790 THE CENTRAL AXIS OF THE NERVOUS SYSTEM. 



The superior face, free in the middle, and corresponding to the bottom of the 

 interlobular fissure, is covered right and left by the substance of the hemispheres. 

 It is traversed from before to behind by two white, and generally very delicate, 

 cords— the tr actus longitudinalis (the chordce longitudinalis of Lancisi) of the 

 corpus callosum — which lie together on the middle line. The infei'ior face is 

 divided — by the insertion of the septum lucidum — into two lateral portions, each 

 of which forms the roof of one of the cerebral ventricles. Behind, it rests on 

 the middle part of the fornix. 



The two lateral borders of the corpus callosum disappear in the central sub- 

 stance of the hemispheres, where it is almost impossible to distinguish their 

 limits. 



The posterior extremity appears at the bottom of the interlobular fissure — 

 after destroying the adhesion usually established above it between the two hemi- 

 spheres— in the form of a thick, rounded enlargement (splenium) doubled below, 

 and confounded with the middle part of the fornix. It is prolonged, laterally, 

 above the ventricular cavities, in forming two angles {linear transversce) which 

 are soon lost in the white central substance of the cerebrum. 



The anterior extremity comports itself in a similar manner between the 

 anterior lobes of the hemispheres. It is named the genu of the corpus callosum, 

 and is doubled more abruptly than the posterior end, terminating by a thin 

 narrow portion — the rostrum. 



2. The Lateral or Cerebral Ventricles (Figs. 429, 435). 



The lateral ventricles are two large elongated cavities excavated in the hemi- 

 spheres, lying against each other in their anterior moiety, and divergent in their 

 posterior part, which is very much curved backwards, outwards, and downwards, 

 to open into the substance of the sphenoidal lobe. 



This disposition permits the division of the cerebral ventricles into two 

 regions — an anterior, and a posterior or reflected. 



The anterior region — anterior cornu, or frontal diverticulum — is separated in 

 the median plane from the opposite ventricle, by the septum lucidum and the 

 summit of the fornix, beneath which is the foramen of Monro establishing a 

 communication between the middle and the two lateral ventricles, and between 

 these latter. Above, it offers a smooth wall formed by the corpus callosum. 

 Below, on its floor, there is first remarked, in front, the corpus striatum ; behind, 

 the internal portion of the hippocampus ; in the middle, an oblique groove 

 running backwards and inwards, at the bottom of which floats the choroid 

 plexus. The anterior extremity of this region, occupied by the base of the 

 corpus striatum, is continued by a narrow opening into the interior of the 

 olfactory lobe. The posterior is prolonged, without any line of demarcation, by 

 the reflected 'portion of the ventricular cavity. 



The latter region {posterior or descending cornu, or sphenoidal diverticulum) 

 occupit'S the most declivitous portion of the posterior lobe of the hemisphere, 

 and is a much-curved canal with its concavity forward ; this canal terminates 

 in a cul-de-sac in the substance of the sphenoidal lobe. On the floor of this 

 canal is marked the posterior portion of the hippocampus and the choroid plexus. 



A very fine membrane — the ventricidar arachnoid — plays the part of a serous 

 membrane and covers the walls of these cavities, being spread everywhere over a 

 layer of white substance, prolonged into the ethmoidal diverticulum, and con- 

 tinuous, through the foramen of Monro, with that of the middle ventricle. This 



