CHAPTER XII. 

 Telencephalon — Rhinencephalon. 



THE telencephalon comprises all those parts of the brain 

 which have not yet been considered — all that lie 

 morphologically anterior to the diencephalon. It 

 consists of a very small median portion which continues the 

 diencephalon forward and encloses the anterior part of the 

 third ventricle, the telencephalon medinm {pars optica hypo- 

 thalami), and of paired lateral evaginations from this the 

 cerebral hemispheres. 



Each hemisphere contains a cavity, the lateral ventricle, 

 which is connected with the anterior part of the third ventricle 

 by a small interventricular foramen {foramen of Monro). The 

 roof of the lateral ventricle with the dorsal parts of its walls 

 form the pallium, which in mammals has an external layer 

 of gray matter, the cerebral cortex, and is greatly expanded so 

 as largely to cover the outer surface of the rest of the hemi- 

 sphere. What were primitively the floor and more ventral 

 parts of the walls form the basal ganglia, which are very 

 largely hidden from view in the gross brain by the expansion 

 of the pallium. Of the structures visible externally, the 

 tuberculum olfactorium belongs to this division, as does also 

 the lower part of the medial wall just in front of the telence- 

 phalon medium. This part is known as the paraterminal 

 body {corpus paraterminale sen precommissurale) , or frequently 

 as the septum, though the latter term is confined in man to a 

 postero-dorsal extension of this region,^ as we shall see 

 presently (p. 93). 



^According to Johnston, the septum pellucidum of higher mammals 

 is not genetically related to the paraterminal body, but is a separate 



