p668
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.
p669
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.
Below this, at the commencement of the Middle Horn it is between the commencing Corpus Fimbriatum of the Fornix and the Pulvinar of 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.
Which are here united through the Foramen Of Monro, and are then prolonged backward on the under surface of the Velum as the Choroid Plexuses of the Third Ventricle.
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.
Veins of the Velum Interpositum
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.
The Venae Galeni unite Posteriorly into a single trunk, the Vena Magna Galeni, which terminates in the Straight Sinus.