1. Forebrain, hemisphere, Sulci and gyri. Olfactory brain. Areas of the cerebral cortex.

2. basal nuclei and white matter of hemispheres. Lateral ventricles.

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

 

Lesson No 18

Theme 1. Forebrain, hemisphere, Sulci and gyri. Olfactory brain. Areas of the cerebral cortex.

The telencephalon includes the cerebral hemispheres with their cavities, the lateral ventricles.

Each cerebral hemisphere may be divided into three fundamental parts: the pallium, the rhinencephalon, and basal nuclei, also fornix and corpus callosum.

Cortex of the Cerebrum

A hemisphere is covered by cortex and has inferior, dorsolateral and medial surfaces. The anterior end of the hemisphere is named the frontal pole; the posterior, the occipital pole; and the anterior end of the temporal lobe, the temporal pole. The right and left hemispheres are separated medially by a deep cleft, named the longitudinal cerebral fissure. The surfaces of the hemispheres are molded into a number of irregular eminencies, named gyri or convolutions, and separated by furrows termed fissures and sulci. The hemispheres consist of 5 lobes: frontal, perietal, occipital, temporal and insula (located in depth of lateral sulcus).

On the dorsolateral surface of the hemisphere can be finding the Central sulcus [Rolandi] that separates frontal and parietal lobes. The Lateral sulcus [Sylvii] that separates temporal lobe from the frontal and parietal lobes. Parietooccipital sulcus passes between parietal and occipital lobes on the medial surface.

There are some sulci on the dorsolateral surface of the frontal lobe: precentral sulcus, superior frontal sulcus, and inferior frontal sulcus. They separate: precentral gyrus, superior frontal gyrus, middle frontal gyrus, and inferior frontal gyrus. The inferior frontal gyrus is divided into opercular, triangular and orbital parts by anterior and ascending rami.

The lateral surface of the parietal lobe is cleft by a well-marked furrow, the intraparietal sulcus and the postcentral sulcus. There are postcentral gyrus, superior parietal lobule and inferior parietal lobule. The last contains the supramarginal and angular gyri.

The temporal lobe is divided into superior, middle, and inferior gyri by the superior and middle temporal sulci. Three or four gyri will be seen springing from the depth of the hinder end of the lateral sulcus, these are named the transverse temporal gyri (Heschl).

The occipital lobe is small and pyramidal in shape. It is traversed by the transverse occipital sulci that border occipital gyri.

On the insula (it is located in depth of lateral sulcus) they distinguish circular and central sulci which border longi and breve gyri.

There are some sulci on the medial surface of the hemisphere: sulcus corporis callosi, cingulate sulcus, hippocampal sulcus, parietooccipital and calcarine sulcus. They separte corporus callosum from gyrus cinguli, superior frontal gyri, gyrus parahippocampalis, dentate gyrus, paracentral lobule (of Bets), precuneus and cuneus. Gyrus cinguli and gyrus parahippocampalis with isthmus form fornicate gyrus that it is the central part of the rhinencephalon.

On the inferior surface of the hemisphere can be finding: collateral sulcus, occipitotemporal sulcus, rhinal sulcus and orbital sulcus. They separate the lateral occipitotemporal gyrus, medial occipitotemporal gyrus, lingual gyrus, gyrus rectus and orbital gyri.

 

Cortical Fields

We distinguish some specific cortical fields that control motor, sensory, language and others functions. They can be divided into more general motor area, sensory area and also some specific sensory centres.

Motor area involved with the control of voluntary muscles. It is located in precentral gyrus and paracentral lobule (motor homunculus).

Centre of conjugate deviaton of the eyes to the opposite side is located in posterior part of the middle frontal gyrus.

Sensory area responsible for cutaneous and muscular sensations (temperature, pain and tactile), located in postcentral gyrus and paracentral lobule (sensory homunculus).

Centre of stereognosia is located in superior parietal lobule closely to intraparietal sulcus. It responsible for the body schema: the right can be differentiated from the left.

Auditory centre located in the transverse temporal gyri (Heschl). Function is interpretation of auditory sensations.

Visual cortical centre situated in calcarine sulcus. Function of the occipital lobe is connscious perception of vision.

Smell and tasting centre located in part of limbic system - uncus.

Centre of praxia is located supramarginal gyrus.

Language areas:

motor -

1.   Motor speech area (Broca's centre). Damage in the region of the lower frontal convolution produces motor aphasia.

2.   Writing (graphic) area located in middle frontal gyrus.

sensory -

3.   Auditory language centre (Wernicke`s area) located in superior temporal gyrus. It responsible for understanding of spoken language.

4.   Reading centre situated in angular gyrus (responsible also for reading, writing, counting and calculating).

 

 

 

The telencephalon includes: (1) the cerebral hemispheres with their cavities, the lateral ventricles; and (2) the pars optica hypothalami and the anterior portion of the third ventricle (already described under the diencephalon). As previously stated (see page 744), each cerebral hemisphere may be divided into three fundamental parts, viz., the rhinencephalon, the corpus striatum, and the neopallium. 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, whereas the neopallium undergoes great development and forms the chief part of the hemisphere.

 

The Cerebral Hemispheres.The cerebral hemispheres constitute the largest part of the brain, and, when viewed together from above, assume the form of an ovoid mass broader behind than in front, the greatest transverse diameter corresponding with a line connecting the two parietal eminences. The hemispheres are separated medially by a deep cleft, named the longitudinal cerebral fissure, and each possesses a central cavity, the lateral ventricle.

  The Longitudinal Cerebral Fissure (fissura cerebri longitudinalis; great longitudinal fissure) contains a sickle-shaped process of dura mater, the falx cerebri. It front and behind, the fissure extends from the upper to the under surfaces of the hemispheres and completely separates them, but its middle portion separates them for only about one-half of their vertical extent; for at this part they are connected across the middle line by a great central white commissure, the corpus callosum.

  In a median sagittal section (720) the cut corpus callosum presents the appearance of a broad, arched band. Its thick posterior end, termed the splenium, overlaps the mid-brain, but is separated from it by the tela chorioidea of the third ventricle and the pineal body. Its anterior curved end, termed the genu, gradually tapers into a thinner portion, the rostrum, which is continued downward and backward in front of the anterior commissure to join the lamina terminalis. Arching backward from immediately behind the anterior commissure to the under surface of the splenium is a second white band named the fornix: between this and the corpus callosum are the laminæ and cavity of the septum pellucidum.

 

Surfaces of the Cerebral Hemispheres.Each hemisphere presents three surfaces: lateral, medial, and inferior.

  The lateral surface is convex in adaptation to the concavity of the corresponding half of the vault of the cranium. The medial surface is flat and vertical, and is separated from that of the opposite hemisphere by the great longitudinal fissure and the falx cerebri. The inferior surface is of an irregular form, and may be divided into three areas: anterior, middle, and posterior. The anterior area, formed by the orbital surface of the frontal lobe, is concave, and rests on the roof of the orbit and nose; the middle area is convex, and consists of the under surface of the temporal lobe: it is adapted to the corresponding half of the middle cranial fossa. The posterior area is concave, directed medialward as well as downward, and is named the tentorial surface, since it rests upon the tentorium cerebelli, which intervenes between it and the upper surface of the cerebellum.

  These three surfaces are separated from each other by the following borders: (a) supero-medial, between the lateral and medial surfaces; (b) infero-lateral, between the lateral and inferior surfaces; the anterior part of this border separating the lateral from the orbital surface, is known as the superciliary border; (c) medial occipital, separating the medial and tentorial surfaces; and (d) medial orbital, separating the orbital from the medial surface. The anterior end of the hemisphere is named the frontal pole; the posterior, the occipital pole; and the anterior end of the temporal lobe, the temporal pole. About 5 cm. in front of the occipital pole on the infero-lateral border is an indentation or notch, named the preoccipital notch.

  The surfaces of the hemispheres are moulded into a number of irregular eminences, named gyri or convolutions, and separated by furrows termed fissures and sulci. The furrows are of two kinds, complete and incomplete. The former appear early in fetal life, are few in number, and are produced by infoldings of the entire thickness of the brain wall, and give rise to corresponding elevations in the interior of the ventricle. They comprise the hippocampal fissure, and parts of the calcarine and collateral fissures. The incomplete furrows are very numerous, and only indent the subjacent white substance, without producing any corresponding elevations in the ventricular cavity.

  The gyri and their intervening fissures and the sulci are fairly constant in their arrangement; at the same time they vary within certain limits, not only in different individuals, but on the two hemispheres of the same brain. The convoluted condition of the surface permits of a great increase of the gray matter without the sacrifice of much additional space. The number and extent of the gyri, as well as the depth of the intervening furrows, appear to bear a direct relation to the intellectual powers of the individual.

  Certain of the fissures and sulci are utilized for the purpose of dividing the hemisphere into lobes, and are therefore termed interlobular; included under this category are the lateral cerebral, parietoöccipital, calcarine, and collateral fissures, the central and cingulate sulci, and the sulcus circularis.

 




Lateral surface of left cerebral hemisphere, viewed from the side.

 

  The Lateral Cerebral Fissure (fissura cerebri lateralis [Sylvii]; fissure of Sylvius) (726) is a well-marked cleft on the inferior and lateral surfaces of the hemisphere, and consists of a short stem which divides into three rami. The stem is situated on the base of the brain, and commences in a depression at the lateral angle of the anterior perforated substance. From this point it extends between the anterior part of the temporal lobe and the orbital surface of the frontal lobe, and reaches the lateral surface of the hemisphere. Here it divides into three rami: an anterior horizontal, an anterior ascending, and a posterior. The anterior horizontal ramus passes foward for about 2.5 cm. into the inferior frontal gyrus, while the anterior ascending ramus extends upward into the same convolution for about an equal distance. The posterior ramus is the longest; it runs backward and slightly upward for about 7 cm., and ends by an upward inflexion in the parietal lobe.

  The Central Sulcus (sulcus centralis [Rolandi]; fissure of Rolando; central fissure) ( 725, 726) is situated about the middle of the lateral surface of the hemisphere, and begins in or near the longitudinal cerebral fissure, a little behind its mid-point. It runs sinuously downward and forward, and ends a little above the posterior ramus of the lateral fissure, and about 2.5 cm. behind the anterior ascending ramus of the same fissure. It described two chief curves: a superior genu with its concavity directed forward, and an inferior genu with its concavity directed backward. The central sulcus forms an angle opening forward of about 70 with the median plane.

 

The Parietoöccipital Fissure (fissura parietoöccipitalis).Only a small part of this fissure is seen on the lateral surface of the hemisphere, its chief part being on the medial surface.

  The lateral part of the parietoöccipital fissure (726) is situated about 5 cm. in front of the occipital pole of the hemisphere, and measures about 1.25 cm. in length.

 

  The Sulcus Circularis (circuminsular fissure) (731) is on the lower and lateral surfaces of the hemisphere: it surrounds the insula and separates it from the frontal, parietal, and temporal lobes.

 

Lobes of the Hemispheres.By means of these fissures and sulci, assisted by certain arbitrary lines, each hemisphere is divided into the following lobes: the frontal, the parietal, the temporal, the occipital, the limbic, and the insula.

 

Frontal Lobe (lobus frontalis).On the lateral surface of the hemisphere this lobe extends from the frontal pole to the central sulcus, the latter separating it from the parietal lobe. Below, it is limited by the posterior ramus of the lateral fissure, which intervenes between it and the central lobe. On the medial surface, it is separated from the cingulate gyrus by the cingulate sulcus; and on the inferior surface, it is bounded behind by the stem of the lateral fissure.

 




Principal fissures and lobes of the cerebrum viewed laterally.

 

  The lateral surface of the frontal lobe (726) is tranversed by three sulci which divide it into four gyri: the sulci are named the precentral, and the superior and inferior frontal; the gyri are the anterior central, and the superior, middle, and inferior frontal. The precentral sulcus runs parallel to the central sulcus, and is usually divided into an upper and a lower part; between it and the central sulcus is the anterior central gyrus. From the precentral sulcus, the superior and inferior frontal sulci run forward and downward, and divide the remainder of the lateral surface of the lobe into three parallel gyri, named, respectively the superior, middle, and inferior frontal gyri.

  The anterior central gyrus (gyrus centralis anterior; ascending frontal convolution; precentral gyre) is bounded in front by the precentral sulcus, behind by the central sulcus; it extends from the supero-medial border of the hemisphere to the posterior ramus of the lateral fissure.

  The superior frontal gyrus (gyrus frontalis superior; superfrontal gyre) is situated above the superior frontal sulcus and is continued on to the medial surface of the hemisphere. The portion on the lateral surface of the hemisphere is usually more or less completely subdivided into an upper and a lower part by an antero-posterior sulcus, the paramedial sulcus, which, however, is frequently interrupted by bridging gyri.

  The middle frontal gyrus (gyrus frontalis medius; medifrontal gyre), between the superior and inferior frontal sulci, is continuous with the anterior orbital gyrus on the inferior surface of the hemisphere; it is frequently subdivided into two by a horizontal sulcus, the medial frontal sulcus of Eberstaller, which ends anteriorly in a wide bifurcation.

  The inferior frontal gyrus (gyrus frontalis inferior; subfrontal gyre) lies below the inferior frontal sulcus, and extends forward from the lower part of the precentral sulcus; it is continuous with the lateral and posterior orbital gyri on the under surface of the lobe. It is subdivided by the anterior horizontal and ascending rami of the lateral fissure into three parts, viz., (1) the orbital part, below the anterior horizontal ramus of the fissure; (2) the triangular part (cap of Broca), between the ascending and horizontal rami; and (3) the basilar part, behind the anterior ascending ramus. The left inferior frontal gyrus is, as a rule, more highly developed than the right, and is named the gyrus of Broca, from the fact that Broca described it as the center for articulate speech.

  The inferior or orbital surface of the frontal lobe is concave, and rests on the orbital plate of the frontal bone (729). It is divided into four orbital gyri by a well-marked H-shaped orbital sulcus. These are named, from their position, the medial, anterior, lateral, and posterior orbital gyri. The medial orbital gyrus presents a well-marked antero-posterior sulcus, the olfactory sulcus, for the olfactory tract; the portion medial to this is named the straight gyrus, and is continuous with the superior frontal gyrus on the medial surface.

 

Temporal Lobe (lobus temporalis).The temporal lobe presents superior, lateral, and inferior surfaces.

  The superior surface forms the lower limit of the lateral fissure and overlaps the insula. On opening out the lateral fissure, three or four gyri will be seen springing from the depth of the hinder end of the fissure, and running obliquely forward and outward on the posterior part of the upper surface of the superior temporal gyrus; these are named the transverse temporal gyri (Heschl) (730).

  The lateral surface (726) is bounded above by the posterior ramus of the lateral fissure, and by the imaginary line continued backward from it; below, it is limited by the infero-lateral border of the hemisphere. It is divided into superior, middle, and inferior gyri by the superior and middle temporal sulci. The superior temporal sulcus runs from before backward across the temporal lobe, some little distance below, but parallel with, the posterior ramus of the lateral fissure; and hence it is often termed the parallel sulcus. The middle temporal sulcus takes the same direction as the superior, but is situated at a lower level, and is usually subdivided into two or more parts. The superior temporal gyrus lies between the posterior ramus of the lateral fissure and the superior temporal sulcus, and is continuous behind with the supramarginal and angular gyri. The middle temporal gyrus is placed between the superior and middle temporal sulci, and is joined posteriorly with the angular gyrus. The inferior temporal gyrus is placed below the middle temporal sulcus, and is connected behind with the inferior occipital gyrus; it also extends around the infero-lateral border on to the inferior surface of the temporal lobe, where it is limited by the inferior sulcus.

 




Section of brain showing upper surface of temporal lobe.

 

  The inferior surface is concave, and is continuous posteriorly with the tentorial surface of the occipital lobe. It is traversed by the inferior temporal sulcus, which extends from near the occipital pole behind, to within a short distance of the temporal pole in front, but is frequently subdivided by bridging gyri. Lateral to this fissure is the narrow tentorial part of the inferior temporal gyrus, and medial to it the fusiform gyrus, which extends from the occipital to the temporal pole; this gyrus is limited medially by the collateral fissure, which separates it from the lingual gyrus behind and from the hippocampal gyrus in front.

  The Insula (island of Reil; central lobe) (731) lies deeply in the lateral or Sylvian fissure, and can only be seen when the lips of that fissure are widely separated, since it is overlapped and hidden by the gyri which bound the fissure. These gyri are termed the opercula of the insula; they are separated from each other by the three rami of the lateral fissure, and are named the orbital, frontal, frontoparietal, and temporal opercula. The orbital operculum lies below the anterior horizontal ramus of the fissure, the frontal between this and the anterior ascending ramus, the parietal between the anterior ascending ramus and the upturned end of the posterior ramus, and the temporal below the posterior ramus. The frontal operculum is of small size in those cases where the anterior horizontal and ascending rami of the lateral fissure arise from a common stem. The insula is surrounded by a deep circular sulcus which separates it from the frontal, parietal, and temporal lobes. When the opercula have been removed, the insula is seen as a triangular eminence, the apex of which is directed toward the anterior perforated substance. It is divided into a larger anterior and a smaller posterior part by a deep sulcus, which runs backward and upward from the apex of the insula. The anterior part is subdivided by shallow sulci into three or four short gyri, while the posterior part is formed by one long gyrus, which is often bifurcated at its upper end. The cortical gray substance of the insula is continuous with that of the different opercula, while its deep surface corresponds with the lentiform nucleus of the corpus striatum.

 




The insula of the left side, exposed by removing the opercula.

 

 

The term limbic lobe was introduced by Broca, and under it he included the cingulate and hippocampal gyri, which together arch around the corpus callosum and the hippocampal fissure. These he separated on the morphological ground that they are well-developed in animals possessing a keen sense of smell (osmatic animals), such as the dog and fox. They were thus regarded as a part of the rhinencephalon, but it is now recognized that they belong to the neopallium; the cingulate gyrus is therefore sometimes described as a part of the frontal lobe, and the hippocampal as a part of the temporal lobe.

  

 

Median sagittal section of brain. Sulci and gyri of medial and lower surfaces of hemisphere

On the medial surface of the brain students should find:

        Cerebral hemispheres communicate each other by corpus callosum. 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




Lateral surface of left cerebral hemisphere, viewed from above.

       

        Above the corpus callosum there is fornix cerebri posteriorly passes into crura fornicis, anteriorly continue as a columna fornicis

        Lamina septi pellucidi is tightened between corpus callosum and columna fornicis

Thalamus positioned under fornix

        Backward from the thalamus there is quadrigeminal plate with two upper and two lower hillocks, the superior and inferior colliculi




Medial surface of left cerebral hemisphere.

 

 

VIDEO

 

  The medial part of the parietoöccipital fissure (727) runs downward and forward as a deep cleft on the medial surface of the hemisphere, and joins the calcarine fissure below and behind the posterior end of the corpus callosum. In most cases it contains a submerged gyrus.

  The Calcarine Fissure (fissura calcarina) (727) is on the medial surface of the hemisphere. It begins near the occipital pole in two converging rami, and runs forward to a point a little below the splenium of the corpus callosum, where it is joined at an acute angle by the medial part of the parietoöccipital fissure. The anterior part of this fissure gives rise to the prominence of the calcar avis in the posterior cornu of the lateral ventricle.

  The Cingulate Sulcus (sulcus cinguli; callosomarginal fissure) (727) is on the medial surface of the hemisphere; it begins below the anterior end of the corpus callosum and runs upward and forward nearly parallel to the rostrum of this body and, curving in front of the genu, is continued backward above the corpus callosum, and finally ascends to the supero-medial border of the hemisphere a short distance behind the upper end of the central sulcus. It separates the superior frontal from the cingulate gyrus.

  The Collateral Fissure (fissura collateralis) (727) is on the tentorial surface of the hemisphere and extends from near the occipital pole to within a short distance of the temporal pole. Behind, it lies below and lateral to the calcarine fissure, from which it is separated by the lingual gyrus; in front, it is situated between the hippocampal gyrus and the anterior part of the fusiform gyrus.

       

        Cerebral aqueduct [Silvii] passes under mesencephalic tectum. It communicates front with the 3d ventricle, back with the 4th ventricle

        The 4th ventricle is bordered upper and lower by the superior medullary velum and inferior medullary velum

        The floor of the IVth ventricle, the rhomboid fossa, occupies the dorsal surface of the medulla oblongata and the pons

        Cerebellum consists of two hemispheres which connected by vermis

Cerebral hemispheres are separated by median longitudinal fissura that passes to the corpus callosum. Cerebrum is separated from the cerebellum by transverse fissura.

  The medial surface of the frontal lobe is occupied by the medial part of the superior frontal gyrus (marginal gyrus) (727). It lies between the cingulate sulcus and the supero-medial margin of the hemisphere. The posterior part of this gyrus is sometimes marked off by a vertical sulcus, and is distinguished as the paracentral lobule, because it is continuous with the anterior and posterior central gyri.

 

Parietal Lobe (lobus parietalis).The parietal lobe is separated from the frontal lobe by the central sulcus, but its boundaries below and behind are not so definite. Posteriorly, it is limited by the parietoöccipital fissure, and by a line carried across the hemisphere from the end of this fissure toward the preoccipital notch. Below, it is separated from the temporal lobe by the posterior ramus of the lateral fissure, and by a line carried backward from it to meet the line passing downward to the preoccipital notch.

 




Orbital surface of left frontal lobe.

 

 

  The lateral surface of the parietal lobe (726) is cleft by a well-marked furrow, the intraparietal sulcus of Turner, which consists of an oblique and a horizontal portion. The oblique part is named the postcentral sulcus, and commences below, about midway between the lower end of the central sulcus and the upturned end of the lateral fissure. It runs upward and backward, parallel to the central sulcus, and is sometimes divided into an upper and a lower ramus. It forms the hinder limit of the posterior central gyrus.

  From about the middle of the postcentral sulcus, or from the upper end of its inferior ramus, the horizontal portion of the intraparietal sulcus is carried backward and slightly upward on the parietal lobe, and is prolonged, under the name of the occipital ramus, on to the occipital lobe, where it divides into two parts, which form nearly a right angle with the main stem and constitute the transverse occipital sulcus. The part of the parietal lobe above the horizontal portion of the intraparietal sulcus is named the superior parietal lobule; the part below, the inferior parietal lobule.

  The posterior central gyrus (gyrus centralis posterior; ascending parietal convolution; postcentral gyre) extends from the longitudinal fissure above to the posterior ramus of the lateral fissure below. It lies parallel with the anterior central gyrus, with which it is connected below, and also, sometimes, above, the central sulcus.

  The superior parietal lobule (lobulus parietalis superior) is bounded in front by the upper part of the postcentral sulcus, but is usually connected with the posterior central gyrus above the end of the sulcus; behind it is the lateral part of the parietoöccipital fissure, around the end of which it is joined to the occipital lobe by a curved gyrus, the arcus parietoöccipitalis; below, it is separated from the inferior parietal lobule by the horizontal portion of the intraparietal sulcus.

  The inferior parietal lobule (lobulus parietalis inferior; subparietal district or lobule) lies below the horizontal portion of the intraparietal sulcus, and behind the lower part of the postcentral sulcus. It is divided from before backward into two gyri. One, the supramarginal, arches over the upturned end of the lateral fissure; it is continuous in front with the postcentral gyrus, and behind with the superior temporal gyrus. The second, the angular, arches over the posterior end of the superior temporal sulcus, behind which it is continuous with the middle temporal gyrus.

  The medial surface of the parietal lobe (727) is bounded behind by the medial part of the parietoöccipital fissure; in front, by the posterior end of the cingulate sulcus; and below, it is separated from the cingulate gyrus by the subparietal sulcus. It is of small size, and consists of a square-shaped convolution, which is termed the precuneus or quadrate lobe.

 

Occipital Lobe (lobus occipitalis).The occipital lobe is small and pyramidal in shape; it presents three surfaces: lateral, medial, and tentorial.

  The lateral surface is limited in front by the lateral part of the parietoöccipital fissure, and by a line carried from the end of this fissure to the preoccipital notch; it is traversed by the transverse occipital and the lateral occipital sulci. The transverse occipital sulcus is continuous with the posterior end of the occipital ramus of the intraparietal sulcus, and runs across the upper part of the lobe, a short distance behind the parietoöccipital fissure. The lateral occipital sulcus extends from behind forward, and divides the lateral surface of the occipital lobe into a superior and an inferior gyrus, which are continuous in front with the parietal and temporal lobes. 125

  The medial surface of the occipital lobe is bounded in front by the medial part of the parietoöccipital fissure, and is traversed by the calcarine fissure, which subdivides it into the cuneus and the lingual gyrus. The cuneus is a wedge-shaped area between the calcarine fissure and the medial part of the parietoöccipital fissure. The lingual gyrus lies between the calcarine fissure and the posterior part of the collateral fissure; behind, it reaches the occipital pole; in front, it is continued on to the tentorial surface of the temporal lobe, and joins the hippocampal gyrus.

  The tentorial surface of the occipital lobe is limited in front by an imaginary transverse line through the preoccipital notch, and consists of the posterior part of the fusiform gyrus (occipitotemporal convolution) and the lower part of the lingual gyrus, which are separated from each other by the posterior segment of the collateral fissure.

The cingulate gyrus (gyrus cinguli; callosal convolution) is an arch-shaped convolution, lying in close relation to the superficial surface of the corpus callosum, from which it is separated by a slit-like fissure, the callosal fissure. It commences below the rostrum of the corpus callosum, curves around in front of the genu, extends along the upper surface of the body, and finally turns downward behind the splenium, where it is connected by a narrow isthmus with the hippocampal gyrus. It is separated from the medial part of the superior frontal gyrus by the cingulate sulcus, and from the precuneus by the subparietal sulcus.

  The hippocampal gyrus (gyrus hippocampi) is bounded above by the hippocampal fissure, and below by the anterior part of the collateral fissure. Behind, it is continuous superiorly, through the isthmus, with the cingulate gyrus and inferiorly with the lingual gyrus. Running in the substance of the cingulate and hippocampal gyri, and connecting them together, is a tract of arched fibers, named the cingulum (page 843). The anterior extremity of the hippocampal gyrus is recurved in the form of a hook (uncus), which is separated from the apex of the temporal lobe by a slight fissure, the incisura temporalis. Although superficially continuous with the hippocampal gyrus, the uncus forms morphologically a part of the rhinencephalon.




Scheme of rhinencephalon.

 

  The Hippocampal sulcus begins immediately behind the splenium of the corpus callosum, and runs forward between the hippocampal and dentate gyri to end in the uncus. It is a complete fissure (page 819), and gives rise to the prominence of the hippocampus in the inferior cornu of the lateral ventricle.

 

 

 

 

 

 

Theme 2. basal nuclei and white matter of hemispheres. Lateral ventricles.

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.




Scheme of rhinencephalon.

 

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.

 

  (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 fimbriathe 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 (733) 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 (720), 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 (720, 747, 748) 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 dAzyr) 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

 

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 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 Pachionis 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 (737).

  The caudate nucleus (nucleus caudatus; caudatum) (741, 742) 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 (743, 744).

 




Horizontal section of right cerebral hemisphere.

 

The lentiform nucleus (nucleus lentiformis; lenticular nucleus; lenticula) (741) 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 (742), 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 (744).

  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 (742, 744) 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) (741), 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) (745, 746) 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 (742) 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) (742) 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.

 




Dissection showing the course of the cerebrospinal fibers.

 

  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. 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 3. Pathways of the brain and spinal cord (associative, commissural, projective). Skin and its derivates. Mammary gland

 

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).

 

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 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 pyramidal and extrapyramidal.

To 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.

 

 

Theme 3. 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:

 

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.

2.     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.

3.    

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.

GENERAL COVER

Includes a skin and hypodermic stratum. Skin forms general bodies cover, which defends the organism from influence of external environment. It carries out regulation of warmly, metabolism, breathing, excretes the sweat and fat and is as depot of power resources. Skin is sensory organ and built from two layers:

1.     epidermis (superficial layer);

2.     dermis (deep layer, or proper skin), which consists of fibrous connective tissue, elastic and muscular fibers.

Blood and lymphatic capillaries, and terminal nervous bodies present in superior dermal layer. Lower stratum of dermis passes into hypodermic base where agglomeration of adipose cells is situated.

Skin color depends on pigment (melanin), which positioned in innermost layer of epidermis. Hair and nails derive from epidermis.

Hairs cover the skin (except palms, soles, transitional part of lips, head and preputium of penis, minor pudenda labia). Hair has a shaft and root. Last lies in deepness of skin and terminates by hair bulb, which provides growth of hair. Root of hair lies hair follicle where sebaceous gland opens and erector pili muscle attaches.

Nail is flattened elastic structures of a horny texture plate lying in connective tissue nail bed where it begins to growth. In nail there distinguish a nail root, nail body and free edge, which stick out form nail bed border, and also cover margin and lateral margin.

Skin Glands. Derivates of skin - sebaceous glands and sweat glands - are learned in histology course carefully.

Mammary gland is modified sweat gland, which lies on fascia of major pectoral muscle on level of the -V ribs. Body of breasts consists of lobes of mammary gland, which contain 15-20 lobules of mammary gland, dissevered one from one by bands of connective tissue. The smallest lobules consist of a cluster of rounded alveoli, which open into the smallest branches of the lactiferous ducts; these ducts communicate by to form larger ducts, and these end in a single canal, corresponding with one of the chief subdivisions of the gland. They converge toward the areola, beneath which they form dilatations sinus, which serve as reservoirs for the milk, and, at the base of the papillæ. Lobes are disposed relatively to mammary nipple radially, and the mammary ducts open on the top of nipple. Mammary areola positioned round mammary nipple covered areolar tubercles where open areolar glands.

 

Hypodermic stratum contains nervous and venous plexus, lymphatic vessels and hypodermic adipose tissue, which carry out function of thermal isolation and depot of energetic reserve.

Also a soft connective tissue there is situated.

 

Prepared by

Reminetskyy B.Y.