LEPUS CUNICULUS 345 



canalis centralis of the spinal cord, and which is derived from the 

 inner cells of the original neural tube. Each lateral ventricle lies 

 below the corpus callosum whose ventral fibres form its roof, and it 

 is a fairly narrow irregularly shaped cleft with very well-marked 

 anterior and descending horns. The anterior horn passes forwards 

 and downwards, being somewhat T-shaped in transverse section, 

 gradually becoming reduced in size until finally as a small slit it 

 passes up the olfactory stalk and enlarges slightly to form the 

 ventricle of the olfactory bulb. The descending or inferior lateral 

 horn of the ventricle runs backwards, outwards and downwards 

 into the hinder region of the hemisphere, i.e. the temporal lobe. 

 The region where the two horns join is sometimes termed the body 

 of the lateral ventricle, and it opens into the third ventricle by a 

 narrow vertical aperture, the foramen interventriculare or foramen 

 of Munro. 



The mesial wall of the anterior cornu below the corpus callosum 

 is formed by a delicate semi-transparent membrane which comes to 

 lie so close to its fellow in the middle line that the two join and form 

 what is known as the septum lucidum, which is limited on the ventral 

 side by a thickened mass of tissue, the fornix. In man there is a 

 cleft-like cavity within the septum. The ventro-lateral wall of this 

 cornu is composed of a thick important mass of nervous tissue 

 known as the corpus striatum, from the striated appearance it 

 presents in transverse section. The lateral and dorso-lateral walls 

 of the anterior cornu is composed of the grey matter of the cerebral 

 cortex and also of white matter arranged in a characteristic radiating 

 manner to form the corona radiata, which is the lateral continuation 

 of the corpus callosum. 



The mesial, posterior and ventro-lateral walls of the descending 

 cornu are formed by a large ganglionic mass, the hippocampus, and 

 attached to this is the posterior pillar of the fornix. Its lateral 

 and dorso-lateral walls are composed of the white matter underlying 

 the cerebral cortex. In front of the pillar of the fornix a narrow 

 strip of the wall of the ventricle becomes very thin, and it is con- 

 tinuous through the interventricular foramen with the epithelium 

 of the roof of the third ventricle, immediately above which the 

 highly vascular pia mater forms the anterior choroid plexus. This 

 thin epithelium with its pia mater is deeply invaginated into the 

 ventricle as a marked folded band of tissue, and so constitutes the 

 choroid plexus of the descending horn. In removing the meninges 

 this portion of the pia mater, together with the epithelium with which 

 it is bound up, is usually torn away, so leaving what appears to be a 

 curved slit-like opening, sometimes spoken of as the " choroidal 

 fissure," leading from the ventricle to the outside. It should be 



