122 THE LATERAL VENTRICLES. 



THE LATERAL VENTRICLES AND THE PARTS OP THE BRAIN 



DEVELOPED IN CONNEXION WITH THEM: CORPORA STRIATA: 



CEREBRAL HEMISPHERES (PROSENCEPHALON). 



The lateral ventricles ; ventricles of the cerebral hemispheres. An 



aperture (foramen of Monro) which has been already mentioned as opening out of 

 the anterior part of the third ventricle (see p. 97) leads on each side into the 

 corresponding ventricle of the cerebral hemisphere or lateral ventricle (figs. 88, 89). 

 Each lateral ventricle is an irregularly curved cavity, extending in the substance of 

 the corresponding hemisphere for about two-thirds of its entire length, and lined 

 by a prolongation of the ciliated epithelium which characterises the inner surface 

 of the true brain-ventricles. It may be described as consisting of a body, an anterior 

 horn, a posterior horn, and a middle, lateral, or descending horn. The anterior horn 

 curves from the foramen of Monro somewhat outwards, with a slight inclination 

 downwards into the frontal lobe ; the body comprises that part of the cavity which 

 extends from the foramen of Monro to its bifurcation into posterior and descending 

 horns opposite the splenium of the corpus callosum, and is separated anteriorly from 

 its fellow of the opposite hemisphere by a thin septum, the septum lucidum ; the 

 posterior horn passes backwards, with a bold curve convex outwards, into the 

 occipital lobe ; and the descending horn passes forwards and slightly downwards also 

 in a bold curve with its convexity outwards, into the temporal lobe, and extends to 

 about an inch from the apex of that lobe. The anterior horn is directly continued 

 from the body of the ventricle, and may therefore be described along with it. 



The body and anterior horn of the ventricle are roofed over by the corpus 

 callosum, with its lateral extension into the substance of the hemisphere. In 

 examining the ventricles from above it is usual to slice the brain horizontally down 

 to the level of the callosum, and then to cut through this commissure a short 

 distance on each side of the middle line, and remove it sufficiently to expose the interior 

 of the ventricle. The following parts are thereby brought to view, from within, out- 

 wards, or laterally : (a) Theedgeof the fornix (figs. 88, 89,/), a flattened tract of white 

 matter with longitudinal fibres, which lies immediately under the callosum, broaden- 

 ing behind as it passes under the splenium, and bifurcating in front into two 

 cylindrical bundles, each of which passes over the foramen of Monro, and in front 

 of that foramen into the mesial part of the subthalamic tegmentum. () If the 

 callosum and fornix are removed, a layer of pia mater is seen underneath. This is 

 the velum interposition (fig. 94), and its edge projects as the choroid plexus of 

 the lateral ventricle from beneath the margin of the fornix in the form of a 

 vascular fringe (figs. 88, 89, pi. ch.), which extends from the foramen of Monro (at the 

 back of which it is continuous with the corresponding plexus of the opposite side as 

 well as with those of the third ventricle) over the surface of the thalamus, as far as 

 the descending cornu, into which it projects along the whole length of the horn 

 (mesial border), (c) Lateral and anterior to the choroid plexus two masses of grey 

 matter, separated by an obliquely-placed white stria, form the floor of the ventricle. 

 The mesial and narrower of these is a part of the dorsal surface of the thalamus (fig. 88, 

 th. opt.) ; it extends from the foramen of Monro to the posterior limit of the body of the 

 ventricle and is covered with a thin layer of white matter. The white stria is the 

 twnia semicircularis or stria terminalis (fig. 8'2,St.). A vein (vein of the corpus striatum) 

 runs superficial to the tasnia, and passes in front, at the foramen of Monro, into the 

 vein of Galen of the velum interpositum. Close to the ependyma and lying over 

 this vein of the corpus striatum is a small greyish band, containing longitudinally 

 running nerve-fibres : this has been named the lamina cornea. The lateral mass of 



