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Gray's Anatomy

Table Of Contents

Hemispheres Of The Brain | Temporal Lobe | Tentorial Surface | Island Of Reil | Olfactory Tract | Limbic Lobe | Cingulum | Olfactory Lobe | Cribriform Plate | Broca's Area | EnCephalon Base | Lamina Cinerea | Optic Commissure | InterPeduncular Space | Pituitary Body | Mammillary Body | Bundle Of Vicq D'Azyr | Tegmentum | Pons | Cerebral Peduncles | InfraTentorial | Cerebellum | Interior Of The Cerebrum | Corpus Callosum | Genu | Raphe | Forceps Minor | Forceps Posterior or Major | Splenium | Lateral Ventricles | Septum Lucidum | Third Ventricle | Cornu | Anterior Cornu | Posterior Cornu | Hippocampus Minor | Bulb Of The Posterior Horn | Middle Cornu | Corpus Striatum | Nucleus Caudatus | Nucleus Amygdalae | Nucleus Lenticularis | External Capsule | Claustrum | Putamen | Globus Pallidus | Anterior Limb | Internal Capsule | Fornix | Foramen Of Monro | Lyra | Crossed Anosmia | Fifth Ventricle | Septum Lucidum | Hippocampus Major | Pes Hippocampi | Uncus or Hook of the Hippocampal Gyrus | Choroid Plexus | Third Ventricle | Velum Interpositum | Veins of the Velum Interpositum


Arrangement Of The Cerebrum

p652

Each Hemisphere consists of a central cavity, the Lateral Ventricle, surrounded by thick and convoluted walls of Nervous Tissue.

The Temporal Lobe, sometimes called the Temporo-Sphenoidal Lobe, presents an outer and an inferior surface. The outer surface is sub-divided by two Fissures, named respectively the First and Second Temporal Sulci.

The First Temporal Sulcus is well marked, and runs from before backward through the Temporal Lobe parallel with, but some little distance below, the horizontal limb of the Fissure Of Sylvius, and hence is often termed the Parallel Sulcus.

The Second Temporal Sulcus takes the same direction as the First, but is situated at a lower level, and is often interrupted by one or more bridging convolutions.

These two Sulci subdivide this surface of the Temporal Lobe into Three Convolutions.

  1. The First or Superior Temporal Convolution is situated between the horizontal limb of the Fissure Of Sylvius and the First Temporal Sulcus and is continuous behind with the Supra-Marginal Convolution.

  2. The Second or Middle Temporal Convolution lies between the First and Second Temporal Sulci, and is continued behind into the Angular and Middle Occipital Convolutions.

  3. The Third or Inferior Temporal Convolution is placed below the Second Temporal Sulcus; it is connected Posteriorly with the Inferior Occipital Convolution.

It is also prolonged on to the under or Tentorial Surface (InfraTentorial) of the Temporal Lobe, where it is limited internally by the Third Temporal Sulcus, about to be described.

The Inferior or Tentorial surface presents two fissures: the Third Temporal Sulcus and the Collateral Fissure - the latter of which has already been described.

The Third Temporal Sulcus extends from near the Occipital Lobe behind, to near the Anterior extremity of the Temporal Lobe in front, but is, however, frequently subdivided by bridging Gyri.

Convolutions on the Inferior Surface Are

  1. The Fourth Temporal or SubCollateral Convolution (sometimes called the External Occipito-Temporal), situated between the Third Temporal Sulcus and the Collateral Fissure.

  2. The SubCalcarine Convolution or Lingual Lobule, lying between the Calcarine Fissure above and the Posterior part of the Collatweal Fissure below and continuous in front with the Hippocampal Convolution, the latter forming part of the Limbic Lobe.


p652

The Central Lobe or Island Of Reil lies deeply in the Sylvian Fissure, and can only be seen when the lips of that Fissure are widely separated, since it is overlapped and hidden by the Convolutions which bound the Fissure.

These convolutions are termed the Opercula of the Insula; they are separated from each other by the Three Limbs of the Sylvian Fissure, and named the Orcital, Frontal, Fronto-Parietal, and Temporal Opercula.

It is almost surrounded by a deep Limiting Sulcus, which separates it from the Frontal, Parietal, and Temporal Lobes.

When the Opercula have been removed, the Insula presents the form of a triangular eminence; its Apex is directed downward and inwards toward the Anterior Perforated Space, and is continuous in front with the Posterior Orbital Convolution and behind with the Hippocampal Convolution.

The Pre-Central Lobe is further subdivided by shallow Sulci into three or four short Convolutions, the Gyri Breves, while the Post-Central Lobe is named the Gyrus Longus and is often bifurcated at its upper extremity.

The Gray Matter of the Insula is continuous with that of the different Opercula, while its Mesial surface corresponds with the Lenticular Nucleus of the Corpus Striatum.


Limbic Lobe

p653

The term Limbic Lobe was introduced by Broca in 1878, under it he included Two Convolutions, the Callosal and Hippocampal; which together arch round the Corpus Callosum and the Hippocampal Fissure.

These he separated on the Morphological grounds that they are well developed in animals possessing a Keen Sense of Smell.

To the Lobe thus defined the following parts must be added:
The Laminae of the Septum Lucidum, together with the Fornix and its Fimbriae, which may be regarded as forming an Inner Arch or Deep Arch.

The Peduncles and Longitudinal Striae of the Corpus Callosum, together with the Gyrus Dentatus, which form a Middle Arch, while the Outer Arch is constituted by the Callosal and Hippocampal Convolutions: the first two arches are separated from each other by the Corpus Callosum. (View Image)


Convolutions of the Limbic Lobe

  1. The Callosal Convolution or Gyrus Cinguli 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 Gyrus Hippocampi.

    It is separated from the Marginal Convolution by the Calloso-Marginal Sulcus, from the Quadrate Lobe by the Post-Limbic Sulcus, and from the SubCalcarine Convolution by the Calcarine Fissure.

  2. The Hippocampal Convolution or Gyrus Hippocampi, is bounded above by the Hippocampal or Dentate Fissure, and below by the Anterior part of the Collateral Fissure.

    Behind it is continuous Superiorly through the Isthmus, with the Callosal Convolution, and Inferiorly with the SubCalcarine or Lingual Convolution. Its Anterior extremity is recurved in the form of a Hook, and is named the Uncus.

    p653

    Running in the subtance of the Callosal and Hippocampal Convolutions, and connecting them together, is a Tract of Arched Fibers, named the Cingulum.

    The outer root of the Olfactory Tract passes into the Anterior Extremity of the Hippocampal Convolution, and the Inner Root into the Commencement of the Callosal Convolution.

    So, these two Convolutions, with the addition of the Olfactory Tract, present a racquet-like appearance: the Olfactory Tract constituting the handle and the Two Convolutions the Circumference of the Blade.

  3. The Dentate Convolution or Dentate Fascia, is situated above the Gyrus Hippocampi, from which it is separated by the Hippocampal or Dentate Fissure.

    It is covered by the Fimbria, and is a narrow, Elongated Convolution, the Free surface of which presents a notched or toothed appearance, hence its name.

    Posteriorly it is Prolonged as a Delicate Lamina, the Fasciola Cinerea, around the Splenium of the Corpus Callosum, and becomes Continuous on the Upper Surface of that body with its Mesial and Lateral Longitudinal Striae.

    Anteriorly it is prolonged into the Notch produced by the Recurving of the Uncus, where it forms a sharp curve; from here it can be traced as a delicate band over the Uncus, on the outer surface of which it is lost.


The Olfactory Lobe
p654

The Olfactory Lobe is situated on the under surface of the Frontal Lobe. It is rudimentary in man and some other mammals.

But in vertebrates generally it is well developed, and consists of a distinct extension of the Cerebral Hemispheres, enclosing a portion of the Anterior Horn of the Lateral Ventricles.

In man it is long and slender and may be described as consisting of two parts: the Anterior and Posterior Olfactory Lobules.

The Anterior Olfactory is made of: the Olfactory Bulb; the Olfactory Tract; the Trigonum Olfactorium; and the Area Of Broca.

  1. The Olfactory Bulb is an oval mass of a reddish-gray color, which rests on the Cribriform Plate of the Ethmoid Bone, 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 Nose.

    p655

  2. The Olfactory Tract is a band of White Matter, triangular on section, the apex being directed upward. It lies in the Olfactory Sulcus on the under surface of the Frontal Lobe.

    Tracked backward, it is seen to divide into Two Roots, an Outer and an Inner. The Outer Root passes across the Outer part of the Anterior Perforated Space to the Nucleus Amygdalae and the Anterior part of the Gyrus Hippocampi.

    The Inner Root turns sharply inward, and ends partly in Broca's Area and partly in the Callosal Convolution; in other words, the Inner Root is continuous with one extremity and the Outer Root with the other extremity of the Limbic Lobe.

  3. The Trigonum Olfactorium is situated between the diverging roots of the Olfactory Tract, and is sometimes described as the Middle or Gray Root of the Tract. It is part of an area of Gray Matter, which forms the base of the Anterior Olfactory Lobule.

  4. The Area Of Broca and a third portion of no special significance, is situated external to the outer root of the Olfactory Tract.

    This area of Gray Matter is bounded internally and posteriorly by a fissure (Fissura Prima) which separates it from the Peduncles of the Corpus Callosum (Hippocampus Bands) and from the Posterior Olfactory Lobule. The Area Of Broca is continuous with the Gyrus Fornicatus.

The Posterior Olfactory Lobule or Anterior Perforated Space is marked off from the Anterior Lobule by the Fissura Prima, and is situated at the commencement of the Fissure Of Sylvius.

Internally, it is bounded by the Peduncles of the Corpus Callosum, and is continuous with the Lamina Cinerea. Posteriorly it is bounded by the Optic Tracts, and it is partially concealed by the Temporal Lobe which overlaps it.

It has received the name of Anterior Perforated Space from its being perforated by numerous openings.

Which transmit Blood Vessels to the Interior of the Brain, and it corresponds to the Under surface of the Lenticular Nucleus and part of the Claustrum.


Base Of The EnCephalon

The base of the Brain presents for examination the under surfaces of the Frontal and Temporal Lobes; the structures contained in the InterPeduncular Space, with the Cura Cerebri or Cerebral Peduncles; the under surfaces of the Pons, Cerebellum, and Medulla Oblongata; and the superficial Origins of the Cranial Nerves.

The various objects exposed to view in the Middle Line and on either Side of the Middle Line, are here arranged in the order they are met with From Before Backward.


In Middle Line
Sides Of Mid-Line
Longitudinal Fissure
Frontal Lobe
Rostrum & Peduncles
of Corpus Callosum

Olfactory Lobe
Fissure Of Sylvius
 
Lamina Cinerea
Optic Tracts
Optic Commissure
Crus Cerebri
Tuber Cinereum
Temporal Lobe
Infundibulum
Cerebellar Hemispheres
Pituitary Body
 
Corpora Albicantia
 
Posterior
Perforated Space

 
Pons
 
Medulla Oblongata
 

The Longitudinal Fissure partially separates the two Hemispheres from each other. It divides completely the Anterior portions of the two Frontal Lobes.

On raising the Cerebellum and Pons, it will be seen to separate completely the two Occipital Lobes; of these two portions of the Longitudinal Fissure, that which separates the Occipital Lobes is the longer.

The intermediate part of the Fissure is filled up by the great Transverse band of White Matter, the Corpus Callosum. In the Fissure between the two Frontal Lobes the Anterior Cerebral Arteries ascend on to the Corpus Callosum.

p656

The Corpus Callosum terminates at the base of the Brain by a concave margin, which is connected with the Tuber Cinereum through the intervention of a thin layer of Gray Substance, the Lamina Cinerea.

This may be exposed by gently raising and drawing back the Optic Commissure.

A white band may be observed on each side, passing backward from the under surface of the Corpus Callossum, across the Posterior Margin of the Anterior Perforated Space to the Hippocampal Gyrus, where each meets the Peduncles of the Corpus Callosum.

They may be traced upward around the Genu to become continuous with the Striae Longitudinales on its upper surface. Laterally, this portion of the Corpus Callosum extends into the Frontal Lobe.

The Lamina Cinerea is a thin layer of gray substance extending backward from the termination of the Corpus Callosum above the Optic Commissure to the Tuber Cinereum.

It is continuous on each side with the Gray Matter of the Anterior Perforated Space, and forms the Anterior part of the Inferior Boundary of the Third Ventricle.

The Optic Commissure is situated in the Middle Line, immediately in front of the Tuber Cinereum and below the Lamina Cinerea; that is to say, the Commissure is Superficial to the Lamina in the order of dissection when the base is uppermost.

It is the point of junction between the two Optic Tracts, and will be described with the Cranial Nerves.

Immediately behind the diverging Optic Tracts, and between them and the Peduncles of the Cerebrum is a lozenge-shaped interval.

The InterPeduncular Space, which is bounded behind by the Pons, and in which are found the following parts: the Tuber Cinereum, Infundibulum, Pituitary Body, Corpora Albicantia and the Posterior Perforated Space

The Tuber Cinereum is an eminence of Gray Matter, situated between the Optic Tracts, and extending from the Corpora Albicantia to the Optic Commissure, to which it is attached.

It is connected with the surrounding parts of the Cerebrum, forms part of the floor of the Third Ventricle, and is continuous with the gray substance in that cavity.

From the Middle of its Under surface a conical tubular process of Gray Matter, about two lines in length, is continued downward and forward to be attached to the Posterior Lobe of the Pituitary Body.

This is the Infundibulum, and its canal, which is funnel shapped, communicates with the Third Ventricle.

The Pituitary Body is a small, reddish-gray, vascular mass, weighing from five to ten grams, and of an oval form, situated in the Sella Turcica, where it is retained by a process of Dura Matter, named the Diaphragma Sellae.

This process covers in the Sella Turcica, and has a small hole in its center through which the Infundibulum passes.


Pituitary's Structure

The Pituitary Body is very Vascular and consists of two Lobes, separated from one another by a Fibrous Lamina.

Of these, the Anterior is the larger, of an oblong form, and somewhat concave behind, where it receives the Posterior Lobe, which is round.

The two Lobes differ both in development and structure. The Anterior Lobe, is of a dark, reddish-brown color, is developed from the Epiblast of the Buccal cavity, and resembles to a considerable extent, in microscopic structure, the Thyroid Body.

p656

It consists of a number of isolated vesicles and slightly convoluted tubules, lined by Epithelium and united together by a very Vascular connective tissue. The Epithelium is always columnar and occasionally Ciliated.

The Alveoli sometimes contain a colloid material, similar to that found in the Thyroid Body, and their walls are surrounded by a close network of Lymphatic and Capillary Blood Vessels.

The Posterior Lobe is developed as an outgrowth from the Embryonic Brain, and during Foetal life contains a cavity which communicates through the Infundibulum with the cavity of the Third Ventricle.

In the adult it becomes firmer and more solid, and consistts of a sponge like connectivve tissue arranged in the form of reticulating bundles, between which are branched cells, some of them containing pigment.

In the lower animals the two lobes are quite distinct, and it is only in the Mammalia that they become fused together.

p657

The Corpora Albicantia or Mammillaria are two small, round, white masses, each about the size of a pea, placed side by side immediately behind the Tuber Cinereum, and connected with each other across the Mesial Plane.

They are mainly formed by the Anterior Crura of the Fornix, which after descending to the base of the Brain, are twisted upon themselves to form loops, and constitute the white covering of the Corpra Albicantia.

A second fasciculus, the Bundle Of Vicq D'Azyr, converges from the Optic Thalamus, and enters the Anterior part of each body on its Dorso-Mesial surface.

They are composed externally of White substance, and internally of Gray Matter; the nerve cells of the Gray Matter are arranged in two sets, inner and outer, the cells of the former set being the smaller.

They are connected to the Tegmentum by a small bundle of fibers, the Peduncle of the Mammillary Body.

At an early period of foetal life they are blended together into one large mass, but become separated about the seventh month. In most vertebrates there is only one Median Corpus Albicans.

The Posterior Perforated Space (Pons Tarini) corresponds to a whitish-gray Fossa placed between the Corpora Albicantia in front, the Pons Varollii behind, and the Crus Cerebri on either side.

It forms the Posterior part of the floor of the Third Ventricle, and is perforated by numerous small orifices for the passage of the Postero-Median Ganglionic branches of the Posterior Cerebral and Posterior Communicating Arteries.

The Pons Varolii is situated immediately behind the two Crura of the Cerebrum. It consists of a broad band of white fibers, which pass transversely from one Cerebellar Hemisphere to the other; the band becoming narrower as it enters the Cerebellum.

In the Middle Line on its under surface a narrow groove runs from before backward and accomodates the Basilar Artery.

The Medulla Oblongata emerges from the Posterior border of the Pons Varolii; it is Pyramidal in form, and is continuous below with the Cervical portion of the Spinal Cord.

It is marked on its Ventral surface by a median fissure, continuous below with the Anterior median fissure of the Cord, and on either side by secondary fissures and columns, which will be described in the sequel.

The Frontal Lobe's under surface is seen on the Anterior part of the base of the Brain on either side of the Median Line. It is sometines called the Orbital Lobe.

p658

The Fissure Of Sylvius at the base of the Brain separates the Frontal from the Temporal Lobe, and lodges the Middle Cerebral Artery.

The Optic Tracts are well marked, flattened bands of Fibers, which run Obliquely across the Crus Cerebri on either side, and unite Anteriorly to form the Optic Commissure.

The Crura Cerebr (Peduncles of the Cerebrum) are two thick cylindrical bundles of White Matter, which appear in front of the Anterior border of the Pons, and diverge as they pass forward and outward to enter the under surface of each Hemisphere.

Each Crura is about three quarters of an inch in length, and is about the same in breadth Anteriorly, but somewhat less Posteriorly.

They are marked upon their surface with Longitudinal Striae, and each is crossed, just before entering the Hemisphere, by the Fourth Nerve and the Optic Tract, the latter of which is adherent by its upper surface to the Peduncle.

The Temporal Lobe's under surface is visible at the base of the Brain, on either side of the Crura and the structures contained in the InterPeduncular Space.

It is separated Anteriorly from the Frontal Lobe by the Fissure Of Sylvius, and behind is limited by the Anterior border of the Lateral Hemispheres of the Cerebellum.

The Fissures and Lobes on its surface have already been described (p652).

p659

The Hemispheres Of The Cerebellum are situated on either side of the Middle Line, and cover the Occipital Lobes of the Cerebrum, when viewed from the base.

The Cerebellum differs much in appearance from the rest of the EnCephalon, being of a darker color, while its Convolutions are smaller and narrower,and arranged like the leaves of a book, and hence called Folia.


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Continued_In 14-02




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