SURFACE AND INTERIOR OF CEREBRUM. 89 



The septum lucidum is a thin, triangular, and almost 

 transparent partition between the lateral ventricles ; it con- 

 sists of two layers, inclosing a space which is the fifth ven- 

 tricle. The septum lucidum is so delicate a structure, that 

 if the decomposition of the brain has at all advanced, it 

 will only be seen as it tears away during the gentle lifting 

 of the anterior portion of the corpus callosum, to the under 

 surface of which it is attached superiorly; its lower border 

 corresponds to the rostrum, and its apex to the genu of the 

 corpus callosum ; its base is applied to the fornix as it 

 arches downward in front. 



The fornix is a thin, horizontal la} 7 er of white substance, 

 triangular in outline, attached along the median line to the 

 under surface of the corpus callosum, behind the septum 

 lucidum ; in front it arches over the foramen of Monro, to 

 form on either side crura, which terminate in the corpora 

 albicantia and optic thalami ; posteriorly it is continuous 

 with the corpus callosum, and it sends off on each side an 

 expansion called its posterior pillars, which lie along the 

 hippocampus major. It is this part of the fornix which is 

 seen in the floor of the lateral ventricle when first laid open, 

 and the thin edge of which, concealed by the choroid plexus, 

 is called the corpus fimbrialum or tsenia hippocampi. If the 

 fornix be divided anteriorly and reflected backward, there 

 will be seen upon its under surface, between its diverging 

 pillars behind and its crura in front, a triangular space 

 marked by transverse lines, and called .the lyra. The re- 

 flection of the fornix exposes the optic thalami. and it will 

 be seen that it rests upon and covers- in these bodies in 

 nearly their whole extent. If we follow the fornix by its 

 free edge or t?enia hippocampi into the descending cornu of 

 the ventricle, we shall find, on lifting it from the optic tha- 

 lamus, that there is a fissure extending downward to the ex- 

 tremity of the posterior cornu, and forward to the foramen 

 of Monro, through which an expansion of the pia mater 

 enters the ventricle, and forms the fringed margin which 

 lies along the tsenia hippocampi, called the choroid plexus. 

 This is called the transverse fissure of the cerebrum, or the 

 fixture of Bichat. The ventricle is made a closed cavity 

 by the union of its lining membrane with that portion of 

 the pia mater which passes through this fissure. 



The central part of the pia mater which thus enters the 

 ventricles, lying beneath the fornix, and corresponding to 

 it in shape and^extent, is called the velum interpositum ; its 



8* 



