THE NERVOUS SYSTEM. 583 



this is the cerebellum, and (d.) When the brain is viewed in the normal 

 position at the bottom of the fissure, between the hemispheres is seen a 

 broad band of white matter connecting one hemisphere with its fellow, 

 the main commissure or corpus callosum (fig. 357). Such parts of the 

 brain are evident even on superficial examination. On dissection, it is 

 found that the central nervous system is not a solid mass of nerve mate- 

 rial; it incloses certain cavities, the cerebral ventricles. Forming the 

 walls and boundaries of these ventricles are very important masses of 

 nervous matter. The cerebrum proper incloses a large central cavity, 

 the lateral ventricle, but separated by a median partition into two. Into 

 the cavity of each lateral ventricle (fig. 355) projects a rounded mass of 

 gray matter anteriorly, which is the caudate nucleus of an important 

 structure known as the corpus striatum, the more external part of which, 

 the lenticular nucleus, is embedded in the mass of the cerebral hemi- 

 sphere. Below, or more posterior to the caudate nucleus, and also pro- 

 jecting into the lateral ventricle, is a second mass of gray matter, called 

 the optic thalamus; the upper part of this only, however, is seen in the 

 lateral ventricle, the lower and more internal part approaching its fellow 

 in the middle line leaves a space which on vertical section is more or less 

 triangular, called the third ventricle. The lateral ventricles are sepa- 

 rated from one another by means of a partition made of two layers of 

 white matter, the septum lucidum. On section the septum is seen to be 

 more or less triangular, and between the two layers there is the space of 

 the fifth ventricle filled with fluid. 



At the posterior part of the septum lucidum, and joining with it, is 

 the fornix. This is a longitudinal commissure; it is arched and its 

 edge is seen in the lateral ventricle on either side. Between its edge 

 and the upper part of the optic thalamus projects a fringe of blood- 

 vessels, which is the upper part of the septum of 'the vascular pia mater, 

 which passes into the interior of the brain, and which is called the cho- 

 roid plexus ; the whole of the projection forming a roof for the third 

 ventricle is called the velum interpositum. 



The fornix (fig. 355, e) is made up of two strands anteriorly, called 

 the anterior pillars, and of two similar pillars posteriorly; the middle 

 portion called the body consists of the parts of the two pillars which are 

 joined together in the middle line. The body of the fornix is triangular 

 in shape, broad and flat behind, where it is connected with the corpus 

 callosum, and narrow in front where it is connected to the septum luci- 

 dum. The anterior pillars pass downward, separated from one another 

 on either side of the third ventricle in front of the foramen, by which 

 the lateral communicates with the third ventricle, called the foramen 

 vf Monro; each pillar then passes forward and down, and twisting upon 

 itself forms the corpus albicans, and then passes in part to join the optic 



