122 Mammalia. Nervous System. 



primitive Myelonal cavity, traversing the Mesencephalitf 

 basis. 



3. The third primary division of the brain, anterior to the 

 Mesencephalon, is termed the Prosencephalon : it includes the 

 Crura Cerebri anterior to the Nates, the Thalami Optici, Cor- 

 pora Striata,and Cerebral Hemispheres with their commissures. 

 The Cerebral Hemispheres attain a greater development than 

 in any other class of Vertebrata : they are connected together 

 in all Mammals, as in Birds, by the cord-like fasciculus of 

 transverse fibres, called the * anterior commissure ' : but the 

 main distinction lies in the superaddition, to the * diverging' 

 or 'crural* fibres, of other commissural tracts ; either longi- 

 tudinal, connecting parts of the same Hemisphere ; or trans- 

 verse, bringing the greater portion of the two Hemispheres 

 into mutual communication. The Hemispheres are always 

 connected by a more or less extensive Corpus Callosum : but 

 in descending the series from Man to those Placental Mammals 

 of lowest cerebral organization, a great change is seen in the 

 condition of the Corpus Callosum, in the disappearance of the 

 rostral portion, and coincident greater development of the 

 posterior folded or psalterial portion ; the latter being con- 

 nected with the relative increase of the hippocampal region 

 of the Cerebrum : a change of precisely the same nature is 

 seen carried to an excess in the brain of the Marsupial. The 

 anterior Commissure diminishes in size in proportion as the 

 Corpus Callosum increases in extent. The Corpus Callosum 

 increases directly as the growth and complexity of the Hemi- 

 spheres. As the Hemispheres increase, in the Placental 

 series, so does the extent of the filmy inner walls of the lateral 

 ventricles (' septum lucidum,' Anthro.) between the body of 

 the Fornix and the Corpus Callosum. Each Hemisphere of the 

 Cerebrum begins as a vesicle of neurine, the'cavity of which 

 receives the growth from the 'crura,' forming the 'Corpus 

 Striatum' ; this^ in Birds, mainly fills the ' Yentricle ' or rem- 



