intracranial lesions may cause herniation of the brain upward or downward through the notch, and distortion of the midbrain may ensue.

Near the apex of the petrous part of the temporal bone, the dura of the posterior cranial fossa bulges anteriorward beneath that of the middle cranial fossa to form a recess, the trigeminal cave, which contains the trigeminal ganglion.

The falx cerebelli, median and sickle shaped, lies below the tentorium and projects between the cerebellar hemispheres.

The diaphragma sellae is the small, circular, horizontal roof of the sella turcica.

Meningeal Innervation and Vessels.

The dura, like the scalp, is supplied by both cranial (chiefly the trigeminal) and cervical nerves. The brain itself is normally insensitive, and headaches are commonly either of vascular (intracranial or extracranial) or dural origin.

The meningeal vessels are nutrient to the bones of the skull. These are outside the brain, between the skull and the dura. Small anterior and posterior branches are provided by the internal carotid and vertebral arteries, but the middle meningeal artery is of much greater significance. The middle meningeal artery is clinically the most important branch of the maxillary artery, because, in head injuries, tearing of this vessel may cause extradural (epidural) hemorrhage. This may result in brain compression and contralateral paralysis and may necessitate trephining (opening the skull). From its origin in the infratemporal fossa (see fig. 48-3C), the artery ascends through the foramen spinosum, runs anterolaterally on the wall of the middle cranial fossa, and divides into a frontal and a parietal branch (see fig. 43-13A). The middle meningeal artery divides at a variable point on a line connecting the midpoint of the zygomatic arch with the posterior end of the pterion (see fig. 43-8). The meningeal vessels occupy grooves and sometimes canals in the bones. The branches include an anastomosis with the lacrimal artery (see fig. 45-6A).

Leptomeninges (fig. 43-16)

The leptomeninges include the arachnoid and pia mater. These layers bound the subarachnoid space, which is limited externally by a water-tight layer of connective tissue, the arachnoid, and internally by a thinner layer, the pia mater. The pia mater adheres intimately to the surface of the brain and spinal cord. There is a trabecular structure of connections between the arachnoid and the pia that bridges the subarachnoid space which is otherwise full of circulating C.S.F.

The arachnoid surrounds the brain loosely and is separable from the dura by a potential space into which subdural hemorrhage may occur. The arachnoid dips into the longitudinal interhemispheric fissure but not into the sulci. Near the dural venous sinuses, the arachnoid has microscopic projections, termed arachnoid villi, which are believed to be concerned with the absorption of C.S.F. Enlargements of the villi, known as arachnoid granulations, enter some of the sinuses (especially the superior sagittal) and their associated lateral lacunae and are visible to the naked eye. Both the granulations and the lacunae lie in granular pits on the internal aspect of the calvaria.

The pia covers the brain intimately and follows the brain into the gyri of the cerebral hemispheres and the folia of the cerebellum.

The subarachnoid space

The subarachnoid space contains the C.S.F. and the cerebral vessels. The subarachnoid space communicates with the fourth ventricle by means of apertures: a median (Magendie) one and two lateral (Luschka) ones. At certain areas on the base of the brain, the subarachnoid space is expanded into cisternae (fig. 43-16B). The most important of these is the cerebellomedullary cisterna (or cisterna magna), which can be "tapped" by a needle inserted through the posterior atlanto-occipital membrane, a procedure known as cisternal puncture (see fig. 41-1). Cisternae that include important vessels are found on the front of the pons (basilar artery), between the cerebral peduncles (arterial circle), and above the cerebellum (great cerebral vein).

Blood supply of brain

Arteries (figs. 43-17 to 43-20, 43-22, and table 43-4)

The brain is supplied by the two internal carotid and the two vertebral arteries. The former supply chiefly the frontal, parietal, and temporal lobes, the latter the temporal and occipital lobes, together with the midbrain and the hindbrain. On the inferior surface of the brain the four arteries form an anastomosis, the arterial circle (circulus arteriosus, of Willis).

The tissues that intervene between the blood and the neurons include capillary endothelial cells (and their basement membranes), which form the "blood-brain barrier."

Internal Carotid Artery (Petrous, Cavernous, and Cerebral Parts)

The cervical part of the internal carotid artery enters the carotid canal in the petrous part of the temporal bone. The petrous part of the artery first ascends and then curves anteriorward and medially. It is closely related to the cochlea, the middle ear, the auditory tube, and the trigeminal ganglion. The subsequent directions of the petrous, cavernous, and cerebral parts of the vessel may be numbered from 5 to 1, as follows (fig. 43-18, inset):

5. At the foramen lacerum, the petrous part of the internal carotid artery ascends to a point medial to the lingula of the sphenoid bone.

4. The artery then enters the cavernous sinus where its surface is covered by an endothelial lining and surounded by the venous blood in the sinus (see fig. 43-22). This is the cavernous part of the artery. In the sinus the vessel passes anteriorward along the side of the sella turcica.

3. It next turns dorsally and pierces the dural roof of the sinus between the anterior and middle clinoid processes.

2. The cerebral part of the internal carotid artery turns posteriorward in the subarachnoid space just inferior to the optic nerve. The U-shaped bend, convex on its anterior aspect and formed by parts 2, 3, and 4 is termed the "carotid siphon" (fig. 43-19).

1. The artery finally ascends and, at the medial end of the lateral sulcus, divides into the anterior and middle cerebral arteries.

The internal carotid artery and its branches, including the cerebral arteries, are surrounded and supplied by a sympathetic plexus of nerves, derived from the superior cervical ganglion.

Branches of the internal carotid artery (fig 43-18).

The internal carotid artery gives no named branches in the neck. Within the cranial cavity, it supplies the hypophysis, the orbit, and much of the brain. The carotid siphon gives off three branches: the ophthalmic, posterior communicating, and anterior choroid arteries.

The ophthalmic artery is described with the orbit (see fig. 45-6A).

The posterior communicating artery connects the internal carotid artery with the posterior cerebral artery and thereby forms a part of the arterial circle.

The anterior choroid artery passes backward along the optic tract and enters the choroid fissure. It gives numerous small branches to the interior of the brain, including the choroid plexus of the lateral ventricle. The anterior choroid artery is frequently the site of thrombosis.

The terminal branches of the internal carotid artery are the anterior and middle cerebral arteries.

The anterior cerebral artery passes medially just superior to the optic chiasma and enters the longitudinal interhemispheric fissure of the brain. Here it is connected with its fellow of the opposite side by the anterior communicating artery (which is frequently double and which sometimes gives off a median anterior cerebral artery). It then runs successively in a rostral, then dorsal, and finally caudal direction. It usually lies on the corpus callosum, and ends by turning dorsally on the medial surface of the hemisphere just before reaching the parieto-occipital sulcus.

The middle cerebral artery, the larger terminal branch of the internal carotid artery, is frequently regarded as the continuation of that vessel. It passes laterally in the lateral fissure and gives rise to numerous branches on the surface of the insula. Small, central branches (lenticulostriate arteries) enter the anterior perforated substance, supplying deeper structures of the hemispheres and are liable to occlusion (lacunar strokes) or rupture (Charcot's "artery of cerebral hemorrhage"). It supplies the motor and premotor areas and the sensory and auditory areas. It also supplies the language areas in the dominant hemisphere. Occlusion of the middle cerebral artery causes a contralateral paralysis (hemiplegia) and a sensory defect. The paralysis is least marked in the lower limb (territory of anterior cerebral artery). When the dominant (usually left) side is involved, there are also disturbances of language (aphasia).

The general distribution of the cerebral arteries is shown in figure 43-4B and C.

The chief branches of the internal carotid artery are summarized in table 43-4.

Vertebral Artery (Intracranial Part) and Basilar Artery (see figs. 43-2, 43-15, 43-17, and 43-18)

The vertebral and basilar arteries and their branches supply the upper part of the spinal cord, the brain stem, the cerebellum; and much of the postero-inferior portion of the cerebral cortex. The branches to the brain stem are functionally end-arteries so occlusion usually results in a stroke.

Vertebral Arteries.

The vertebral artery, a branch of the subclavian artery, may be considered in four parts: cervical, vertebral, suboccipital, and intracranial. The suboccipital part of the vertebral artery perforates the dura and arachnoid and passes through the foramen magnum (see fig. 43-2). The intracranial part of each vertebral artery procedes rostrally and medially to reach a position anterior to the medulla. At approximately the caudal border of the pons, the two vertebral arteries unite to form the basilar artery (see fig. 50-23).


The vertebral artery, which gives off muscular and spinal branches in the neck, supplies chiefly the posterior part of the brain, both directly and, of greater importance, by way of the basilar artery.

The anterior spinal artery runs caudally just anterior to the medulla and unites with the vessel of the opposite side to form a median trunk. This contributes to the supply of the ventral side of the medulla and spinal cord.

The posterior inferior cerebellar artery winds dorsally around the olive and gives branches to the lateral medulla, the choroid plexus of the fourth ventricle, and the cerebellum. The posterior spinal artery is usually a branch of the posterior inferior cerebellar artery, but it may come directly from the vertebral artery.

Basilar Artery.

The basilar artery is formed by the union of the right and left vertebral arteries. It begins at approximately the caudal border of the pons and ends near the rostral border by dividing into the two posterior cerebral arteries (fig. 43-17). It passes through the pontine cistern and frequently lies in a longitudinal groove on the ventral pons.

Branches (fig. 43-18).

Branches of the basilar artery are distributed to the pons, cerebellum, internal ear, midbrain, and cerebral hemispheres.

The paired anterior inferior cerebellar arteries pass dorsally on the inferior surface of the cerebellum and supply the cerebellum and pons.

The paired labyrinthine (internal auditory) arteries may arise from either the basilar or the anterior inferior cerebellar artery, more commonly the latter. Each enters the corresponding internal acoustic meatus and is distributed to the internal ear.

The paired superior cerebellar arteries pass laterally just inferior to the oculomotor and trochlear nerves and are distributed to the cerebellum.

The two posterior cerebral arteries are the terminal branches of the basilar artery. They supply much of the medial temporal and most of the occipital lobes (see fig. 43-4). Each is connected with the corresponding internal carotid artery by a posterior communicating artery; occasionally the posterior cerebral arises as a branch of the internal carotid artery (an arrangement referred to as trifurcation of the internal carotid artery or persistnat fetal circulation). The posterior cerebral artery runs posteriorward, superior and parallel to the superior cerebellar artery, from which it is separated by the oculomotor and trochlear nerves. Among the branches are the posterior choroidal branches, which supply the choroid plexuses of the third and lateral ventricles.

Arterial Circle (fig. 43-17)

Branches of the three cerebral arteries to the cerebral cortex have important anastomoses with one another on the surface of the brain. The arterial system in the brain, therefore, is not strictly terminal. However, in the event of occlusion, these microscopic anastomoses are not capable of providing an alternate circulation for the ischemic brain tissue.

The arterial circle, described by Thomas Willis in 1664, is an important polygonal anastomosis between the four arteries that supply the brain: the two vertebral and the two internal carotid arteries. It is formed by the posterior cerebral, posterior communicating, internal carotid, anterior cerebral, and anterior communicating arteries. The circle forms an important means of collateral circulation in the event of obstruction of a major vessel. Variations in the size of the vessels that constitute the circulus are very common.

The blood vessels of the brain may be demonstrated radiographically by cerebral angiography (figs. 43-18, 43-19 and 43-20).

Venous drainage

Veins of Brain (see fig. 43-13B and C)

The veins of the brain pierce the arachnoid and dura and open into the venous sinuses of the dura.

The superior cerebral veins drain into the superior sagittal sinus. The superficial middle cerebral vein follows the lateral fissure, sends superior and inferior anastomotic veins to the superior sagittal and transverse sinuses, respectively, and ends in the cavernous sinus. The inferior cerebral veins drain the inferior aspect of the hemispheres and join nearby sinuses.

The basal vein is formed by the union of several veins, including those that accompany the anterior and middle cerebral arteries. It winds around the cerebral peduncle and ends in the great cerebral vein.

The single great cerebral vein (see fig. 43-13C) is formed by the union of two internal cerebral veins. It receives, directly or indirectly, a number of vessels from the interior of the cerebral hemispheres and also the basal veins. It ends in the straight sinus.

Venous Sinuses of Dura Mater (fig. 43-21)

The blood from the brain drains into sinuses that are situated within the dura mater and that empty ultimately into the internal jugular veins.

The superior sagittal sinus (see fig. 43-13B and C) lies in the convex border of the falx cerebri. From its commencement near the crista galli, the sinus runs posteriorward and, near the internal occipital protuberance, enters in a variable manner one or both transverse sinuses. It receives the superior cerebral veins and communicates with lateral lacunae that contain arachnoid granulations.

The confluence of the sinuses (or torcular) is the junction of the superior sagittal, straight, and right and left transverse sinuses (fig. 43-21). It is situated near the internal occipital protuberance. The pattern of the constituent sinuses varies, and dominance of one side in drainage (e.g., the right) is usual.

The inferior sagittal sinus lies in the concave, free border of the falx cerebri (the blade of the sickle) and ends in the straight sinus, which also receives the great cerebral vein. The straight sinus runs posteriorward between the falx and tentorium, and joins the confluence.

The transverse sinuses begin in the confluence, and each curves laterally in the convex border of the tentorium, where it attaches to the skull. At the petrous part of the temporal bone, the transverse becomes the sigmoid sinus (see fig. 43-13A), which grooves the mastoid part of the temporal bone and traverses the jugular foramen to become the internal jugular vein. Smaller channels (petrosal sinuses) connect the cavernous sinus with the transverse sinus and jugular vein.

The cavernous sinus comprises one or more venous channels (sometimes a plexus). * It is located in a dural compartment bounded by the body of the sphenoid bone and the anterior portion of the tentorium. In addition to the venous channels, the dural compartment contains (outside the endothelium) the internal carotid artery, sympathetic plexus, abducent nerve, and, further laterally, the oculomotor, trochlear, and ophthalmic nerves (fig. 43-22). The cavernous sinus extends posterorly from the superior orbital fissure to the apex of the petrous part of the temporal bone. It receives several veins (superior ophthalmic, superficial middle cerebral, and sphenoparietal sinus) and communicates (by the petrosal sinuses) with the transverse sinus and internal jugular vein, as well as with the opposite cavernous sinus. The facial vein (via the superior ophthalmic vein) communicates with the cavernous sinus and hence allows infection around the nose and upper lip ("danger area") to spread to intracranial structures.

Lateral lacunae (see fig. 43-11) are venous meshworks within the dura near the superior sagittal sinus, and both the lacunae and sinus occupy the granular pits of the calvaria. The lacunae receive (1) emissary veins, (2) diploic veins, (3) meningeal veins, and (4) occasionally some cerebral veins. It should be noted that the emissary veins, which pass through foramina in the skull, connect the deeper vessels with the veins of the scalp and hence also allow infection to spread from the scalp to intracranial structures.

Additional reading

Blinkov, S. M., and Glezer, I. I., The Human Brain in Figures and Tables, trans. by B. Haigh, Basic Books, New York, 1968.

Bossy, J., Atlas du systeme nerveux. Aspects macroscopiques de l'encephale. Editions Offidoc, Paris, 1972. Beautiful photographs of the brain.

Gardner, E., Fundamentals of Neurology 6th ed., W. B. Saunders Company, Philadelphia, 1975. A good introduction to the nervous system.

Ludwig, E., and Klingler, J.,Atlas cerebri humani. Karger, Basel, 1956. Excellent photographs of superb dissections.

Stephens, R. B., and Stilwell, D. L.,Arteries and Veins of The Human Brain. Thomas, Springfield, Illinois, 1969.


43-1 Review the major divisions of the brain.

43-1 The major divisions of the brain are listed in table 43-1.

43-2 Why is the pons so designated?

43-2 The pons (L., a bridge), associated with the name of Varolius (1573) but better illustrated previously by Eustachius, appears superficially to bridge the two cerebellar hemispheres. These fibers actually connect one cerebellar hemisphere with the contralateral half of the pons. They are a part of the corticopontocerebellar pathway.

43-3 What are the roots of the trigeminal nerve?

43-3 The roots of the trigeminal nerve are sensory (formerly termed portio major) and motor (formerly called portio minor). Intermediate fibers are probably accessory to the motor root (R. L. Saunders and E. Sachs, J. Neurosurg., 33:317, 1970; M. Laude et al., Bull. Assoc. Anat., 60:151, 1976), although a tactile function has also been proposed (J. Provost and J. Hardy, Neurochirurgie 16:459, 1970).

43-4 What are the main parts of the midbrain?

43-4 The main parts of the midbrain are the tectum (colliculi) dorsally and the cerebral peduncles ventrally. Each peduncle consists of crus cerebri, substantia nigra, and tegmentum (see fig. 42-16A).

43-5 Which parts of the diencephalon are visible from the surface of an intact brain?

43-5 The parts of the diencephalon that can be seen from the surface of an intact brain include the optic chiasm, hypophysis, and mamillary bodies.

43-6 What are the parts of the corpus callosum?

43-6 The corpus callosum consists of the rostrum, genu, body, and splenium.

43-7 List the main lobes of the cerebral hemisphere.

43-7 The main lobes of the cerebral hemisphere are the frontal, parietal, occipital, and temporal (see fig. 43-4A).

43-8 Which are the two most frequently mentioned sulci?

43-8 The central and lateral sulci are important landmarks. The lateral fissure (Sylvius, seventeenth century) is evident (see fig. 43-8). The central sulcus (Rolando, nineteenth century) is not as easily identified. It begins on the medial surface of the hemisphere, where it can be detected by following the cingulate sulcus backward and upward and then finding the next groove anteriorly (see fig. 43-5B).

43-9 Where is the insula?

43-9 The insula is an island (Reil, 1796) of cerebral cortex buried in the depths of the lateral sulcus.

43-10 How are the various areas of the cerebral cortex numbered?

43-10 Some 57 varieties of cerebral cortex, only some of which are known to have discrete functional significance, were numbered by K. Brodmann in 1907 on a somewhat arbitrary histological basis. Areas 1,2,3 (sensory areas), 4,6,8 (motor areas), 17, 18, 19 (visual areas), 22 (receptive language area of Wernicke), 41, 42 (auditory projection area of Heschl), and 44 (motor language area of Broca) are functionally the most important (see fig. 43-6A and B).

43-11 What are the first pair of cranial nerves?

43-11 The first pair of cranial nerves, in the accepted but arbitrary enumeration, are the olfactory filaments. It should be noted that the olfactory bulbs and tracts are parts of the cerebral hemispheres and are not cranial nerves.

43-12 How are the ventricles numbered?

43-12 The two lateral ventricles are followed by the third ventricle, cerebral aqueduct (Sylvius), fourth ventricle, and central canal. The lateral ventricles communicate with the third ventricle by the interventricular foramina (Monro, 1783). The fourth ventricle communicates with the subarachnoid space by median (Magendie, 1828) and lateral (Luschka, 1855) apertures.

43-13 What are the main parts of the lateral ventricles?

43-13 Each lateral ventricle consists of a frontal (anterior) horn, central part, and occipital (posterior) and temporal (inferior) horns.

43-14 List the chief recesses of the third ventricle.

43-14 The chief recesses of the third ventricle are the optic, infundibular, pineal, and suprapineal (see fig. 43-9).

43-15 Where are the lateral recesses of the fourth ventricle?

43-15 The lateral recesses of the fourth ventricle curve lateral and anteriorward to present near the ventral aspect of the brain stem, close to cranial nerves VII and VIII (cerebellopontine angle) and IX (see fig. 43-1). The end of each recess opens as the lateral aperture.

43-16 What are the subdivisions of the hypophysis cerebri?

43-16 The hypophysis comprises the adenohypophysis (infundibular, intermediate, and distal parts) and the neurohypophysis (median eminence, infundibular stem, and infundibular process) (see fig. 43-12A).

43-17 Review the cranial nerves and their components.

43-17 The cranial nerves and their components are summarized in table 43-2.

43-18 List the chief parasympathetic ganglia associated with cranial nerves.

43-18 The chief parasympathetic ganglia associated with cranial nerves are the ciliary, pterygopalatine, otic, and submandibular (see table 43-3), but ganglionic cells are present diffusely in the cranial nerves (J.-A. Baumann and S. Gajisin, Bull. Assoc. Anal., 59:329, 1975).

43-19 What are the chief processes of the cerebral dura mater?

43-19 The chief processes of the dura are the falx (L., sickle) cerebri, tentorium (L., tent) cerebelli, falx cerebelli, and diaphragma sellae.

43-20 What is the tentorial notch?

43-20 The tentorial notch is an important gap between the tentorium and the dorsum sellae. It permits passage of the midbrain (see fig. 43-15).

43-21 Which is the most important meningeal vessel clinically?

43-21 The most important meningeal vessel clinically is the middle meningeal artery (see fig. 43-8). Middle meningeal hemorrhage is usually caused by an injury (potentially trivial) usually (although not always) associated with fracture of the temporal bone.

43-22 What is the subarachnoid space?

43-22 The subarachnoid space is the meshwork in the leptomeninges that contains cerebrospinal fluid (see fig. 43-11). Excessive C.S.F., either within the ventricular system or in the subarachnoid space, is termed hydrocephaly. The most frequent cause is a blockage of the normal circulation of C.S.F.

43-23 Which is the most important subarachnoid cisterna?

43-23 The most important subarachnoid cistern is the cerebellomedullary (cisterna magna), which can be "tapped" through the posterior atlanto-occipital membrane (see fig. 41-1).

43-24 What is the carotid siphon?

43-24 The term carotid siphon is used for the horizontal U-shaped bend (parts 2, 3, and 4) of the internal carotid artery, as seen on an angiogram (see figs. 43-18 and 43-19A).

43-25 What is the continuation of the internal carotid artery?

43-25 The middle cerebral artery may be regarded as the continuation of the internal carotid artery. The term carotid (Gk, karos, heavy sleep) refers to the stupor that may result from compression of the carotid arteries. Under good conditions, however, one internal carotid artery can be occluded without any residual symptoms.

43-26 Which vessel supplies the medulla behind the olive?

43-26 The posterior inferior cerebellar artery (often spoken of as "PICA") a branch of the vertebral artery, supplies the medulla dorsolateral to the olive, an area that includes important nuclei and tracts. Vascular disease of this dorsolateral part ofthe medulla may result in widespread signs and symptoms (Wallenberg's syndrome, 1895).

43-27 What is the circulus arteriosus?

43-27 The arterial circle (circulus arteriosus), described by Thomas Willis in 1664, is a polygonal anastomosis between the vertebral and internal carotid arteries (see fig. 43-17). Beautiful illustrations of normal and abnormal cerebral circulation can be found in M. Waddington, Atlas of Cerebral Angiography with Anatomic Correlation. Little, Brown, Boston, 1974. In certain mammals (e.g., sheep) part of the course of the internal carotid artery is taken over by a network (rete mirabile) that is supplied by an external vessel, e.g., the maxillary artery (P. M. Daniel, J. D. K. Dawes, and M. M. L. Prichard, Philos. Trans. R. Soc. Lond., 237B:173, 1953; H. Minagi and T. H. Newton, Radiology, 86:100, 1966).

43-28 How is the great cerebral vein formed?

43-28 The great cerebral vein (Galen, second century) is formed by the union of the left and right internal cerebral veins.

43-29 What is the confluence of the sinuses?

43-29 The confluence of the sinuses is the highly variable junction of the superior sagittal, straight, and left and right transverse sinuses. For variations see H. Browning (Am. J. AnaL, 93:307, 1953); A. Elmohamed and K.-J. Hempel (Frankfurt. Z. Pathol., 75:321, 1966). The confluence was formerly called the torcular (L., wine press) Herophili (Herophilus, fourth century B.C.)

43-30 Where is the cavernous sinus?

43-30 The cavernous sinus consists of one or more venous channels (sometimes a plexus) that extend between the superior orbital fissure and the apex of the petrous part of the temporal bone. Important relations are the internal carotid artery and cranial nerves III, IV, V, and VI (see fig. 43-22).

43-31 Which veins drain into the lateral lacunae?

43-31 The lateral lacunae receive emissary, diploic, meningeal, and sometimes cerebral veins (see fig. 43-11, inset). Detailed accounts are available, and these include H. M. Duvernoy, The Superficial Veins of the Human Brain, Springer, Berlin, 1975; and J. Browder and H. A. Kaplan, Cerebral Dural Sinuses and their Tributaries, Thomas, Springfield, Illinois, 1976.

* The cavernous sinus may be a venous plexus in the newborn (P.-E. Duroux, A. Bouchet, J. Bossy, and F. Calas, C. R. Assoc. Anat.,42:486-490, 1956), but it is usually a non-cavernous orbitotemporal sinus in the adult, with few or no trabeculae (M. A. Bedford, Br. J. Ophthalmol., 50:41-46, 1966).

Figure legends

Figure 43-1 The brain stem, anterior aspect, showing the cranial nerves.

Figure 43-2 Posterior aspect of the brain stem and the rostral part of the spinal cord after removal of the cerebellum, which displays the floor of the fourth ventricle. The vertebral artery is visible on each side, together with certain cranial and spinal nerves.

Figure 43-3 The cerebellum, inferior aspect. In the inset, the brain stem has been removed by sectioning of the cerebellar peduncles. 1, primary fissure; 5, trigeminal nerve.

Figure 43-4 A, The lobes of the brain, left lateral aspect. B and C, The territories supplied by the cerebral arteries, lateral and medial aspects of the left cerebral hemisphere.

Figure 43-5 Sulci. A, B, and C: lateral, medial, and inferior aspects, respectively, of the left cerebral hemisphere. In B, T represents the cut surface of the thalamus.

Figure 43-6 Gyri. A, B, and C: lateral, medial, and inferior aspects, respectively, of the left cerebral hemisphere. The chief speech area (of Broca) is mainly area 44 and is usually on the left side of the brain. A receptive speech area (of Wernicke) is found mainly in area 22.

Figure 43-7 The interpeduncular fossa and surroundings, antero-inferior aspect. The left half of the hypophysis has been removed. (From a photograph by David Bassett, M.D.).

Figure 43-8 Craniocerebral topography. The middle meningeal artery proceeds from the middle of the zygomatic arch upward (dotted line) about 4 to 5 cm to the pterion (P). The external acoustic meatus is shown in black. B, bregma; I, inion; L, lambda; XY, orbitomeatal plane. (After von Lanz and Wachsmuth.)

Figure 43-9 Cast of the ventricles of the brain, left lateral aspect. In this brain, the right occipital horn is considerably longer than the left. The key drawing indicates related solid structures in parentheses. (Courtesy of David Tompsett, Ph.D., London.)

Figure 43-10 Computerized axial tomograms of the head, showing the ventricular system as seen in horizontal sections. A and B are from the same subject. A shows the body and posterior horn of each lateral ventricle. The median white line indicates the falx cerebri. The following structures are visible in B, from anterior to posterior: the frontal horns curved around the head of each caudate nucleus; the interventricular foramina, barely visible between the fronal horns and the third ventricle, which appears as a median slit; a calcified pineal gland, seen as a white dot immediately posterior to the third ventricle; and a transverse, crescentic area behind this, which is the subarachnoid space behind the tectum of the midbrain. In C, the lesser wings of the sphenoid bone and the petrous portions of the temporal bones delimit the cranial fossae. The dorsum sellae appears as a white band between the shadows of the temporal lobes. Between the cerebellar hemispheres, the fourth ventricle is visible as a dark, inverted U. (Courtesy of Giovanni Di Chiro, M.D., Bethesda, Maryland.) In D, the approximate planes of section are indicated. XY, the orbitomeatal plane.

Figure 43-11 The course of the cerebrospinal fluid (C.S.F.). Arrows lead from the choroid plexuses of the lateral and third ventricles toward the aqueduct. The fluid thereby formed is joined by that produced in the fourth ventricle and passes through the median aperture to the cerebellomedullary cistern of the subarachnoid space. The fluid then extends (1) dorsally around the brain and (2) caudally around the spinal cord. The inset (a coronal section at the sagittal suture) shows the drainage of the C.S.F. into the venous system (superior sagittal sinus, S, and lateral lacunae, L, by way of arachnoid granulations, G). Various adjacent vessels are also included. A, cerebral artery; C, cerebral vein; D, diploic vein; E, emissary vein; M, meningeal vein; 3 and 4, third and fourth ventricles. The lowermost part of the figure shows the caudal end of the spinal cord. lumbar puncture (L.P.) is performed in the part of the subarachnoid space that lies caudal to the termination of the spinal cord. Ct. fig. 41-1.

Figure 43-12 The hypophysis cerebri. A and B illustrate the terminology. C shows the blood supply. Arteries in the hypophysial stalk break up into capillary loops, which drain into hypophysial portal vessels. These, on reaching the pars distalis, drain into sinusoids, which enter the venous sinuses around the gland. D illustrates schematically the hypothalamohypophysial tract.

Figure 43-13 The meninges and associated vessels. A, lateral aspect of the intact dural sac. B, cerebral veins as seen through the arachnoid after removal of the dura. C, processes of the dura after removal of the brain and spinal cord. (A is based on Strong and Elwyn.)

Figure 43-14 Processes of the dura. A, Right lateral aspect seen also from above and behind. B, Coronal section through the foramen magnum.

Figure 43-15 The tentorial notch, superior aspect. The notch is bounded anteriorly by the dorsum sellae, between the two posterior clinoid processes.

Figure 43-16 Median sections. A shows important features of the brain stem and a cross section of the midbrain. The interthalamic adhesion is indicated by an asterisk. B emphasizes the meninges, subarachnoid space, and cisterns. Aq., aqueduct; Ep., epiphysis (pineal body); 1, primary fissure; 2, secondary fissure; 3 and 4, third and fourth ventricles. The interrupted line indicates the orbitomeatal plane, which has been used for orientation, thereby rendering the brain stem almost vertical.

Figure 43-17 The carotid-vertebral circulation and the arterial circle. The optic chiasm and the anterior and posterior perforated substances have been included.

Figure 43-18 The chief arteries to the brain, as seen on angiography. The main vessels are shaded. A and B, Carotid arteriograms in lateral and anteroposterior projections. The internal carotid pierces the dura at d. The inset shows, in left lateral view, the numbered portions of the internal carotid artery. C and D, Vertebral arteriograms in lateral and half-axial (fronto-occipital) projections. In C, a is the site of the foramen transversarium of the atlas, b is the foramen magnum, and c is at the junction of the right and left vertebral arteries, i.e., the beginning of the basilar artery. AC, anterior cerebral artery; MC, middle cerebral artery; PC, posterior cerebral artery. (Based largely on Greitz and Lindgren.)

Figure 43-19 Internal carotid angiograms in vivo. A, lateral view. B, Postero-anterior view. The numerals from 1 to 7 refer to successive parts of the internal carotid artery. Number 7 is in the neck, number 6 lies within the carotid canal, number 5 is medial to the trigeminal ganglion, numbers 4 to 2 constitute the carotid siphon (which gives off the ophthalmic artery, Oph.), and number 1 ascends to the division into anterior (A) and middle (M) cerebral arteries. (Courtesy of Arthur B. Dublin, M.D., Sacramento, California.)

Figure 43-20 Vertebral angiograms in vivo. A, lateral view. B, Postero-anterior view. The right and left vertebral arteries (V) unite to form the basilar artery (B), which divides into the two posterior cerebral arteries (P). The posterior communicating arteries (asterisk in A) run anteriorward to take part in the arterial circle (see fig. 43-17). An important branch of the vertebral, the posterior inferior cerebellar artery (P.I.C.A.), forms a characteristic loop. (Courtesy of Arthur B. Dublin, M.D., Sacramento, California.)

Figure 43-21 The venous sinuses of the dura. The orientation is similar to that of fig. 43-14A.

Figure 43-22 A, Right lateral aspect showing the structures related to the cavernous sinus. The numerals 2 to 6 on the internal carotid artery indicate the successive parts of the artery; parts 2 to 4 constitute the anteriorly directed carotid siphon. (After Cunningham.) B, Coronal section through the cavernous sinus along the plane XY in A. Although the maxillary nerve is shown here in rather close relation to the sinus (as it is usually depicted), according to W. R. Henderson (J. Anat., 100:905, 1966) the nerve is embedded in the dura of the middle cranial fossa, lateral to the sinus.

* See L. C. Rogers and E. E. Payne (J. Anat., 95:586-588, 1961), whose views have been disputed by A. B. Beasley and H. Kuhlenbeck (Anal. Rec., 154:315, 1966)

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