The fornix is a triangular layer of longitudinal white fibres, with its apex directed forward and its base attached behind to the posterior extremity of tile corpus callosum. Its anterior extremity divides into two rounded cords, one on each side, which descend in front of the thalamii optici to the base of the brain, and obliquely- back-ward to the cor pora albicautia in the interpecluncular space, and from these latter are tracts of white fibres passing outward to the internal capsules.
The apex of the fornix is continued forward to the gnu of the corpus callosum the median line, by a thin membrane, the. septum lucidum. This membrane fills up the interval between the fornix and the germ of the corpus callosum dividing the lateral ventricles from each other. The fibres of the septum lucidum pass in a longitudinal direction from the fornix to the corpus callos um. The septum is composed of two layers, side by side, enclosing a vertical slit, the fifth ventricle. The posterior angles of the fornix are -extended outward, backward, and downward along the broad groove behind. the arch or germ of -the internal capsule, to be continuous with the layer of white fibres which cover the floor of the middle aud posterior cornua. These extensions are called its posterior pillars, each apparently dividing to form the hippocampus major aud minor of the same side. The edge of the fornix is called the corpus fimbriatuna, and rests upon the thalamus optieus. It extends behind the posterior extremity of the thalamus downward along the internal margin of the floor of the middle cornu and terminates in the apex of the ungual convolution.
The choroid plexus is a capillary network lying upon the thalamus and attached to the edge of the velum interpositum, from which it receives the blood vessels that supply it. It extends from the foramen of Monro in front over the thalamus opticus into the middle cornu of the lateral ventri cle. It lies parallel with the fissure of Bichat and the corpus fimbriatum to their lower extremities, which terminate at the apex of the middle cornu. The choroid plexuses of the lateral ventricles and those of the third ventricle are con nected through the foramina of Monro.
The fornix forms the roof the third ventricle. If it is divided transversely and its parts reflected, its anterior pillars will be seen descending in front as two rounded cords; and upon the under surface of its posterior part, crossing in lateral direction, are several transverse strife called the lyra.
These latter are fibres of the corpus callosum crossing in a transverse direction beneath the attachment of the posterior margin of the fornix to that of the corpus callosum.
Beneath the foruix, and exposed by its removal, is a pro cess of pia mater extending into the ventricles of the brain, and called the velum interpositum on account of its being interposed between the fornix and the third ventricle, which is beneath it. The velum interpositum is triangular in shape, corresponding to the under surface of the fornis, with which it is in contact. The velum contains numerous blood vessels which supply the choroid plexuses. Near its centre, side by side, are two large veins, the vena Galeni, which receive blood from the venue corpora striata and convey it to the straight sinus, at the junction of the falx cerebri and tentor um cerebelli. Suspended along the middle line upon the under surface of the velum are two fringes, the choroid plexuses of the third ventricle, which are continuous with those of the lateral ventricles at the foramina Monro. The velum is derived from the pia mater covering the occipital and temporal lobes of the brain. It is admitted into the third ventricle beneath the posterior border of the corpus callosum, and passing forward under the fornix, beneath its lateral mar gins,it extends into the lateral ventricles through the fissures of Bichat. At the outer sides of the cruri cerebri, the pia mater passes directly from the under surface of the brain into the middle cornua of the lateral ventricles upon each side. This membrane embraces the pineal body, and the latter may be torn off by its careless removal. TLe parts exposed by its removal are, the cavity of the third ventricle, pineal body, optic lobes and the thalamus. These, together with the basal ganglia and internal capsule, will now be described together, in order that their relations may be more clearly understood. The removal of the cerebellum, as seen in Fig. 8, is advantageous to this dissection, as the parts above men tioned are then seen in connection with those below.