84 INTRODUCTION. 



separated longitudinally, by a strong membrane cal- 

 led the falx, which is a prolongation of the dura 

 mater, or lining membrane of the skull ; but beneath 

 the lobes, are united by a white medullary mass, cal- 

 led the corpus callosum ; nearly on a level with this 

 substance, and on each side of it, we find the lateral 

 ventricles, consisting of three sinuses, one anterior, 

 one inferior or middle, and the other posterior ; the 

 walls, or sides of these cavities, are in contact, un- 

 less separated by a small quantity of limpid fluid, 

 which is not unfrequent : in the centre of each we 

 find a rounded tubercular-like elevation, termed the 

 corpus strictum, and attached to it posteriorly the 

 optic thalamus, or origin of the optic nerve : the 

 ventricles are divided from each other by the septum 

 lucidum, or their medullary partition, which de- 

 scends from the under surface of the corpus callosum, 

 and is united to the fornix ; this partition, on being 

 torn, is found to consist of two layers ; and the 

 cavity between has been called the fifth ventricle : 

 the fornix is a white medullary band, connected at 

 each extremity to the anterior and middle lobes, by 

 two cords or crura : under the anterior crura there 

 is a small aperture, which forms a communication 

 between the ventricles ; between the fornix and cor- 

 pora striata, is the plexus choroides, consisting of a 

 fasciculus of minute blood vessels, extending into the 

 inferior and posterior corners of the ventricles, and 

 communicating with the blood vessels of the mem- 

 brane which covers the base of the brain. On sepa- 

 rating the optic thalami, we see a white cord, termed 



