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 | 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
Interior Of The Cerebrum
If the upper part of either Hemisphere is removed with a knife, about half an inch above the level of the Corpus Callosum, its internal White Matter will be exposed.
It is an oval shaped center of white substance, surrounded on all sides by a narrow convoluted margin of Gray Matter, which presents an equal thickness in nearly every part.
This White central mass has been called the Centrum Ovale Minus. Its surface is studded with numerous minute red dots (Puncta Vasculosa), produced by the escape of Blood from divided blood vessels.
In inflammation or great congestion of the Brain these are very numerous and of a dark color.
If the remaining portion of one Hemisphere is slightly separated from the other, a broad band of white substance will be observed connecting them at the bottom of the Longitudinal Fissure; this is the Corpus Callosum.
The margins of the Hemispheres which overlap this portion of the Brain are called the Labii Cerebri.
Each Labium is part of the Callosal Convolution already described; and the space between it and the upper surface of the Corpus Callosum is termed the Callosal Fissure.
The Hemispheres should now be sliced off to a level with the upper surface of the Corpus Callosum, when the white substance of that structure will be seen connecting the two Hemispheres.
The large expanse of Medullary matter now exposed, surrounded by the convoluted margin of gray substance, called the Centrum Ovale Majus of Vieussens (aka Centrum SemiOvale: right and left).
The Corpus Callosum, a thick stratum of transversely directed nerve fibers, by which almost every part of one Hemisphere is connected with the corresponding part of the other Hemisphere.
The fibers of this body, when they pass from it into the Hemisphere radiate in various directions, to terminate in the Gray Matter of the Periphery.
It thus connects the two Hemispheres of the Brain, forming their great Transverse Commissure and at the same time roofs in the Lateral Ventricles.
The best conception of its size and form is obtained by making an anterior posterior vertical section, through the center of the Brain.
It is then seen to be a long, thick, irregularly flattened arch; in front taking a sharp bend, Genu, and dipping downward and backward to the base of the Brain by a reflected portion, the Rostrum.
Which is connected with the Lamina Cinerea; behind it terminates by a rounded end, which is folded over and is named the Splenium.
It is about four inches in length, and extends to within an inch and a half of the Anterior, and two inches and a half of the Posterior extremity of the Cerebrum.
It is somewhat broader behind than in front and is thicker at either end than in its central part, being thickest behind.
The Reflected Anterior portion of the Corpus Callosum, called the Beak or Rostrum becomes gradually thinner as it descends, and is attached by its lateral margins to the Frontal Lobes.
At its termination, in addition to joining the Lamina Cinerea, the Corpus Callosum gives off two bands of white substance, the Peduncles of the Corpus Callosum, already described (p656).
Posteriorly, the Corpus Callosum forms a thick rounded fold, called the Splenium or Pad, which is free for a distance as it curves forward and is then continuous by its under surface with the Fornix.
The Pad or Splenium overlaps the MesenCephalon, but is separated from it by the Pia Mater, which is prolonged forward to form the Velum Interpositum.
On its upper surface, the structure of the Corpus Callosum is very apparant, being collected into coarse transverse bundles.
Along the Middle Line is a longitudinal depression, the Raphe, bounded laterally by two or more slightly elevated longitudinal bands.
These are called the Striae Longitudinales or Nerves of Lancisi; still more externally, other Longitudinal Striae are seen, beneath the Callosal Convolutions.
These are the Striae Longitudinales Laterales or Tania Tectae. On each side of the middle line the under surface of the Corpus Callosum forms the roof of the Lateral Ventricles, while in the Mesial Plane it is continuous behind with the Fornix.
It is separated from it in front by the Septum Lucidum, which forms a vertical partition between the two Ventricles.
On each side the Fibers of the Corpus Callosum extend into the substance of the Hemispheres connecting them together.
The greater thickness of the two extremities of this Commissure is explained by the fact that the fibers from the Anterior and Posterior parts of each Hemisphere cannot pass directly across, but have to take a curved direction.
The part of the Corpus Callosum which curves forward on each side from the Genu into the Frontal Lobe and covers the front part of the Anterior Cornu of the Lateral Ventricle is called the Forceps Anterior or Minor.
The part which curves backwards from each side of the Splenium into the Occipital Lobe is known as the Forceps Posterior or Major.
Between these two parts on each side is the main body of the Lateral Ventricle. These are known as the Tapetum or Mat.
The Lateral Ventricles, two in number, right and left, are irregular cavities situated in the lower and inner parts of the Cerebral Hemisphere, one on either side of the Middle Line.
They are lined by a thin Diaphanous membrane, the Ependyma, which is covered by Ciliated Epithelium, and are moistened by a serous fluid, which, even in health, may be secreted in considerable amount.
Each Lateral Ventricle consists of a Central Cavity or Body, and Three Prolongations from it, termed Cornu:
The Central Cavity or Body of the Lateral Ventricle is situated in the lower part of the Parietal Lobe. It is an irregularly curved cavity, triangular in shape on transverse section, and presents a Roof, a Floor, and an Inner Wall.
Its Roof is formed by the under surface of the Corpus Callosum; its Inner Wall is the Septum Lucidum, which separates it from the opposite Ventricle and connects the under surface of the Corpus Callosum with the Fornix
Its Floor is formed by the following parts, enumerated in their order of position from before backward: the Caudate Nucleus of the Corpus Striatum, Taenia Semicircularis, Optic Thalamus, Chorid Plexus, one-half of the Fornix and its Posterior Pillar.
The Anterior Cornu passes forward and outward with a slight inclination downward, from the Foramen Of Monroe into the Frontal Lobe, curving round the anterior extremity of the Caudate Nucleus.
It is bounded above by the Corpus Callosum, and below by the upper surface of its reflected portion the Rostrum.
It is bounded laterally by the Anterior portion of the Septum Lucidum, and externally by the head of the Caudate Nucleus of the Corpus Striatum. Its Apex reaches the Posterior surface of the Genu of the Corpus Callosum.
The Posterior Cornu curves backward into the substance of the Occipital Lobe, its direction being backward and outward, and then inward; its concavity is therefore directed inward.
Its Roof is formed by the fibers of the Corpus Callosum passing to the Temporal and Occipital Lobes. On its inner wall is seen a Longitudinal Eminence, which is in an involution of the Ventricular all produced by the Calcarine Sulcus; this is called the Hippocampus Minor, or Calcar Avis.
Just above this the Forceps Major of the Corpus Callosum, sweeping round to enter the Occipital Lobe, causes another Projection, which is known as the Bulb Of The Posterior Horn.
The Hippocampus Minor and Bulb of the Posterior Horn are extremely variable in their degree of development, being in some casses ill defined, while in others they are unusually prominent.
Between the Middle and Posterior Cornu is a triangular area, called the Trigonum Ventriculi (see Descending Horn).
The Middle or Descending Cornu, the largest of the three, traverses the Temporal Lobe, forming in its course a remarkable curve round the back of the Optic Thalamus.
It passes at first backward, outward, and downward, and then curves round the Crus Cerebri, forward and inward, to within an inch of the Apex of the Temporal Lobe its direction being fairly well indicated on the surface of the Brain by that of the Parallel Sulcus.
Its upper boundary,or Roof, is formed chiefly by the under surface of the Tapetum of the Corpus Callosum, but the Tail of the Nucleus Caudatus of the Corpus Striatum and the Tania Semicircularis are also prolonged into it.
And extend forward in the roof of the Descending Horn to its extremity where they end in a mass of Gray Matter, the Amygdaloid Nucleus; this Nucleus is merely a localized thickening of the adjacent Gray Cortex.
Its lower boundary, or Floor, presents for examination the following parts: the Hippocampus Major, Pes Hippocampi, Eminentia Collateralis, Corpus Fimbriatum, prolonged from the Posterior Pillar of the Fornix, and the Chorid Plexus.
Along the mesial aspect of the Descending Cornu there is a cleft-like opening, which is the lower part of the Transverse Fissure, through which the Chorid Plexus of the Pia Mater is invaginated into the Ventricle, but covered by the Ependyma, which is pushed in before it.
The Corpus Striatum received its name from the striped appearance which its section presents, in consequence of diverging White Fibers being mixed with the Gray Matter which forms the greater part of its substance.
The larger portion of this body is embedded in the White Matter of the Hemisphere and is therefore external to the Ventricle.
It is termed the Extra-Ventricular portion or the Nucleus Lenticularis; a part, however, is visible in the Ventricle and its Anterior Cornu: this is the Intra-Ventricular portion, or the Nucleus Caudatus.
Its narrow end is directed outward and backward on the outer side of the Optic Thalamus; it is continued downward into the roof of the Descending Cornu, where it terminates in the Nucleus Amygdalae, a collection of Gray Matter in 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 Extra-Ventricular portion, in the greater part of its extent, by a Lamina of White Matter, which is called the Internal Capsule, but the two portions of the Corpus Striatum are united in front.
The Nucleus Lenticularis, or Extra-Ventricular portion of the Corpus Striatum, is only seen in sections of the Hemisphere.
When divided horizontally, it presents to some extent, the appearance of a biconvex lens, while a vertical transverse section of it gives a somewhat triangular outline. It does not extend as far forward or backward as the Nucleus Caudatus.
It is bounded externally by a Lamina of White Matter called the External Capsule, on the outer surface of which is a thin layer of matter termed the Claustrum.
The Claustrum presents ridges and furrows on its outer surface, corresponding to the Convolutions and Sulci of the island of Reil, from which it is separated by a thin white Lamina.
Upon making a transverse vertical section through the middle of the Nucleus Lenticularis it is seen to present two white lines, parallel with its lateral border, which divide it up into three zones, of which the outer and largest is of a reddish color, and is known as the Putamen.
While the two inner are paler and of a yellowish tint, and are termed the Globus Pallidus. All three zones are marked by fine radiating white fibers, which are most distinct in the Putamen. (View Image)
The Gray Matter of the Corpus Striatum is transversed by nerve fibers, some of which are believed to originate in it.
The cells are multipolar, both large and small; those of the Lenticular Nucleus containing yellow pigment.
The Internal Capsule is formed by fibers of the Crusta of the Crus Cerebri, supplemented by fibers derived from the Corpus Striatum and Optic Thalamus on each side.
In horizontal section it is seen to be somewhat abruptly curved, with its convexity inward; the prominence of the curve is called the Genu, and projects between the Caudate Nucleus and the Optic Thalamus.
The portion in front of the Genu is termed the Anterior Limb, and separates the Lenticular from the Caudatus Nucleus; the portion behind the Genu is the Posterior Limb, and separates the Lenticular Nucleus from the Optic Thalamus.
The Internal Capsule is composed largely of fibers, which derived from the Crusta of the Crus Cerebri, are continued through it to the Cortex of the Cerebral Hemispheres.
The fibers of the Anterior Limb passing to the Frontal region; those from the Genu and the anterior two-thirds of the Posterior Limb pass to the Rolandic Area of the Cortex, while those in the hindermost third of the Same Limb pass to the Temporal-Occipital Region.
In addition to these, there are fibers which terminate in the Corpus Striatum and the Optic Thalamus; and other fibers derived from the Gray Matter of these two bodies, from the SubThalamic region, and from the Hemisphere of the opposite side through the Corpus Callosum, which pass through the Internal Capsule to the Cerebral Cortex.
The External Capsule is a Lamina of White Matter, situated on the outer side of the Lenticular Nucleus, between it and the Claustrum, and is continuous with the Internal Capsule below and behind the Lenticular Nucleus.
It is made up of fibers derived partly from the anterior white commissure and partly from the SubThalamic region.
The Claustrum is a thin layer of Gray Matter, situated on the outer surface of the External Capsule. On transverse section it is seen to be triangular, with its apex directed upward and its base downward.
Its inner surface, which is continuous to the Outer Capsule, is smooth but its outer surface presents ridges and furrows which correspond with the convolutions and Sulci of the Island of Reil, with which it is in close relationship.
The Claustrum is regarded as a detached portion of the Gray Matter of the Island Of Reil, from which it is separated by a layer of white fibers, the Capsula Extrema or Band Of Baillarger.
Its cells are small and spindle shaped, and contain yellow pigment; they are similar to those found in the deepest layers of the Cortex.
The Taenia Semicircularis is a narrow, whitish band of Medullary substance, situated in the depression between the Caudate Nucleus and the Optic Thalamus.
Anteriorly its fibers are partly continued into the anterior Pillar of the Fornix; some, however, pass over the Anterior Commissure to the Gray Matter between the Caudate Nucleus and Septum Lucidum, while others penetrate the Caudate Nucleus.
Posteriorly it is continued into the roof of the middle or Descending Horn of the Lateral Ventricle, at the extremity of which it enters the Nucleus Amygdalae, an oval mass of Gray Matter, situated in the roof of the lower extremity of the Descending Horn.
Like the Corpus Striatum, it is formed by a localized thickening of the Gray Matter of the Cortex Cerebri. Superficial to it is a large vein, Vena Corpus Striati, which receives numerous small veins from the surface of the Corpus Striatum and Optic Thalamus.
It runs forward and passes through the Foramen of Monro to join the corresponding Vena Galeni. On the surface of the vein of the Corpus Striatum is a narrow band of white fibers, named the Lamina Cornea.
The Fornix is a longitudinal, arch shaped lamella of White Matter, situated beneath the Corpus Callosum, with which it is continuous behind, but separated in front by the Septum Lucidum.
It may be described as consisting of two symmetrical halves, 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 two anterior, separated parts are called the Anterior Pillars (Columnae Fornicis); the intermediate united portions constitute the body of the Fornix; and the Posterior parts, which are also separated from each other, are called the Posterior Pillars (Crura Fornicis).
The body of the Fornix is triangular, narrow in front, broad behind. Its upper surface is connected, in the Middle Line, to the Septum Lucidum in front, and the Corpus Callosum behind; laterally this surface forms part of the floor of each Lateral Ventricle.
Its under surface rests upon the Velum Interpositum, which separates it from the Third Ventricle, and the inner portion of the upper surface of the Optic Thalami. Its outer edge, on each side, is free, and is connected with the Choroid Plexuses.
The Anterior Pillars arch downward toward the base of the Brain, separated from each other by a narrow interval.
They are composed of White Fibers, which descend through the Gray Matter in the Lateral wall of the Third Ventricle, and are placed immediately behind the Anterior Commissure.
At the base of the Brain, each Pillar becomes twisted upon itself to form a loop, somewhat resembling the figure 8. The lowest part of the loop constitutes the White Matter of the corresponding Corpus Albicans.
From which the fibers can apparently be traced upward and backward, as the Bundle of Vicq d' Azur, into the substance of the corresponding Optic Thalamus.
It must be stated, however, that there is probably no direct continuity between this bundle and the Anterior Pillar of the Fornix - the latter possibly ending in the Gray Matter of the Corpus Albicans.
The Anterior Crura of the Fornix are joined in their course by the Peduncles of the Pineal Gland and the superficial fibers of the Taenia Semicirularis, and receive fibers from the Septum Lucidum.
Zuckerkand describes an Olfactory Fasciculus, which becomes detached from the main portion of the Anterior Pillar 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 inner root of the Olfactory Tract; the other, the Peduncle of the Corpus Callosum, and through it reaching the Hippocampal Convolution.
Between the Anterior Pillars of the Fornix and the Anterior extremity of the Optic Thalamus, an oval aperture is seen on each side; this is the Foramen Of Monro.
Through this Foramen the Lateral Ventricles communicate with the Third Ventricle,and consequently with each other; through it also the two Choroid Plexuses become joined with each other across the Middle Line.
The boundaries of the opening are: above and in front, the Anterior Pillars of the Fornix; behind, the Anterior extremity of the Optic Thalamus.
The Posterior Pillars are the backward prolongnations of the two halves of the Body of the Fornix. They are flattened bands, and, at their commencement, are intimately connected by their upper surfaces with the Corpus Callosum.
Diverging from one another, each curves round the posterior extremity of the Optic Thalamus, and then passes downward & forward into the Descending Horn of the Lateral Ventricle.
Here it lies along the concavity of the Hippocampus Major, on the surface of which some of its fibers are spread out, while the remainder are continued, as the Corpus Fimbriatum or Taenia Hippocampi, into the Hook or Uncus of the Hippcampal Convolution.
Upon examining the under surface of the Fornix, between its diverging Posterior Pillars, a triangular portion of the under surface of the Corpus Callosum may be seen.
On it are a number of curved or oblique lines passing between the two Pillars of the Fornix. This portion has been termed the Lyra, from the fancied resemblance it bears to a Harp.
The Anterior Commissure is a bundle of white fibers, placed in front of the Anterior Pillars of the Fornix, and appears to connect together the Corpora Striata.
On transverse section it is oval in shape, its long diameter being vertical in direction and is about one-fifth of an inch.
Its fibers can be traced backward and downward through the Globus Pallidus and below the Putamen on each side into the substance of the Temporal Lobe.
It serves to connect the two Temporal Lobes, but it also contains fibers from the Olfactory Tract of the opposite side.
The Decussation of which in the Anterior Commissure may serve to explain the condition of Crossed Anosmia - where there is a lesion in one Temporal Lobe with a loss of Smell in the Olfactory area of the opposite side of the Nose.
The Septum Lucidum is a thin, double, vertically placed partition, which forms the interior boundary of the body and Anterior Horn of the Lateral Ventricle.
It consists of two distinct laminae, separated in part of their extent by a narrow chink or interval, called the Fifth Ventricle.
It is a thin, semitransparent Septum, 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 outer surface of each Lamina is directed toward the Lateral Ventricle, and is covered by the Ependyma of that cavity, while its mesial surface bounds the cavity of the Fifth Ventricle.
The Fifth Ventricle was originally a part of the great Longitudinal 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 is therefore formed by the Median Wall of the Hemisphere, and consists of an internal layer of Gray Matter, derived from the Gray Matter of the Cortex, and an external layer of White Matter continuous with the White Matter of the Cerebral Hemispheres.
The Fifth Ventricle Differs from the other Ventricles of the Brain, it is Not developed from the cavity of the Cerebraal Vesicles, it is Not lined by Ciliated Epithelium but by Altered Pia Mater, and it does Not communicate with the general Ventricular Cavity; further the Fluid it contains is of the nature of Lymph.
Descending Horn Floor
The Hippocampus Major or Cornu Ammonis, is a white eminence, about two inches in length, of a curved elongated form, extending throughout the entire length of the floor of the Descending Horn of the Lateral Ventricle.
At its lower extremity it becomes enlarged, and presenting two or three rounded elevations with intervening depressions it resembles the paw of an animal, and is called the Pes Hippocampi.
If a transverse section is made through the Hippocampus Major, it will be seen that this Eminence is produced by the folding of the Cortex of the Brain to form the Dentate (Hippocampal) Sulcus.
To the outer side and parallel with the Hippocampus Major an elongated eminaence, The Eminentia Collateralis, is frequently recognized.
It corresponds with the middle part of the Collateral Fissure, and its size depends on the direction and depth of this fissure.
The main mass of the Hippocampus Major consists of Gray Matter, but on its Ventricular surface is a thin layer of White Matter, known as the Alveus, which is continuous with the Corpus Fimbriatum of the Fornix and covered by the Ependyma of the Ventricle.
If the Alveus and Superficial Strata of Gray Matter be reflected from the surface of the Hippocampus by an incision carried along its convexity, the core of the Hippocampus, presents in many cases a corrugated or crimped appearance.
The Corpus Fimbriatum or Fimbria (Taenia Hippocampi), has already been mentioned as a part of the Posterior Pillar of the Fornix. It consists of a narrow White Band, which is placed immediately below the Choroid Plexus.
It is attached by its deep surface to the White Matter (Alveus) of the Hippocampus Major as it courses through the descending Cornu of the Lateral Ventricle.
It can be traced as far as the Uncus or Hook of the Hippocampal Gyrus. Its inner margin is free, and rests upon the Dentate Convolution, from which it is separated by a slit like fissure, the Fimbrio-Dentate Fissure.
Its outer margin is attenuated and irregular and forms the line along which the Ependyma is reflected over the Choroid Plexus as the latter is invaginated through the inferior part of the Transverse Fissure.
When the Choroid Plexus is pulled away it carries the Ependyma with it and the Descending Horn opens on to the surface of the Brain through the Transverse Fissure.
If now the inner border of the Corpus Fimbriatum be raised, a notched band of Gray Matter, the Gyrus Dentatus, will be exposed; this has already been described as forming part of the Limbic Lobe (p.653).
The Choroid Plexus is a highly vascular fringe-like structure which is situated partly in the body and partly in the Descending Cornu of the Lateral Ventricle. It will be desirable to consider these two portions separately, in order to get a just conception of how they are formed.
The portion in the body of the Ventricle is the Vascular, fringed border of a triangular fold of Pia Mater, the Velum Interpositum, which lies on the under surface of the Fornix and forms the Roof of the The Third Ventricle.
It will be remembered that the developing Brain Vesicles are covered by Pia Mater.
As the Prolongation from the First Vesicle, which is to form the Cerebral Hemispheres, increases in size, it grows backward and downward and covers the other Vesicles, with the result that the Pia Mater covering the Hemisphere comes in contact with that covering the upper surface of the Second Vesicle.
A portion of the two layers which are in contact, forms the Velum Interpositum. Immediately above is the body of the Fornix, which is formed by the fusion of the Cerebral Hemispheres in the middle line and below is the Cavity of the Second Vesicle (The Third Ventricle).
With the Optic Thalamus on either side. Just beyond the free Lateral border of the Fornix, between it and the Taenia Semicircularis, is a portion of the First Cerebral Vesicle, which is Not developed into Nervous Matter, but is made up only of Ependyma covered by Pia Mater.
The Vessels of this portion of the highly Vascular Pia Mater become dilated and prolonged, amd grow into the Ventricle, pushing the Ependyma before them, and forming an irregular Congeries of Vessels.
Apparently incroaching on the Cavity of tha Lateral Ventricle, but in reality being external to it, because they are separated from it by the lining membrane of the cavity, the Ependyma. This vascular structure is the Choroid Plexus of the Body of the Ventricle.
The part of the Choroid Plexus seen in the Descending Cornu is formed in exactly the same way, by an ingrowth of the vessels of the Pia Mater into the Cavity, pushing the Ependyma before it.
At a part of the wall of the Horn where there is a similar Absence of Nervous Tissue and where it consists simply of Pia Mater and Ependyma in close contact.
This portion lies between the Corpus Fimbriatum in the Floor and the Taenia Semicircularis in the Roof of the Descending Horn.
This area, Destitute of Nervous Matter, is continuous with the area in the Body of the Ventricle, from which the Choroid Plexus of this region originated, and in it the Vessels of its Pia Mater increase and invaginating the Ependyma, appear in the Descending Horn as its Choroid Plexus.
In the Body of the Ventricle the Choroid Plexus is really the Vascular Fringed Margin of the Velum; beyond the Posterior margin of the Velum the Plexus of the Descending Horn is continuous with the Pia Mater on the surface of the Gyrus Hippocampi.
The two portions of the Plexus are, however, directly continuous with each other. The Gap or Clef through which the invagination of the Pia Mater takes place is called the Transverse Fissure.
In front, the Choroid Plexus of the Lateral Ventricle is small and tapering, and communicates with that of the oposite side through the Foramen Of Monro.
In structure it consists of minute and highly vascular villous processes, containing an Afferent and Efferent vessel, and covered by a single layer of flattened Epithelium, the cells of which often contain a yellowish fat molecule.
The Anterior Choroidal Artery is derived from the Internal Carotid, and enters the Ventricle at the extremity of the Descending Cornu, and, after ramifying in the Plexus, sends branches into the adjacent parts of the Brain.
The Posterior Choroidal Arteries, one or two in number, are derived from the Posterior Cerebral Artery, and reach the plexus by passing forward under the Splenium of the Corpus Callosum.
The veins of the Choroid Plexus unite to form a prominent vein which courses from behind forward to the Foramen Of Monro, and joins with the vein of the Corpus Striatum to form the corresponding Vein Of Galen.
The Transverse Fissure is not a real fissure or cleft, because it is filled by the invagination of the Pia Mater, forming the Velum Interpositum and the Choroid Plexus, covered by the lining of the Ventricular cavities.
If this involution of Pia Mater is pulled out, the Ventricular Lining will necessarily be torn away with it and a cleft-like space will be left on either side, extending from the Foramen Of Monro to the bottom of the Descending Horn of the Lateral Ventricle.
The upper part of this cleft, that is to say, the part nearest the Foramen Of Monro, is between the Lateral Border of the Body of the Fornix and the Optic Thalamus.
Lower still in the Descending Horn, between the Corpus Fimbriatum on the floor and the Taenia Semicircularis in the roof of the Cornu.
Posteriorly the Transverse Fissure opens between the Splenium of the Corpus Callosum above, and the Corpora Quadrigemina and Pineal Gland below. Through the fissure the Venae Galeni emerge to join the Straight Sinus.
The Velum Interpositum or Tela Choroidea Superior is a Vascular Membrane, and is a Prolongation of the Pia Mater into the Interior of the Brain through the Middle part of the Transverse Fissure.
It is of a triangular form,and separates the under surface of the body and Posterior Pillars of the Fornix from the cavity of the Third Vntricle.
Laterally it covers the inner part of the upper surface of the Optic Thalamus.
Its posterior border or base lies beneath the Splenium of the Corpus Callosum above, and the Optic Thalamus, the Corpora Quadrigemina, and Pineal Body below.
Its Anterior extremity, or Apex ends just behind the Anterior Pillars of the Fornix, where it is connected with the Anterior extremities of the Choroid Plexuses.
In front, these Plexuses of the Third Ventricle lie close to the Middle Line, but diverge from each other behind.
The Lateral Margins of the Velum Interpositum form the Choroid Plexuses of the Lateral Ventricles. It is supplied by the Anterior and Posterior Choroidal Arteries already described.
The Venae Galeni, two in number, run between its layers, each being formed by the union of the Vein of the Corpus Striatum with the Choroid Vein.
Veins of the Velum Interpositum
The Venae Galeni unite Posteriorly into a single trunk, the Vena Magna Galeni, which terminates in the Straight Sinus.