PKOSENCEPIIALON OF MAMMALS. 



Ill 



fissure now defines a c convolution of the corpus callosum,' * the 

 lower margin of which, resting on that body, is the * labium cerebri' 

 of anthropotomy : in Sloths, as in Shrews and Rodents, it is an- 

 terior to and distinct from the ' hippocampal ' fissure and fold. 



In all Lissencephala the hemispheres present the following 

 structure, the rise in which, as compared with that in Lyence- 

 phala, is independent of the smallness and smoothness of those 

 divisions of the brain. 



Taking that of the Beaver ( Castor fiber), e. g., and comparing 

 its prosencephalon with the sub-convolute one in Phaseolomys or 

 Macropus, we find, on divaricating the hemispheres, that the 

 6 corpus callosum ' is brought into view, and on removing the 

 cerebral substance to a level with this body, as in fig. 78, its 

 fibres are observed to diverge into the substance of each hemi- 

 sphere, some bending upward, but a greater proportion arching 

 downward and commingling with those that diverge from the cere- 

 bral nuclei. The portions of the brain which are removed in 

 thus tracing the extent of the corpus callosum bring into view the 

 corpora bigemina, B, and the pineal gland, u ; but the optic thalami 

 are concealed by the great com- 

 missure above described. If the 

 posterior margin of the corpus 

 callosum be raised, its inferior 

 surface is found to be closely 

 connected or continuous with the 

 transverse commissural band of 

 fibres, arching over the anterior 

 part of the optic thalami, and 

 passing outward and backward 

 along the floor of the lateral ven- 

 tricles into the substance of the 

 hippocampi, which are almost as 

 large as in the Wombat. The 

 anterior part of the corpus cal- 

 losum is bent downward, and it 

 is attached along the middle line 

 of its inferior surface by a uniting 

 medium of medullary substance, 

 the beginning of the c septum lucidum,' to the hippocampal 

 commissure or fornix; the taeniae hippocampi send forward, 

 as in the Wombat, a delicate layer of medullary fibres which 



78 



Dissection of brain to show corpus callosum, 

 Beaver. 



Ourlct ' and ■ internal convolution ' of Fovillc, xxv". 



