THE LATERAL VENTRICLES. 543 



Although it presents a few longitudinal white fibres on its surface, the corpus cal- 

 losum consists almost entirely of fibres having a transverse course towards each side, 

 and spreading in a radiating manner into the substance of the two hemispheres. As the 

 transverse fibres from the anterior and posterior lobes of the cerebrum are necessarily 

 aggregated in large numbers near the corresponding ends of the corpus callosum, its 

 greater thickness at those points, in comparison with the rest of its extent, is 

 accounted for ; and, since the posterior lobe reaches further beyond the corpus callosum 

 than the anterior, the greater thickness behind is also explained. 



LATERAL VENTRICLES, or ventriculi tricornes. By dividing the fibres of 

 the corpus callosum in a longitudinal direction at a short distance on each 

 side of the middle line, and about midway between the two ends of the 

 hemispheres, an opening is made into the right and left lateral ventricles of 

 the brain. These ventricles form part of the general ventricular space 

 within the cerebrum ; they are serous cavities, and are lined by a delicate 

 epitheliated structure, the ependyma ventriculorum, which at certain parts in 

 the adult, and probably throughout its whole extent in the foetus, is pro- 

 vided with cilia. In the natural state, the walls of the ventricles are moist- 

 ened internally with a serous fluid, which sometimes exists in considerable 

 quantity, even in a healthy brain. 



It was formerly a subject of dispute whether the lining of the ventricles consisted 

 of epithelium only, or also of a membrane. The progress of the histology of the 

 brain has solved the problem in a manner which leaves the disputants on both sides 

 partially in the right. It is now recognised that a peculiar form of connective tissue is 

 found throughout the substance of the brain, similar to that which has been described 

 in the spinal cord. A layer of this substance, unmixed with nerve-tissues, but in direct 

 continuity with the interstitial web, and not a distinct membrane, supports the 

 epithelium. It is of the same nature as the substance immediately surrounding the 

 central canal of the spinal cord, and is named by Virchow neuroglia (Virchow's 

 " Cellular Pathology," by Chance, p. 273). 



The form of the epithelial cells appears to vary in different parts ; these cells 

 being, according to Kb'lliker, of the flat pavement kind in the third ventricle, and 

 more spherical in the lateral ventricles ; and, according to Gerlach, cylindrical in the 

 aqueductus Sylvii. 



From the central part or body of each lateral ventricle the cavity is 

 extended into each of the three lobes of the hemisphere, thus forming 

 an anterior, a posterior, and a middle or descending cornu. 



The body of each lateral ventricle is roofed in by the corpus callosum, and 

 is separated from its fellow by a vertical partition, the septum lucidum, 

 which descends from the corpus callosum to the fornix. In the floor of the 

 ventricle there is seen most posteriorly one half of the fornix, which is a 

 thin layer of white brain-substance, broad behind and narrow in front : 

 external and anterior to this is the choroid plexus of the lateral ventricle, a 

 red vascular fringe, forming the border of the velum interpositum, a fold 

 of pia mater extending inwards, on which the fornix rests : external and 

 anterior to the choroid plexus is the anterior and outer part of the optic 

 thalamus, appearing from beneath it : outside and in front of the thalamus 

 is the corpus striatum ; and between those two bodies is a narrow flat band, 

 the tcpnia semicircularis. 



The anterior cornu is the blind anterior extremity of the ventricle, pro- 

 jecting a little way into the anterior lobe. It is covered by the corpus 

 callosum, and turns forwards and outwards round the anterior free extremity 

 of the corpus striatum, descending as it proceeds, and bounded behind by 

 that body, and in front by the reflected part of the corpus callosum. 



The middle or descending cornu turns round the back part of the optic 



