THE EAPHE AND STELE LONGITUDINALES.—In the middle line upon the upper surface of the corpus callosum is a slight linear longitudinal depression, the raphe, and upon each side, and parallel to it, are two longitudinal white lines, the stria; longitudinales, or the nerves of Lancisi. The anterior fibres of the corpus callosum are arched forward into the frontal lobe, the middle fibres are transverse, and the posterior form a broad arch behind and are directed back wards beneath the posterior cornea of the lateral ventri cles to be distributed to the occipital, basalar and temporal convolutions. The fibres of the corpus callosurn decussate continuously with those of the internal capsule above the outer margin of the lateral ventricle and the roof of the pos terior cornu, forming the crest. The posterior margin of the corpus callosum is thick, is inverted beneath the posterior edge of the fornix, to which it is attached and is called the splenium.
The splenium forms a part of the upper boundary of the intraventricular portion of the transverse fissure of Bechat.
The anterior margin of the corpus callosum is arched downwards to the base of the brain, where it divides into two peduncles that diverge outward obliquely across the anterior perforated spaces of opposite hemispheres to termin ate in the anterior part of the temporal lobe upon each side. At their angle of divergence they bound the lamina cinerea in front. The downward deflection of the corpus callosum is called its genu or knee.
A vertical section of the corpus callosum shows that it is about double in thickness in front and behind than in the middle, and in separating its fibres, the inferior layer is observed to be turned downward at its lateral extremi ties into the caudate nucleii, while the upper fibres pass upward into the crest on either side. The under surface of the corpus callosum forms the roof of the lateral ventri cles and is attached below along the median line from before backwards, behind to the fornix, and in front to the septum lncidum. Its attachment to these parts bounds the lateral ventricles internally and separates them from each other.
The removal of the corpus callosum exposes the lateral ventricles, except the middle and posterior cornua which are uncovered by a section of the temporal and occipital lobes.
The cavities thus exposed together present a broad depressed surface between the internal capsules. These project upward and outward, and form its outer boundries on each side for two-thirds of the distance in front.
Projections of the lateral ventricles extend into the anter ior, middle and posterior lobes of the cerebrum, and are called the anterior, middle and posterior cornua of the lateral ventri cles. The anterior cornua are directed forward and outward, the middle backwards, outwards, downward, forward and in ward, encircling the crura of the cerebrum and upon the floor of each is a rounded longitudinal eminence, the hippocampus major which terminates below in a broad convoluted extremity, the pes hippocampi. The posterior cornu is directed outward, downward, backward and inward, and projecting from its in ternal wall is a longitudinal eminence, the hippocampus minor. An oval eminence is situated externally at the junction of the middle and posterior corona and is called the pes accessories, or eminentia collateralis. Behind the reflection of the internal capsule, as it heads downward into the roof of the middle cornu, is a broad groove, directed outwards and downwards, which is the common communication between the middle and posterior cornua with the body of the lateral ventricle.
The parts seen upon the floor of ft lateral ventricle, from before backwards, are the caudate nucleus, the 'Emilia semi circularis, a portion of the thalamus opticus, the choroid plexus, corpus fitnbriatum and the fornix. Between the anterior pillars of the fornix and the anterior extremity of the thalamus is an oval opening through which the lateral ventricle communicates with the third ventricle, and with the lateral ventricle of the opposite side. This opening transmits the choroid plexus of one ventricle to be contin uous with those of the others, and is called the foramen of Monro. Extending backwards from the foramen of Monro, beneath the corpus fimbriatum or edge of the fornix, as far as the apex of the middle coruu of the lateral ventri cle, is a narrow fissure which transmits the velum interposi tum to the lateral ventricle. This fissure is called the fissure of Bichat, and is the ventricular termination of the great transverse fissure.