1. Limbic system. Structure of white matter. Corpus callosum, fornix. Lateral ventricles. Basal nuclei

2. Pathways of the brain and spinal cord (associative, commissural, projective). Topography of pathways on section of spinal cord

3. Meninges of the brain. Dural sinuses. Circulation of spinocerebral fluid. Blood vessels of the brain and spinal cord

Lesson No 22

Theme 1. Limbic system. Structure of white matter. Corpus callosum, fornix. Lateral ventricles. Basal nuclei

The Rhinencephalon, associated with the sense of smell, is the oldest part of the telencephalon, and forms almost the whole of the hemisphere in some of the lower animals, e. g., fishes, amphibians, and reptiles. In man it is rudimentary. It divides into central and peripheral parts. The central part includes the hippocampus, gyrus fornicatus (gyrus cinguli + gyrus hyppocampi), gyrus dentatus, septum pellucidum and uncus. The peripheral part includes the olfactory bulb, olfactory tract, olfactory trigone and anterior perforated substance. Cortical smell analyzer located in the uncus. Rhinencephalon is a center of emotional colouring of sensible perception of external environment (Limbic system). Together from all subcortical centers it is by energy source for cortex and answers for vitally important man reactions regulates activity of internal organs: hunger feeling and thirst, sounds perceptions and smells. Here are the memory mechanisms.

The rhinencephalon comprises the olfactory lobe, the uncus, the subcallosal and supracallosal gyri, the fascia dentata hippocampi, the septum pellucidum, the fornix, and the hippocampus.

1. The Olfactory Lobe (lobus olfactorius) is situated under the inferior or orbital surface of the frontal lobe. In many vertebrates it constitutes a well-marked portion of the hemisphere and contains an extension of the lateral ventricle; but in man and some other mammals it is rudimentary. It consists of the olfactory bulb and tract, the olfactory trigone, the parolfactory area of Broca, and the anterior perforated substance.

(a) The olfactory bulb (bulbus olfactorius) is an oval, reddish-gray mass which rests on the cribriform plate of the ethmoid and forms the anterior expanded extremity of the olfactory tract. Its under surface receives the olfactory nerves, which pass upward through the cribriform plate from the olfactory region of the nasal cavity.

Scheme of rhinencephalon.

(b) The olfactory tract (tractus olfactorius) is a narrow white band, triangular on coronal section, the apex being directed upward. It lies in the olfactory sulcus on the inferior surface of the frontal lobe, and divides posteriorly into two striæ, a medial and a lateral. The lateral stria is directed across the lateral part of the anterior perforated substance and then bends abruptly medialward toward the uncus of the hippocampal gyrus. The medial stria turns medialward behind the parolfactory area and ends in the subcallosal gyrus; in some cases a small intermediate stria is seen running backward to the anterior perforated substance.

(c) The olfactory trigone (trigonum olfactorium) is a small triangular area in front of the anterior perforated substance. Its apex, directed forward, occupies the posterior part of the olfactory sulcus, and is brought into view by throwing back the olfactory tract.

(d) The parolfactory area of Broca (area parolfactoria) is a small triangular field on the medial surface of the hemisphere in front of the subcallosal gyrus, from which it is separated by the posterior parolfactory sulcus; it is continuous below with the olfactory trigone, and above and in front with the cingulate gyrus; it is limited anteriorly by the anterior parolfactory sulcus.

(e) The anterior perforated substance (substantia perforata anterior) is an irregularly quadrilateral area in front of the optic tract and behind the olfactory trigone, from which it is separated by the fissure prima; medially and in front it is continuous with the subcallosal gyrus; laterally it is bounded by the lateral stria of the olfactory tract and is continued into the uncus. Its gray substance is confluent above with that of the corpus striatum, and is perforated anteriorly by numerous small bloodvessels.

2. The Uncus has already been described (page 826) as the recurved, hook-like portion of the hippocampal gyrus.

3. The Subcallosal, Supracallosal, and Dentate Gyri form a rudimentary arch-shaped lamina of gray substance extending over the corpus callosum and above the hippocampal gyrus from the anterior perforated substance to the uncus.

(a) The subcallosal gyrus (gyrus subcallosus; peduncle of the corpus callosum) is a narrow lamina on the medial surface of the hemisphere in front of the lamina terminalis, behind the parolfactory area, and below the rostrum of the corpus callosum. It is continuous around the genu of the corpus callosum with the supracallosal gyrus.

(b) The supracallosal gyrus (indusium griseum; gyrus epicallosus) consists of a thin layer of gray substance in contact with the upper surface of the corpus callosum and continuous laterally with the gray substance of the cingulate gyrus. It contains two longitudinally directed strands of fibers termed respectively the medial and lateral longitudinal striæ. The supracallosal gyrus is prolonged around the splenium of the corpus callosum as a delicate lamina, the fasciola cinerea, which is continuous below with the fascia dentata hippocampi.

(c) The fascia dentata hippocampi (gyrus dentatus) is a narrow band extending downward and forward above the hippocampal gyrus but separated from it by the hippocampal fissure; its free margin is notched and overlapped by the fimbria—the fimbriodentate fissure intervening. Anteriorly it is continued into the notch of the uncus, where it forms a sharp bend and is then prolonged as a delicate band, the band of Giacomini, over the uncus, on the lateral surface of which it is lost.

The remaining parts of the rhinencephalon, viz., the septum pellucidum, fornix, and hippocampus, will be described in connection with the lateral ventricle.

White Matter of the Cerebrum. The external capsule located between putamen and claustrum. The extrema capsule separates the claustrum and cortex of the insula.

The corpus callosum connect right and left cerebral hemispheres. Corpus callosum anteriorly carries a genu that passes into rostrum. Last continue as a lamina rostralis and lamina terminalis. Back part of the corpus callosum called splemium. Anterior fibers of the corpus callosum form the frontal forceps, posterior fibers of the corpus callosum - the occipital forceps.

The Corpus Callosum is the great transverse commissure which unites the cerebral hemispheres and roofs in the lateral ventricles. A good conception of its position and size is obtained by examining a median sagittal section of the brain, when it is seen to form an arched structure about 10 cm. long. Its anterior end is about 4 cm. from the frontal pole, and its posterior end about 6 cm. from the occipital pole of the hemisphere.

Corpus callosum from above.

The anterior end is named the genu, and is bent downward and backward in front of the septum pellucidum; diminishing rapidly in thickness, it is prolonged backward under the name of the rostrum, which is connected below with the lamina terminalis. The anterior cerebral arteries are in contact with the under surface of the rostrum; they then arch over the front of the genu, and are carried backward above the body of the corpus callosum.

The posterior end is termed the splenium and constitutes the thickest part of the corpus callosum. It overlaps the tela chorioidea of the third ventricle and the mid-brain, and ends in a thick, convex, free border. A sagittal section of the splenium shows that the posterior end of the corpus callosum is acutely bent forward, the upper and lower parts being applied to each other.

The superior surface is convex from before backward, and is about 2.5 cm. wide. Its medial part forms the bottom of the longitudinal fissure, and is in contact posteriorly with the lower border of the falx cerebri. Laterally it is overlapped by the cingulate gyrus, but is separated from it by the slit-like callosal fissure. It is traversed by numerous transverse ridges and furrows, and is covered by a thin layer of gray matter, the supracallosal gyrus, which exhibits on either side of the middle line the medial and lateral longitudinal striæ, already described (page 827).

The inferior surface is concave, and forms on either side of the middle line the roof of the lateral ventricle. Medially, this surface is attached in front to the septum pellucidum; behind this it is fused with the upper surface of the body of the fornix, while the splenium is in contact with the tela chorioidea.

On either side, the fibers of the corpus callosum radiate in the white substance and pass to the various parts of the cerebral cortex; those curving forward from the genu into the frontal lobe constitute the forceps anterior, and those curving backward into the occipital lobe, the forceps posterior. Between these two parts is the main body of the fibers which constitute the tapetum and extend laterally on either side into the temporal lobe, and cover in the central part of the lateral ventricle.

VIDEO

The fornix cerebri located under corpus callosum and has a body, columna fornicis (anteriorly) and crura fornicis (posteriorly). Crus fused with the hippocampus and form the fimbria hippocampi. Anterior commissura positioned closly to the columna fornicis.

The Fornix is a longitudinal, arch-shaped lamella of white substance, situated below the corpus callosum, and continuous with it behind, but separated from it in front by the septum pellucidum. It may be described as consisting of two symmetrical bands, one for either hemisphere. The two portions are not united to each other in front and behind, but their central parts are joined together in the middle line. The anterior parts are called the columns of the fornix; the intermediate united portions, the body; and the posterior parts, the crura.

Diagram of the tracts in the internal capsule. Motor tract red. The sensory tract (blue) is not direct, but formed of neurons receiving impulses from below in the thalamus and transmitting them to the cortex. The optic radiation (occipitothalamic) is shown in violet.

The body (corpus fornicis) of the fornix is triangular, narrow in front, and broad behind. The medial part of its upper surface is connected to the septum pellucidum in front and to the corpus callosum behind. The lateral portion of this surface forms part of the floor of the lateral ventricle, and is covered by the ventricular epithelium. Its lateral edge overlaps the choroid plexus, and is continuous with the epithelial covering of this structure. The under surface rests upon the tela chorioidea of the third ventricle, which separates it from the epithelial roof of that cavity, and from the medial portions of the upper surfaces of the thalami. Below, the lateral portions of the body of the fornix are joined by a thin triangular lamina, named the psalterium (lyra). This lamina contains some transverse fibers which connect the two hippocampi across the middle line and constitute the hippocampal commissure. Between the psalterium and the corpus callosum a horizontal cleft, the so-called ventricle of the fornix (ventricle of Verga), is sometimes found.

The columns (columna fornicis; anterior pillars; fornicolumns) of the fornix arch downward in front of the interventricular foramen and behind the anterior commissure, and each descends through the gray substance in the lateral wall of the third ventricle to the base of the brain, where it ends in the corpus mammillare. From the cells of the corpus mammillare the thalamomammillary fasciculus (bundle of Vicq d’Azyr) takes origin and is prolonged into the anterior nucleus of the thalamus. The column of the fornix and the thalamomammillary fasciculus together form a loop resembling the figure 8, but the continuity of the loop is broken in the corpus mammillare. The column of the fornix is joined by the stria medullaris of the pineal body and by the superficial fibers of the stria terminalis, and is said to receive also fibers from the septum pellucidum. Zuckerkandl describes an olfactory fasciculus which becomes detached from the main portion of the column of the fornix, and passes downward in front of the anterior commissure to the base of the brain, where it divides into two bundles, one joining the medial stria of the olfactory tract; the other joins the subcallosal gyrus, and through it reaches the hippocampal gyrus.

Diagram of the fornix.

The fornix and corpus callosum from below.

The crura (crus fornicis; posterior pillars) of the fornix are prolonged backward from the body. They are flattened bands, and at their commencement are intimately connected with the under surface of the corpus callosum. Diverging from one another, each curves around the posterior end of the thalamus, and passes downward and forward into the inferior cornu of the lateral ventricle (750). Here it lies along the concavity of the hippocampus, on the surface of which some of its fibers are spread out to form the alveus, while the remainder are continued as a narrow white band, the fimbria hippocampi, which is prolonged into the uncus of the hippocampal gyrus. The inner edge of the fimbria overlaps the fascia dentata hippocampi (dentate gyrus) (page 827), from which it is separated by the fimbriodentate fissure; from its lateral margin, which is thin and ragged, the ventricular epithelium is reflected over the choroid plexus as the latter projects into the chorioidal fissure.

Interventricular Foramen (foramen of Monro).—Between the columns of the fornix and the anterior ends of the thalami, an oval aperture is present on either side: this is the interventricular foramen, and through it the lateral ventricles communicate with the third ventricle. Behind the epithelial lining of the foramen the choroid plexuses of the lateral ventricles are joined across the middle line.

The Anterior Commissure (precommissure) is a bundle of white fibers, connecting the two cerebral hemispheres across the middle line, and placed in front of the columns of the fornix. On sagittal section it is oval in shape, its long diameter being vertical and measuring about 5 mm. Its fibers can be traced lateralward and backward on either side beneath the corpus striatum into the substance of the temporal lobe. It serves in this way to connect the two temporal lobes, but it also contains decussating fibers from the olfactory tracts.

The Septum Pellucidum (septum lucidum) (720) is a thin, vertically placed partition consisting of two laminæ, separated in the greater part of their extent by a narrow chink or interval, the cavity of the septum pellucidum. It is attached, above, to the under surface of the corpus callosum; below, to the anterior part of the fornix behind, and the reflected portion of the corpus callosum in front. It is triangular in form, broad in front and narrow behind; its inferior angle corresponds with the upper part of the anterior commissure. The lateral surface of each lamina is directed toward the body and anterior cornu of the lateral ventricle, and is covered by the ependyma of that cavity.

The cavity of the septum pellucidum (cavum septi pellucidi; pseudocele; fifth ventricle) is generally regarded as part of the longitudinal cerebral fissure, which has become shut off by the union of the hemispheres in the formation of the corpus callosum above and the fornix below. Each half of the septum therefore forms part of the medial wall of the hemisphere, and consists of a medial layer of gray substance, derived from that of the cortex, and a lateral layer of white substance continuous with that of the cerebral hemispheres. This cavity is not developed from the cavity of the cerebral vesicles, and never communicates with the ventricles of the brain.

The septum pellucidum is tightened between corpus callosum and fornix. It consists of the 2 laminae and cavity between them.

Lateral ventricles

The two lateral ventricles are cavities situated in the lower and medial parts of the cerebral hemispheres. They are separated from each other by a median vertical partition, the septum pellucidum, but communicate with the third ventricle and indirectly with each other through the interventricular foramen. Each lateral ventricle consists of a central part, and three prolongations from it, termed anterior, posterior and inferior cornua.

The anterior horns passes forward into the frontal lobe. It bordered:

* medially - by lamina of septum pellucidum

* laterally - by head of caudate nucleus

* anteriorly and roof - by corpus callosum

The Lateral Ventricles (ventriculus lateralis) are irregular cavities situated in the lower and medial parts of the cerebral hemispheres, one on either side of the middle line. They are separated from each other by a median vertical partition, the septum pellucidum, but communicate with the third ventricle and indirectly with each other through the interventricular foramen. They are lined by a thin, diaphanous membrane, the ependyma, covered by ciliated epithelium, and contain cerebrospinal fluid, which, even in health, may be secreted in considerable amount. Each lateral ventricle consists of a central part or body, and three prolongations from it, termed cornua.

The central part (pars centralis ventriculi lateralis; cella) of the lateral ventricle extends from the interventricular foramen to the splenium of the corpus callosum. It is an irregularly curved cavity, triangular on transverse section, with a roof, a floor, and a medial wall. The roof is formed by the under surface of the corpus callosum; the floor by the following parts, enumerated in their order of position, from before backward: the caudate nucleus of the corpus striatum, the stria terminalis and the terminal vein, the lateral portion of the upper surface of the thalamus, the choroid plexus, and the lateral part of the fornix; the medial wall is the posterior part of the septum pellucidum, which separates it from the opposite ventricle.

Central part is found in parietal lobe. It is limited:

· below - by body of caudate nucleus and dorsal surface of the thalamus;

· upwards and laterally - by fibers of corpus callosum, which form a roof.

The anterior cornu (cornu anterius; anterior horn; precornu) (Fig. 736) passes forward and lateralward, with a slight inclination downward, from the interventricular foramen into the frontal lobe, curving around the anterior end of the caudate nucleus. Its floor is formed by the upper surface of the reflected portion of the corpus callosum, the rostrum. It is bounded medially by the anterior portion of the septum pellucidum, and laterally by the head of the caudate nucleus. Its apex reaches the posterior surface of the genu of the corpus callosum.

Drawing of a cast of the ventricular cavities, viewed from the side. (Retzius.)

The posterior cornu (cornu posterius; postcornu) passes into the occipital lobe, its direction being backward and lateralward, and then medialward. Its roof is formed by the fibers of the corpus callosum passing to the temporal and occipital lobes. On its medial wall is a longitudinal eminence, the calcar avis (hippocampus minor), which is an involution of the ventricular wall produced by the calcarine fissure. Above this the forceps posterior of the corpus callosum, sweeping around to enter the occipital lobe, causes another projection, termed the bulb of the posterior cornu. The calcar avis and bulb of the posterior cornu are extremely variable in their degree of development; in some cases they are ill-defined, in others prominent.

Central part and anterior and posterior cornua of lateral ventricles exposed from above

The inferior cornu (cornu inferior; descending horn; middle horn; medicornu) the largest of the three, traverses the temporal lobe of the brain, forming in its course a curve around the posterior end of the thalamus. It passes at first backward, lateralward, and downward, and then curves forward to within 2.5 cm. of the apex of the temporal lobe, its direction being fairly well indicated on the surface of the brain by that of the superior temporal sulcus. Its roof is formed chiefly by the inferior surface of the tapetum of the corpus callosum, but the tail of the caudate nucleus and the stria terminalis also extend forward in the roof of the inferior cornu to its extremity; the tail of the caudate nucleus joins the putamen. Its floor presents the following parts: the hippocampus, the fimbria hippocampi, the collateral eminence, and the choroid plexus. When the choroid plexus is removed, a cleft-like opening is left along the medial wall of the inferior cornu; this cleft constitutes the lower part of the choroidal fissure.

Coronal section through posterior cornua of lateral ventricle

Posterior and inferior cornua of left lateral ventricle exposed from the side

The posterior horns localised in occipital lobes and cover by white matter ‘tapetum’. They carry the bulb and the calcar avis on medial wall, and a collateral triangle on the floor.

The inferior horns are found in temporal lobe. They are boundered:

· medially - by hippocampus;

· below - by white matter, which forms collateral eminence;

· superolaterally - by white matter;

· superomedially - by a tail of caudate nucleus.

The central part and temporal horn of lateral ventricle contain choroid plexus of lateral ventricle generated of penetration pia mater by vessels. Choroid plexus passes to the III ventricle through interventricular foramen. Choroid plexus takes part in formation of larger half of cerebrospinal fluid.

A cerebrospinal fluid passes from lateral ventricles through the interventricular foramen into third ventricle, where its amount increases. Then it flows from third ventricle through the cerebral aqueduct into fourth ventricle. In the fourth ventricle a cerebrospinal fluid passes the subarachnoid space through the median (of Magendie) and lateral (of Luschka) aperture and also to the central canal of the spinal cord. Cerebrospinal fluid of the subarachnoid space returns to the venous blood in the venous sinuses through the Pachioni’s arachnoid granulation.

Basal nuclei are the paired masses of gray matter located deep within the white matter in base of the forebrain. Basal nuclei include 1) corpus striatum, 2) claustrum and 3) nucleus amygdaloideus.

Corpus striatum is composed of caudate nucleus (it has a head, body and tail) and lentiform nucleus (it consists of medial and lateral globus pallidus and putamen). There is a thick lamina of white substance, the internal capsule between caudate nucleus and globus pallidus. It has the prominence of the curve is called the genu, the frontal crus and the occipital crus. The occipital crus separates the lentiform nucleus from the thalamus and carries optic and acoustic radiation.

The nucleus amygdaloideus is an ovoid gray mass situated at the lower end of the inferior cornu of the lateral ventricle (in temporal lobe).

The corpus striatum has received its name from the striped appearance which a section of its anterior part presents, in consequence of diverging white fibers being mixed with the gray substance which forms its chief mass. A part of the corpus striatum is imbedded in the white substance of the hemisphere, and is therefore external to the ventricle; it is termed the extraventricular portion, or the lentiform nucleus; the remainder, however, projects into the ventricle, and is named the intraventricular portion, or the caudate nucleus.

The caudate nucleus (nucleus caudatus; caudatum) is a pear-shaped, highly arched gray mass; its broad extremity, or head, is directed forward into the anterior cornu of the lateral ventricle, and is continuous with the anterior perforated substance and with the anterior end of the lentiform nucleus; its narrow end, or tail, is directed backward on the lateral side of the thalamus, from which it is separated by the stria terminalis and the terminal vein. It is then continued downward into the roof of the inferior cornu, and ends in the putamen near the apex of the temporal lobe. It is covered by the lining of the ventricle, and crossed by some veins of considerable size. It is separated from the lentiform nucleus, in the greater part of its extent, by a thick lamina of white substance, called the

internal capsule, but the two portions of the corpus striatum are united in front.

Horizontal section of right cerebral hemisphere.

The lentiform nucleus (nucleus lentiformis; lenticular nucleus; lenticula) is lateral to the caudate nucleus and thalamus, and is seen only in sections of the hemisphere. When divided horizontally, it exhibits, to some extent, the appearance of a biconvex lens. while a coronal section of its central part presents a somewhat triangular outline. It is shorter than the caudate nucleus and does not extend as far forward. It is bounded laterally by a lamina of white substance called the external capsule, and lateral to this is a thin layer of gray substance termed the claustrum. Its anterior end is continuous with the lower part of the head of the caudate nucleus and with the anterior perforated substance.

In a coronal section through the middle of the lentiform nucleus, two medullary laminæ are seen dividing it into three parts. The lateral and largest part is of a reddish color, and is known as the putamen, while the medial and intermediate are of a yellowish tint, and together constitute the globus pallidus; all three are marked by fine radiating white fibers, which are most distinct in the putamen.

The gray substance of the corpus striatum is traversed by nerve fibers, some of which originate in it. The cells are multipolar, both large and small; those of the lentiform nucleus contain yellow pigment. The caudate and lentiform nuclei are not only directly continuous with each other anteriorly, but are connected to each other by numerous fibers. The corpus striatum is also connected: (1) to the cerebral cortex, by what are termed the corticostriate fibers; (2) to the thalamus, by fibers which pass through the internal capsule, and by a strand named the ansa lentiformis; (3) to the cerebral peduncle, by fibers which leave the lower aspect of the caudate and lentiform nuclei.

Coronal section through anterior cornua of lateral ventricles.

The claustrum is a thin layer of gray substance, situated on the lateral surface of the external capsule. Its transverse section is triangular, with the apex directed upward. Its medial surface, contiguous to the external capsule, is smooth, but its lateral surface presents ridges and furrows corresponding with the gyri and sulci of the insula, with which it is in close relationship. The claustrum is regarded as a detached portion of the gray substance of the insula, from which it is separated by a layer of white fibers, the capsula extrema (band of Baillarger). Its cells are small and spindle-shaped, and contain yellow pigment; they are similar to those of the deepest layer of the cortex.

The nucleus amygdalæ (amygdala) is an ovoid gray mass, situated at the lower end of the roof of the inferior cornu. It is merely a localized thickening of the gray cortex, continuous with that of the uncus; in front it is continuous with the putamen, behind with the stria terminalis and the tail of the caudate nucleus.

The internal capsule (capsula interna) is a flattened band of white fibers, between the lentiform nucleus on the lateral side and the caudate nucleus and thalamus on the medial side. In horizontal section it is seen to be somewhat abruptly curved, with its convexity inward; the prominence of the curve is called the genu, and projects between the caudate nucleus and the thalamus. The portion in front of the genu is termed the frontal part, and separates the lentiform from the caudate nucleus; the portion behind the genu is the occipital part, and separates the lentiform nucleus from the thalamus.

Coronal section of brain through anterior commissure.

The frontal part of the internal capsule contains: (1) fibers running from the thalamus to the frontal lobe; (2) fibers connecting the lentiform and caudate nuclei; (3) fibers connecting the cortex with the corpus striatum; and (4) fibers passing from the frontal lobe through the medial fifth of the base of the cerebral peduncle to the nuclei pontis. The fibers in the region of the genu are named the geniculate fibers; they originate in the motor part of the cerebral cortex, and, after passing downward through the base of the cerebral peduncle with the cerebrospinal fibers, undergo decussation and end in the motor nuclei of the cranial nerves of the opposite side. The anterior two-thirds of the occipital part of the internal capsule contains the cerebrospinal fibers, which arise in the motor area of the cerebral cortex and, passing downward through the middle three-fifths of the base of the cerebral peduncle, are continued into the pyramids of the medulla oblongata. The posterior third of the occipital part contains: (1) sensory fibers, largely derived from the thalamus, though some may be continued upward from the medial lemniscus; (2) the fibers of optic radiation, from the lower visual centers to the cortex of the occipital lobe; (3) acoustic fibers, from the lateral lemniscus to the temporal lobe; and (4) fibers which pass from the occipital and temporal lobes to the nuclei pontis.

The fibers of the internal capsule radiate widely as they pass to and from the various parts of the cerebral cortex, forming the corona radiata (745) and intermingling with the fibers of the corpus callosum.

The external capsule (capsula externa) is a lamina of white substance, situated lateral to the lentiform nucleus, between it and the claustrum, and continuous with the internal capsule below and behind the lentiform nucleus. It probably contains fibers derived from the thalamus, the anterior commissure, and the subthalamic region.

The substantia innominata of Meynert is a stratum consisting partly of gray and partly of white substance, which lies below the anter

ior part of the thalamus and lentiform nucleus. It consists of three layers, superior, middle, and inferior. The superior layer is named the ansa lentiformis, and its fibers, derived from the medullary lamina of the lentiform nucleus, pass medially to end in the thalamus and subthalamic region, while others are said to end in the tegmentum and red nucleus.

Dissection showing the course of the cerebrospinal fibers.

The middle layer consists of nerve cells and nerve fibers; fibers enter it from the parietal lobe through the external capsule, while others are said to connect it with the medial longitudinal fasciculus. The inferior layer forms the main part of the inferior stalk of the thalamus, and connects this body with the temporal lobe and the insula.

The stria terminalis (tænia semicircularis) is a narrow band of white substance situated in the depression between the caudate nucleus and the thalamus. Anteriorly, its fibers are partly continued into the column of the fornix; some, however, pass over the anterior commissure to the gray substance between the caudate nucleus and septum pellucidum, while others are said to enter the caudate nucleus. Posteriorly, it is continued into the roof of the inferior cornu of the lateral ventricle, at the extremity of which it enters the nucleus amygdalæ. Superficial to it is a large vein, the terminal vein (vein of the corpus striatum), which receives numerous tributaries from the corpus striatum and thalamus; it runs forward to the interventricular foramen and there joins with the vein of the choroid plexus to form the corresponding internal cerebral vein.

Theme 2. Pathways of the brain and spinal cord (associative, commissural, projective). Ascending pathways.

Pathways of the brain and spinal cord subdivide into: associative, commissural and projection fibers.

Associative pathways are the tracts communicating functional areas of one hemisphere. They can be divided into long associative fibres and short associative fibres. The long associative pathways include:

1. Superior longitudinal fascicle communicates frontal, parietal and occipital lobes;

2. Inferior longitudinal fascicle communicates parietal, occipital, temporal lobes;

3. Uncinate fascicle - communicates frontal, parietal and temporal lobes;

4. Fornicate fascicle - communicates central areas of rhinencephalon;

To short associative tracts are arcuate fibres of the cerebrum, which communicate neighbouring convolutions within hemisphere.

associationis breves).

The association fibers unite different parts of the same hemisphere, and are of two kinds: (1) those connecting adjacent gyri, short association fibers; (2) those passing between more distant parts, long association fibers.

The short association fibers lie immediately beneath the gray substance of the cortex of the hemispheres, and connect together adjacent gyri.

The long association fibers include the following: (a) the uncinate fasciculus; (b) the cingulum; (c) the superior longitudinal fasciculus; (d) the inferior longitudinal fasciculus; (e) the perpendicular fasciculus; (f) the occipitofrontal fasciculus; and (g) the fornix.

(a) The uncinate fasciculus passes across the bottom of the lateral fissure, and unites the gyri of the frontal lobe with the anterior end of the temporal lobe.

(b) The cingulum is a band of white matter contained within the cingulate gyrus. Beginning in front at the anterior perforated substance, it passes forward and upward parallel with the rostrum, winds around the genu, runs backward above the corpus callosum, turns around the splenium, and ends in the hippocampal gyrus.

(c) The superior longitudinal fasciculus passes backward from the frontal lobe above the lentiform nucleus and insula; some of its fibers end in the occipital lobe, and others curve downward and forward into the temporal lobe.

Dissection of cortex and brain-stem showing association fibers and island of Reil after removal of its superficial gray substance

(d) The inferior longitudinal fasciculus connects the temporal and occipital lobes, running along the lateral walls of the inferior and posterior cornua of the lateral ventricle.

Deep dissection of cortex and brain-stem

(e) The perpendicular fasciculus runs vertically through the front part of the occipital lobe, and connects the inferior parietal lobule with the fusiform gyrus.

(f) The occipitofrontal fasciculus passes backward from the frontal lobe, along the lateral border of the caudate nucleus, and on the mesial aspect of the corona radiata; its fibers radiate in a fan-like manner and pass into the occipital and temporal lobes lateral to the posterior and inferior cornua. Déjerine regards the fibers of the tapetum as being derived from this fasciculus, and not from the corpus callosum.

(g) The fornix connects the hippocampal gyrus with the corpus mammillare and, by means of the thalamomammillary fasciculus, with the thalamus (see page 839). Through the fibers of the hippocampal commissure it probably also unites the opposite hippocampal gyri.

VIDEO

The Commissural pathways communicate symmetric areas of both hemispheres of cerebrum and both halves of spinal cord for co-ordination of their activity. There are:

1. corpus callosum;

2. anterior cerebral commissura;

3. posterior cerebral commissura;

4. habenular commissura ;

5. fornicate commissura ;

6. interthalamic adhesion;

7. posterior white commissura of spinal cord.

The corpus callosum connects right and left cerebral hemispheres. Corpus callosum anteriorly carries a genu that passes into rostrum. Last continue as a lamina rostralis and lamina terminalis. Back part of the corpus callosum called splemium. Anterior fibers of the corpus callosum form the frontal forceps, posterior fibers of the corpus callosum - the occipital forceps.

Corpus callosum from above

The Projecting pathways are the tracts, which communicate the cerebrum and spinal cord with working organs. They subdivided into ascending (sensible) and descending (motor). The sensible projecting tracts into its turn subdivide into exteroreceptive, interoceptive and proprioreceptive. The motor projecting fibers tracts are pyramidalі and extrapyramidal.

Sensory Pathways from the Spinal Cord to the Brain.—The posterior root fibers conducting the impulses of conscious muscle sense, tendon sense and joint sense, those impulses which have to do with the coördination and adjustment of muscular movements, ascend in the fasciculus gracilis and fasciculus cuneatus to the nucleus gracilis and nucleus cuneatus in the medulla oblongata.

In the nucleus gracilis and nucleus cuneatus synaptic relations are found with neurons whose cell bodies are located in these nuclei and whose axons pass by way of the internal arcuate fibers, cross in the raphé to the opposite side in the region between the olives and turn abruptly upward to form the medial lemniscus or medial fillet. The medial fillet passes upward in the ventral part of the formatio reticularis through the medulla oblongata, pons and mid-brain to the principal sensory nucleus of the ventro-lateral region of the thalamus. Here the terminals form synapses with neurons of the third order whose axons pass through the internal capsule and corona radiata to the somatic sensory area of the cortex in the post-central gyrus.

Fibers conducting the impulses of unconscious muscle sense pass to the cerebellum partly by way of the fasciculus gracilis and fasciculus cuneatus to the nucleus gracilis and nucleus cuneatus, thence neurons of the second order convey the impulses either via the dorsal external arcuate fibers directly into the inferior peduncle of the cerebellum or via the ventral external arcuate fibers which are continued from the internal arcuate fibers through the ventral part of the raphé and after crossing the midline emerge on the surface of the medulla in the ventral sulcus between the pyramids or in the groove between the pyramid and the olive. They pass over the lateral surface of the medulla and olive to reach the inferior peduncle through which they pass to the cerebellum.

Other fibers conducting impulses of unconscious muscle sense pass upward in the dorsal spinocerebellar fasciculus, which arises from cells in the nucleus dorsalis. The posterior root fibers conducting these impulses pass into the fasciculus cuneatus and the collaterals from them to the nucleus dorsalis are said to come almost exclusively from the middle area of the fasciculus cuneatus. They form by their multiple division baskets about the individual cells of the nucleus dorsalis, each fiber coming in relation with the bodies and dendrites of several cells. The axons of the second order pass into the dorsal spinocerebellar fasciculus of the same side and ascend along the lateral surface of the spinal cord and medulla oblongata until they arrive at the level of the olive, they then curve backward beneath the external arcuate fibers into the inferior peduncle and pass into the cerebellum. Here they give off collaterals to the dentate nucleus and finally terminate in the cortex of the dorsal and superior portion of the vermis, partly on the same side, but to a great extent by way of a large commissure to the opposite side. The fibers lose their myelin sheaths as they enter the gray substance and terminate by end ramifications among the nerve cells and their processes. Some of the fibers are said to end in the nucleus dentatus and the roof nuclei of the cerebellum (the nucleus globosus, nucleus emboliformis and nucleus fastigius) and others pass through them to terminate in the inferior vermis. A few fibers of the dorsal spinocerebellar fasciculus are said not to enter the inferior peduncle but to pass with the ventral spinocerebellar fasciculus. The cerebellar reflex are is supposed to be completed by the fibers of the superior peduncle which pass from the cerebellum to the red nucleus of the mid-brain where some of their terminals and collaterals form synapses with neurons whose axons descend to the spinal cord in the rubrospinal fasciculus. The terminal and collaterals of this fasciculus end either directly or indirectly about the motor cells in the anterior column.

The ventral spinocerebellar fasciculus, since most of its fibers pass to the cerebellum, is also supposed to be concerned in the conduction of unconscious muscle sense. The location of its cells of origin is uncertain. They are probably in or near the dorsal nucleus of the same and the opposite side; various other locations are given, the dorsal column, the intermediate zone of the gray matter and the central portion of the anterior column. The neurons of the first order whose central fibers enter the fasciculus cuneatus from the dorsal roots send collaterals and terminals to form synapses with these cells. The fibers which come from the opposite gray columns cross some in the white and some in the gray commissure and pass with fibers from the same side through the lateral funiculus to the marginal region ventral to the dorsal spinocerebellar fasciculus. The fasciculus begins about the level of the third lumbar nerve and continues upward on the lateral surface of the spinal cord and medulla oblongata until it passes under cover of the external arcuate fibers. It passes just dorsal to the olive and above this joins the lateral edge of the lateral lemniscus along which it runs, ventral to the roots of the trigeminal nerve, almost to the level of the superior colliculus, it then crosses over the superior peduncle, turns abruptly backward along its medial border, enters the cerebellum with it and ends in the vermis of the same and the opposite side. Some of its fibers are said to join the dorsal spinocerebellar fasciculus in the medulla oblongata and enter the cerebellum through the inferior peduncle. A number of fibers are said to continue upward in the dorsolateral part of the tegmentum as far as the superior colliculus and a few pass to the thalamus. They probably form part of the sensory or higher reflex path.

The posterior root fibers conducting impulses of pain and temperature probably terminate in the posterior column or the intermediate region of the gray matter soon after they enter the spinal cord. The neurons of the second order are supposed to pass through the anterior commissure to the superficial antero-lateral fasciculus (tract of Gowers) and pass upward in that portion of it known as the lateral spinothalamic fasciculus. This fasciculus lies along the medial side of the ventral spinocerebellar fasciculus. It is stated by some authors that the pain fibers pass upward in the antero-lateral ground bundles. In some of the lower mammals this pathway carries the pain fibers upward by a series of neurons some of which cross to the opposite side, so that in part there is a double path. In man, however, the lateral spinothalamic fasciculus is probably the most important pathway. On reaching the medulla these fibers continue upward through the formatio reticularis in the neighborhood of the median fillet to the thalamus, probably its ventro-lateral region. Whether higher neurons convey the pain impulses to the cortex through the internal capsule is uncertain. The pathway is probably more complex and Head is of the opinion that our sensations of pain are essentially thalamic. The pain and temperature pathways in the lateral spinothalamic fasciculus are not so closely intermingled but that one can be destroyed without injury to the other.

Ransom suggests that the non-medullated fibers of the posterior roots, which turn into Lissauer’s tract and ascend or descend for short distances not exceeding one or two segments and finally end in the substantia gelatinosa, are in part at least pain fibers and that the fasciculus of Lissauer and the substantia gelatinosa represent part of the mechanism for reflexes associated with pain conduction and reception while the fibers to the higher centers pass up in the spinothalamic tract.

The fibers of tactile discrimination, according to Head and Thompson, pass up in the fasciculus cuneatus and fasciculus gracilis of the same side and follow the path of the muscle-sense fibers. The axons of the second order arising in the nucleus cuneatus and gracilis cross with the internal arcuate fibers and ascend to the thalamus with the medial lemniscus, thence by neurons of higher order the impulses are carried to the somatic sensory area of the cortex through the internal capsule. The other touch fibers, shortly after entering the spinal cord, terminate in the dorsal column or intermediate gray matter. Neurons of the second order send their axons through the anterior commissure to pass upward in the antero-lateral funiculus probably in the ventral spinothalamic fasciculus. In the medulla they join or pass upward in the neighborhood of the medial lemniscus to the thalamus and thence by neurons of higher order to the somatic sensory area of the cortex.

The remaining ascending fasciculi form a part of the complex known as the superficial antero-lateral fasciculus (tract of Gowers). The spinotectal fasciculus, as its name indicates, is supposed to have its origin in the gray matter of the cord and terminations in the superior and inferior (?) colliculi of the mid-brain serving for reflexes between the cord and the visceral and auditory centers of the mid-brain.

The spino-olivary fasciculus (olivospinal; bulbospinal, Helweg’s bundle) is likewise of unknown constitution and function; there is uncertainty even in regard to the direction of its fibers.

Sympathetic afferent fibers (visceral afferent; viscero-sensory; splanchnic afferent) enter the spinal cord by the posterior roots of the thoracic and first two or three lumbar nerves and the second to the fourth sacral nerves. The fibers pass to these nerves from the peripheral sympathetic system through the white rami communicantes. Some of the cell bodies of these afferent fibers are located in the spinal ganglia and others are in the sympathetic ganglia. Some of the afferent sympathetic fibers end about the cell bodies of somatic sensory neurons and visceral impulses are thus transmitted to these neurons which conduct them as well as their own special impulses to the spinal cord. Other sympathetic afferent neurons whose cell bodies are located in the spinal ganglia send collaterals to neighboring cells of somatic sensory neurons and thus have a double path of transmission to the spinal cord. Such an arrangement provides a mechanism for some of the referred pains.

To sernsory exteroreceptive (E) tracts belong the tracts temperatural, palpable, pain sensivity, also vision, hearing, olfaction. The proprioreceptive (P) tracts subdivided into tracts of cortical direction (Goll tracts and Burdach) and cerebellar directions (Flechsig and Gowers tracts). Anterior and lateral corticospinal tracts and also corticonuclear tract belong to pyramidal pathways. The extrapyramidal tracts include rubrospinal, vestibulospinal, reticulospinal, olivospinal pathways.

(E) Pain and temperature sensation pathway (Lateral spinothalamic tract).

1. Body of first neuron is found in spinal ganglion. The peripheral process of this neuron terminates by exteroreceptor in skin, and central - on the strength of posterior rootlets into posterior horns of spinal cord, where contacts with second neuron.

2. The second neuron localised in proper nucleus of posterior horns of spinal cord. Its axons make a crossing in grey commissura of spinal cord and pass in lateral funiculus (lateral spinothalamic tract). Axons within the medial of lemniscus reach the thalamus, where terminate by synapse with body of third neuron.

3. The third neuron is in lateral nucleus of thalamus, and its axons ‘thalamocortical tract’ pass to postcentral gyrus (cortical analyser of skin sensation) through the back third of internal capsule.

(E) Pressure and touch sensation pathway (Anterior spinothalamic tract).

1. Body of first neuron is found in spinal ganglion. The peripheral process of this neuron terminates by exteroreceptor in skin, and central – in the gelatinose substance of spinal cord, where contacts with second neuron.

2. The second neuron axons make a crossing in grey commissura of spinal cord and pass in lateral funiculus (anterior spinothalamic tract). Axons reach the thalamus, where terminate by synapse with body of third neuron.

3. The third neuron is in thalamus, it passes to postcentral gyrus (cortical analyser of skin sensation) through the back third of internal capsule:

Proprioreceptive pathway (cortical direction) - Bulbothalamic tract.

1. The first neuron of this tract lies in spinal ganglion. Its peripheral process terminates by proprioreceptor in muscles, tendoms, jonts capsules and ligaments. The central process passeswith posterior rootlets into spinal cord and form in white matter Goll tract - fascicle (upper 11 segments). The process passes to medulla oblongata, where terminate by synapse with second neuron:

2. The second neurons are in gracilis and cuneate nuclei of medulla oblongata. Axons this neuron form bulbothalamic tract, which composes a base of medial lemniscus. Axons of second neuron cross (deccusation of lemniscus) and reach the thalamus, where terminate by synapse with third neuron.

3. The third neurons pass through the internal capsule and form the thalamocortical tract:

(P) Proprioreceptive pathways with cerebellar direction (Spinocerebellar tracts).

Posterior spinocerebellar tract (Flechsig tract)

1. The first neuron is in spinal ganglion. Its peripheral process terminates by proprioreceptor and central with the posterior rootlets of spinal cord passes to the grey matter.

2. The second neuron lies in thoracic nucleus of posterior horns (Clarc-Steiling). Its axons pass in lateral funiculus. Then it reaches the cortex of the cerebellar vermis through the inferior cerebellar pedunculi and dentate nucleus. Nervous impulse passes to the ruber nucleus:

Anterior spinocerebellar tract (Gowers tract). Two-neurons way. This tract differs from previous by its second neuron, a body of which is found in medial intermediate nucleus of spinal cord. Axons of the second neuron get across and pass into lateral funiculus, reaching the superior cerebral velum. They cross here and pass to the vermis and dentate nucleus. Nervous impulse also passes to the ruber nucleus.

Descending pathways: pyramidal, exrapyramidal. Topography of pathways on section of spinal cord

Lateral corticospinal (pyramidal) tract

1. A Body of first neuron is found in gigantic pyramidal cells (Bets) of top and middle third of precentral gyrus of the cortex. Axons pass through anterior third part of internal capsule, pons, medulla oblongata; part of fibres gets across forming the pyramids. The crossed fibres of first neuron pass in lateral funiculus of the spinal cord ‘lateral corticospinal (pyramidal) tract’ and terminate in anterior horns of spinal cord.

2. The second neuron lies in motor nucleus of anterior horns, and its axons pass with the anterior rootlets and spinal nerves as far as skeletal muscles of the trunk and extremities:

The descending fasciculi which convey impulses from the higher centers to the spinal cord and located in the lateral and ventral funiculi.

The Motor Tract conveying voluntary impulses, arises from the pyramid cells situated in the motor area of the cortex, the anterior central and the posterior portions of the frontal gyri and the paracentral lobule. The fibers are at first somewhat widely diffused, but as they descend through the corona radiata they gradually approach each other, and pass between the lentiform nucleus and thalamus, in the genu and anterior two-thirds of the occipital part of the internal capsule; those in the genu are named the geniculate fibers, while the remainder constitute the cerebrospinal fibers; proceeding downward they enter the middle three-fifths of the base of the cerebral peduncle. The geniculate fibers cross the middle line, and end by arborizing around the cells of the motor nuclei of the cranial nerves. The cerebrospinal fibers are continued downward into the pyramids of the medulla oblongata, and the transit of the fibers from the medulla oblongata is effected by two paths. The fibers nearest to the anterior median fissure cross the middle line, forming the decussation of the pyramids, and descend in the opposite side of the medulla spinalis, as the lateral cerebrospinal fasciculus (crossed pyramidal tract). Throughout the length of the medulla spinalis fibers from this column pass into the gray substance, to terminate either directly or indirectly around the motor cells of the anterior column. The more laterally placed portion of the tract does not decussate in the medulla oblongata, but descends as the anterior cerebrospinal fasciculus (direct pyramidal tract); these fibers, however, end in the anterior gray column of the opposite side of the medulla spinalis by passing across in the anterior white commissure. There is considerable variation in the extent to which decussation takes place in the medulla oblongata; about two-thirds or three-fourths of the fibers usually decussate in the medulla oblongata and the remainder in the medulla spinalis.

The axons of the motor cells in the anterior column pass out as the fibers of the anterior roots of the spinal nerves, along which the impulses are conducted to the muscles of the trunk and limbs.

From this it will be seen that all the fibers of the motor tract pass to the nuclei of the motor nerves on the opposite side of the brain or medulla spinalis, a fact which explains why a lesion involving the motor area of one side causes paralysis of the muscles of the opposite side of the body. Further, it will be seen that there is a break in the continuity of the motor chain; in the case of the cranial nerves this break occurs in the nuclei of these nerves; and in the case of the spinal nerves, in the anterior gray column of the medulla spinalis. For clinical purposes it is convenient to emphasize this break and divide the motor tract into two portions: (1) a series of upper motor neurons which comprises the motor cells in the cortex and their descending fibers down to the nuclei of the motor nerves; (2) a series of lower motor neurons which includes the cells of the nuclei of the motor cerebral nerves or the cells of the anterior columns of the medulla spinalis and their axiscylinder processes to the periphery.

The rubrospinal fasciculus arises from the large cells of the red nucleus. The fibers cross the raphé of the mid-brain in the decussation of Forel and descend in the formatio reticularis of the pons and medulla dorsal to the medial lemniscus and as they pass into the spinal cord come to lie in a position ventral to the crossed pyramidal tracts in the lateral funiculus. The rubrospinal fibers end either directly or indirectly by terminals and collaterals about the motor cells in the anterior column on the side opposite from their origin in the red nucleus. A few are said to pass down on the same side. Since the red nucleus is intimately related to the cerebellum by terminals and collaterals of the superior peduncle which arises in the dentate nucleus of the cerebellum, the rubrospinal fasciculus is supposed to be concerned with cerebellar reflexes, complex motor coördinations necessary in locomotion and equilibrium. The afferent paths concerned in these reflexes have already been partly considered, namely, the dorsal and ventral spinocerebellar fasciculi, and probably some of the fibers of the posterior funiculi which reach the cerebellum by the inferior peduncle.

The tectospinal fasciculus arises from the superior colliculus of the roof (tectum) of the mid-brain. The axons come from large cells in the stratum opticum and stratum lemnisci and sweep ventrally around the central gray matter of the aqueduct, cross the raphé in the fountain decussation of Meynert and turn downward in the tegmentum in the ventral longitudinal bundle. Some of the fibers do not cross in the raphé but pass down on the same side; it is uncertain whether they come from the superior colliculus of the same side or arch over the aqueduct from the colliculus of the opposite side. The tectospinal fasciculus which comprises the major part of the ventral longitudinal bundle passes down through the tegmentum and reticular formation of the pons and medulla oblongata ventral to the medial longitudinal bundle. In the medulla the two bundles are more or less intermingled and the tectospinal portion is continued into the antero-lateral funiculus of the spinal cord ventral to the rubrospinal fasciculus with which some of its fibers are intermingled. Some of the fibers of the tectospinal fasciculus pass through the red nucleus giving off collaterals to it, others are given off to the motor nuclei of the cranial nerves and in the spinal cord they terminate either directly or indirectly by terminals and collaterals among the nuclei of the anterior column. Since the superior colliculus is an important optic reflex center, this tract is probably concerned in optic reflexes; and possibly also with auditory reflexes since some of the fibers of the central auditory path, the lateral lemniscus, terminate in the superior colliculus.

The vestibulospinal fasciculus (part of the anterior marginal fasciculus or Loewenthal’s tract) situated chiefly in the marginal part of the anterior funiculus is mainly derived from the cells of the terminal nuclei of the vestibular nerve, probably Deiters’s and Bechterew’s, and some of its fibers are supposed to come from the nucleus fastigius (roof nucleus of the cerebellum). The latter nucleus is intimately connected with Dieters’s and Bechterew’s nuclei. The vestibulospinal fasciculus is concerned with equilibratory reflexes. Its terminals and collaterals end about the motor cells in the anterior column. It extends to the sacral region of the cord. Its fibers are intermingled with the ascending spinothalamic fasciculus, with the anterior proper fasciculus and laterally with the tectospinal fasciculus. Its fibers are supposed to be both crossed and uncrossed. In the brain-stem it is associated with the dorsal longitudinal bundle.

The pontospinal fasciculus (Bechterew) arises from the cells in the reticular formation of the pons from the same and the opposite side and is associated in the brain-stem with the ventral longitudinal bundle. In the cord it is intermingled with the fibers of the vestibulospinal fasciculus in the anterior funiculus. Not much is known about this tract.

There are probably other descending fasciculi such as the thalamospinal but not much is known about them.

Anterior corticospinal (pyramidal) tract

1. Major part of fibres of first neurons of anterior corticospinal tract does not cross in pyramids of medulla oblongata, and passes in anterior funiculus of the spinal cord. Axons cross in grey commissura terminate by synapse with bodies of second neurons.first

2. The second neuron lies in motor nucleus of anterior horns, and its axons pass within the anterior rootlets and spinal nerves as far as skeletal muscles of the trunk and extremities.

Corticonuclear tract

1. The first neuron of this tract is found in gigantic pyramidal cells of cortex in lower third of precentral gyrus. Axons pass through the genu of internal capsule, base of cerebral peduncles and terminate in motor nucleus of rhomboid fossa and midbrain, previously passing across partially on opposite side.

The second neuron lies in motor nucleus of rhomboid fossa and midbrain of opposite side, and its axons with cranial nerves pass as far as stripped muscles of the head and superficial neck muscles:

Extrapyramidal pathways transmit impulses providing muscles tone and reflexes of equilibrium and execution of automated motions. They include rubrospinal, tectospinal, vestibulospinal, reticulospinal and olivospinal tracts.

Rubrospinal tract

1. The first neuron is found in red nucleus (n. ruber). Its axons make tegmental deccusation and pass through the base of cerebral peduncles, pons, and medulla oblongata. They form tract in lateral funiculus and reach motor nuclei in anterior horns of spinal cord.

2. The second neuron lies in motor nucleus of anterior horns of spinal cord, and its axons on the strength of anterior rootlets and spinal nerves reach trunk and extremities muscles.

This 2-neuron tract is descending link for reflex link of unconscious motion coordination. Spinocerebellar tracts (Flechsig and Gowers tracts) are the ascending links for this regulation. Spinocerebellar tracts send impulses for rubrospinal tracts through the intermediate link (from cerebellar vermis and dentate nuclei to the ruber nuclei).

Corticopontocerebellar tract (way of cortical correction of the cerebellum)

This 2-neuron tract starts from all lobes of cerebral hemispheres. Axons run through the internal capsula to the proper pontini nuclei. Second neurons cross and pass to the cerebellar vermis through the middle cerebellar pedunculi. Some links directed also to dentate and ruber nuclei. This pathway materializes cortical correction of unconscious motion co-ordination.

Theme 1. Meninges of the brain. Dural sinuses. Circulation of spinocerebral fluid. Blood vessels of the brain and spinal cord

The brain is enclosed by in 3 meninges: dura mater, arachnoid and pia mater. Dura mater is the periosteum for internal surface of the skull bones and there is no space between dura mater and bones. There is subarachnoid space between choroid and pia mater, which is filled in by cerebrospinal fluid. This space has expansions, which are called ‘cisterns’:

· cerebellomedullary cistern;

· chiasmatic cistern;

· interpeduncular cistern;

· cistern of the lateral fossa;

· pontocerebellar cistern;

· superior cistern.

Diagram showing the positions of the three principal subarachnoid cisternæ.

Dura mater has some processes extend into the cavity of the scull and separating the brain parts:

· Falx cerebri occupies a longitudinal fissura cerebri and separates the brain to right and left hemispheres;

· Falx cerebelli lies in longitudinal cerebellum furrow and separates it into right and left cerebellum hemispheres;

· Diaphragma sellae closes hypophysial fossa, separating a hypophysis from diencephalon.

· Tentorium cerebelli occupies a tranverse fissura cerebri and separates the cerebellum from the occipital lobe of the telencephalon.

· Dura mater forms a trigeminal cavity on anterior surface of the temporal pyramide.

The processes of dura mater approaching sulcuses on the skull bones slit and fasten to sulcus edges, forming the venous sinuses where venous blood flows:

Diagrammatic representation of a section across the top of the skull, showing the membranes of the brain,

· superior sagittal sinus;

· inferior sagittal sinus;

· straight [rectus] sinus;

· occipital sinus;

· inferior petrosal sinus;

· superior petrosal sinus;

· transverse sinus;

· cavernous sinus;

· intercavernous sinus;

· sphenoperietal sinus;

· sygmoid sinus.

These sinuses flow together in confluence sinuum. Venous blood from all sinuses passes into sygmoid sinus, and then - into internal jugular vein. The arachnoid forms the Pachioni’s arachnoid granulation. They extend from venous sinuses in internal surface skull vault bone and form foveoli granulares. Arachnoid granulations provide a cerebrospinal fluid flow from subarachnoid space into venous blood. Emissary veins and diploic veins communicate the sinuses of dura mater with the superficial head veins.

The brain and medulla spinalis are enclosed within three membranes. These are named from without inward: the dura mater, the arachnoid, and the pia mater.

The Dura Mater

The dura mater is a thick and dense inelastic membrane. The portion which encloses the brain differs in several essential particulars from that which surrounds the medulla spinalis, and therefore it is necessary to describe them separately; but at the same time it must be distinctly understood that the two form one complete membrane, and are continuous with each other at the foramen magnum.

The Cranial Dura Mater (dura mater encephali; dura of the brain) lines the interior of the skull, and serves the twofold purpose of an internal periosteum to the bones, and a membrane for the protection of the brain. It is composed of two layers, an inner or meningeal and an outer or endosteal, closely connected together, except in certain situations, where, as already described (page 654), they separate to form sinuses for the passage of venous blood. Its outer surface is rough and fibrillated, and adheres closely to the inner surfaces of the bones, the adhesions being most marked opposite the sutures and at the base of the skull its inner surface is smooth and lined by a layer of endothelium. It sends inward four processes which divide the cavity of the skull into a series of freely communicating compartments, for the lodgement and protection of the different parts of the brain; and it is prolonged to the outer surface of the skull, through the various foramina which exist at the base, and thus becomes continuous with the pericranium; its fibrous layer forms sheaths for the nerves which pass through these apertures. Around the margin of the foramen magnum it is closely adherent to the bone, and is continuous with the spinal dura mater.

Dura mater and its processes exposed by removing part of the right half of the skull and the brain.

Processes.—The processes of the cranial dura mater, which projects into the cavity of the skull, are formed by reduplications of the inner or meningeal layer of the membrane, and are four in number: the falx cerebri, the tentorium cerebelli, the falx cerebelli, and the diaphragma sellæ.

The falx cerebri (765), so named from its sickle-like form, is a strong, arched process which descends vertically in the longitudinal fissure between the cerebral hemispheres. It is narrow in front, where it is attached to the crista galli of the ethmoid; and broad behind, where it is connected with the upper surface of the tentorium cerebelli. Its upper margin is convex, and attached to the inner surface of the skull in the middle line, as far back as the internal occipital protuberance; it contains the superior sagittal sinus. Its lower margin is free and concave, and contains the inferior sagittal sinus.

The tentorium cerebelli (766) is an arched lamina, elevated in the middle, and inclining downward toward the circumference. It covers the superior surface of the cerebellum, and supports the occipital lobes of the brain. Its anterior border is free and concave, and bounds a large oval opening, the incisura tentorii, for the transmission of the cerebral peduncles. It is attached, behind, by its convex border, to the transverse ridges upon the inner surface of the occipital bone, and there encloses the transverse sinuses; in front, to the superior angle of the petrous part of the temporal bone on either side, enclosing the superior petrosal sinuses. At the apex of the petrous part of the temporal bone the free and attached borders meet, and, crossing one another, are continued forward to be fixed to the anterior and posterior clinoid processes respectively. To the middle line of its upper surface the posterior border of the falx cerebri is attached, the straight sinus being placed at their line of junction.

Tentorium cerebelli seen from above.

The falx cerebelli is a small triangular process of dura mater, received into the posterior cerebellar notch. Its base is attached, above, to the under and back part of the tentorium; its posterior margin, to the lower division of the vertical crest on the inner surface of the occipital bone. As it descends, it sometimes divides into two smaller folds, which are lost on the sides of the foramen magnum.

The diaphragma sellæ is a small circular horizontal fold, which roofs in the sella turcica and almost completely covers the hypophysis; a small central opening transmits the infundibulum.

Structure.—The cranial dura mater consists of white fibrous tissue and elastic fibers arranged in flattened laminæ which are imperfectly separated by lacunar spaces and bloodvessels into two layers, endosteal and meningeal. The endosteal layer is the internal periosteum for the cranial bones, and contains the bloodvessels for their supply. At the margin of the foramen magnum it is continuous with the periosteum lining the vertebral canal. The meningeal or supporting layer is lined on its inner surface by a layer of nucleated flattened mesothelium, similar to that found on serous membranes.

The arteries of the dura mater are very numerous. Those in the anterior fossa are the anterior meningeal branches of the anterior and posterior ethmoidal and internal carotid, and a branch from the middle meningeal. Those in the middle fossa are the middle and accessory meningeal of the internal maxillary; a branch from the ascending pharyngeal, which enters the skull through the foramen lacerum; branches from the internal carotid, and a recurrent branch from the lacrimal. Those in the posterior fossa are meningeal branches from the occipital, one entering the skull through the jugular foramen, and another through the mastoid foramen; the posterior meningeal from the vertebral; occasional meningeal branches from the ascending pharyngeal, entering the skull through the jugular foramen and hypoglossal canal; and a branch from the middle meningeal.

The veins returning the blood from the cranial dura mater anastomose with the diploic veins and end in the various sinuses. Many of the meningeal veins do not open directly into the sinuses, but indirectly through a series of ampullæ, termed venous lacunæ. These are found on either side of the superior sagittal sinus, especially near its middle portion, and are often invaginated by arachnoid granulations; they also exist near the transverse and straight sinuses. They communicate with the underlying cerebral veins, and also with the diploic and emissary veins.

The nerves of the cranial dura mater are filaments from the semilunar ganglion, from the ophthalmic, maxillary, mandibular, vagus, and hypoglossal nerves, and from the sympathetic.

The Spinal Dura Mater (dura mater spinalis; spinal dura) (767) forms a loose sheath around the medulla spinalis, and represents only the inner or meningeal layer of the cranial dura mater; the outer or endosteal layer ceases at the foramen magnum, its place being taken by the periosteum lining the vertebral canal. The spinal dura mater is separated from the arachnoid by a potential cavity, the subdural cavity; the two membranes are, in fact, in contact with each other, except where they are separated by a minute quantity of fluid, which serves to moisten the apposed surfaces. It is separated from the wall of the vertebral canal by a space, the epidural space, which contains a quantity of loose areolar tissue and a plexus of veins; the situation of these veins between the dura mater and the periosteum of the vertebræ corresponds therefore to that of the cranial sinuses between the meningeal and endosteal layers of the cranial dura mater. The spinal dura mater is attached to the circumference of the foramen magnum, and to the second and third cervical vertebræ; it is also connected to the posterior longitudinal ligament, especially near the lower end of the vertebral canal, by fibrous slips. The subdural cavity ends at the lower border of the second sacral vertebra; below this level the dura mater closely invests the filum terminale and descends to the back of the coccyx, where it blends with the periosteum. The sheath of dura mater is much larger than is necessary for the accommodation of its contents, and its size is greater in the cervical and lumbar regions than in the thoracic. On each side may be seen the double openings which transmit the two roots of the corresponding spinal nerve, the dura mater being continued in the form of tubular prolongations on them as they pass through the intervertebral foramina. These prolongations are short in the upper part of the vertebral column, but gradually become longer below, forming a number of tubes of fibrous membrane, which enclose the lower spinal nerves and are contained in the vertebral canal.

The medulla spinalis and its membranes.

Structure.—The spinal dura mater resembles in structure the meningeal or supporting layer of the cranial dura mater, consisting of white fibrous and elastic tissue arranged in bands or lamellæ which, for the most part, are parallel with one another and have a longitudinal arrangement. Its internal surface is smooth and covered by a layer of mesothelium. It is sparingly supplied with bloodvessels, and a few nerves have been traced into it.

The Arachnoid—The arachnoid is a delicate membrane enveloping the brain and medulla spinalis and lying between the pia mater internally and the dura mater externally; it is separated from the pia mater by the subarachnoid cavity, which is filled with cerebrospinal fluid.

The Cranial Part (arachnoidea encephali) of the arachnoid invests the brain loosely, and does not dip into the sulci between the gyri, nor into the fissures, with the exception of the longitudinal. On the upper surface of the brain the arachnoid is thin and transparent; at the base it is thicker, and slightly opaque toward the central part, where it extends across between the two temporal lobes in front of the pons, so as to leave a considerable interval between it and the brain.

The Spinal Part (arachnoidea spinalis) of the arachnoid is a thin, delicate, tubular membrane loosely investing the medulla spinalis. Above, it is continuous with the cranial arachnoid; below, it widens out and invests the cauda equina and the nerves proceeding from it. It is separated from the dura mater by the subdural space, but here and there this space is traversed by isolated connective-tissue trabeculæ, which are most numerous on the posterior surface of the medulla spinalis.

The arachnoid surrounds the cranial and spinal nerves, and encloses them in loose sheaths as far as their points of exit from the skull and vertebral canal.

Structure.—The arachnoid consists of bundles of white fibrous and elastic tissue intimately blended together. Its outer surface is covered with a layer of low cuboidal mesothelium. The inner surface and the trabeculæ are likewise covered by a somewhat low type of cuboidal mesothelium which in places are flattened to a pavement type. Vessels of considerable size, but few in number, and, according to Bochdalek, a rich plexus of nerves derived from the motor root of the trigeminal, the facial, and the accessory nerves, are found in the arachnoid.

The Subarachnoid Cavity (cavum subarachnoideale; subarachnoid space) is the interval between the arachnoid and pia mater. It is occupied by a spongy tissue consisting of trabeculæ of delicate connective tissue, and intercommunicating channels in which the subarachnoid fluid is contained. This cavity is small on the surface of the hemispheres of the brain; on the summit of each gyrus the pia mater and the arachnoid are in close contact; but in the sulci between the gyri, triangular spaces are left, in which the subarachnoid trabecular tissue is found, for the pia mater dips into the sulci, whereas the arachnoid bridges across them from gyrus to gyrus. At certain parts of the base of the brain, the arachnoid is separated from the pia mater by wide intervals, which communicate freely with each other and are named subarachnoid cisternæ; in these the subarachnoid tissue is less abundant.

Subarachnoid Cisternæ (cisternæ subarachnoidales)

—The cisterna cerebellomedullaris (cisterna magna) is triangular on sagittal section, and results from the arachnoid bridging over the interval between the medulla oblongata and the under surfaces of the hemispheres of the cerebellum; it is continuous with the subarachnoid cavity of the medulla spinalis at the level of the foramen magnum. The cisterna pontis is a considerable space on the ventral aspect of the pons. It contains the basilar artery, and is continuous behind with the subarachnoid cavity of the medulla spinalis, and with the cisterna cerebellomedullaris; and in front of the pons with the cisterna interpeduncularis. The cisterna interpeduncularis (cisterna basalis) is a wide cavity where the arachnoid extends across between the two temporal lobes. It encloses the cerebral peduncles and the structures contained in the interpeduncular fossa, and contains the arterial circle of Willis. In front, the cisterna interpeduncularis extends forward across the optic chiasma, forming the cisterna chiasmatis, and on to the upper surface of the corpus callosum, for the arachnoid stretches across from one cerebral hemisphere to the other immediately beneath the free border of the falx cerebri, and thus leaves a space in which the anterior cerebral arteries are contained. The cisterna fossæ cerebri lateralis is formed in front of either temporal lobe by the arachnoid bridging across the lateral fissure. This cavity contains the middle cerebral artery. The cisterna venæ magnæ cerebri occupies the interval between the splenium of the corpus callosum and the superior surface of the cerebellum; it extends between the layers of the tela chorioidea of the third ventricle and contains the great cerebral vein.

The subarachnoid cavity communicates with the general ventricular cavity of the brain by three openings; one, the foramen of Majendie, is in the middle line at the inferior part of the roof of the fourth ventricle; the other two are at the extremities of the lateral recesses of that ventricle, behind the upper roots of the glossopharyngeal nerves and are known as the foramina of Luschka. It is still somewhat uncertain whether these foramina are actual openings or merely modified areas of the inferior velum which permit the passage of the cerebrospinal fluid from the ventricle into the subarachnoid spaces as through a permeable membrane.

The spinal part of the subarachnoid cavity is a very wide interval, and is the largest at the lower part of the vertebral canal, where the arachnoid encloses the nerves which form the cauda equina. Above, it is continuous with the cranial subarachnoid cavity; below, it ends at the level of the lower border of the second sacral vertebra. It is partially divided by a longitudinal septum, the subarachnoid septum, which connects the arachnoid with the pia mater opposite the posterior median sulcus of the medulla spinalis, and forms a partition, incomplete and cribriform above, but more perfect in the thoracic region. The spinal subarachnoid cavity is further subdivided by the ligamentum denticulatum, which will be described with the pia mater.

The cerebrospinal fluid is a clear limpid fluid, having a saltish taste, and a slightly alkaline reaction. According to Lassaigne, it consists of 98.5 parts of water, the remaining 1.5 per cent. being solid matters, animal and saline. It varies in quantity, being most abundant in old persons, and is quickly secreted.

The Arachnoid Villi (granulationes arachnoideales; glandulæ Pacchioni; Pacchionian bodies) (769) are small, fleshy-looking elevations, usually collected into clusters of variable size, which are present upon the outer surface of the dura mater, in the vicinity of the superior sagittal sinus, and in some other situations. Upon laying open the sagittal sinus and the venous lacunæ on either side of it villi will be found protruding into its interior. They are not seen in infancy, and very rarely until the third year. They are usually found after the seventh year; and from this period they increase in number and size as age advances. They are not glandular in structure, but are enlarged normal villi of the arachnoid. As they grow they push the thinned dura mater before them, and cause absorption of the bone from pressure, and so produce the pits or depressions on the inner wall of the calvarium.

Structure.—An arachnoidal villus represents an invasion of the dura by the arachnoid membrane, the latter penetrates the dura in such a manner that the arachnoid mesothelial cells come to lie directly beneath the vascular endothelium of the great dural sinuses. It consists of the following parts: (1) In the interior is a core of subarachnoid tissue, continuous with the meshwork of the general subarachnoid tissue through a narrow pedicle, by which the villus is attached to the arachnoid. (2) Around this tissue is a layer of arachnoid membrane, limiting and enclosing the subarachnoid tissue. (3) Outside this is the thinned wall of the lacuna, which is separated from the arachnoid by a potential space which corresponds to and is continuous with the subdural cavity. (4) And finally, if the villus projects into the sagittal sinus, it will be covered by the greatly thinned wall of the sinus which may consist merely of endothelium. It will be seen, therefore, that fluid injected into the subarachnoid cavity will find its way into these villi, and it has been found experimentally that it passes from the villi into the venous sinuses into which they project.

The Pia Mater—The pia mater is a vascular membrane, consisting of a minute plexus of bloodvessels, held together by an extremely fine areolar tissue and covered by a reflexion of the mesothelial cells from the arachnoid trabeculæ. It is an incomplete membrane, absent probably at the foramen of Majendie and the two foramina of Luschka and perforated in a peculiar manner by all the bloodvessels as they enter or leave the nervous system. In the perivascular spaces, the pia apparently enters as a mesothelial lining of the outer surface of the space; a variable distance from the exterior these cells become unrecognizable and are apparently lacking, replaced by neuroglia elements. The inner walls of these perivascular spaces seem likewise covered for a certain distance by the mesothelial cells, reflected with the vessels from the arachnoid covering of these vascular channels as they traverse the subarachnoid spaces.

The Cranial Pia Mater (pia mater encephali; pia of the brain) invests the entire surface of the brain, dips between the cerebral gyri and cerebellar laminæ, and is invaginated to form the tela chorioidea of the third ventricle, and the choroid plexuses of the lateral and third ventricles (pages 840 and 841); as it passes over the roof of the fourth ventricle, it forms the tela chorioidea and the choroid plexuses of this ventricle. On the cerebellum the membrane is more delicate; the vessels from its deep surface are shorter, and its relations to the cortex are not so intimate.

Diagrammatic transverse section of the medulla spinalis and its membranes.