THE BEAIN 355 



the base of the skull when the brain is removed : 

 in this case there is at the apex of the infundi- 

 btdum a slit-like aperture, leading into the third 

 ventricle. 



d. The optic cMasma is a transverse band of nerve- 



fibres, lying immediately in front of the infimdi- 

 bulum. From its anterior border the optic nerves 

 arise, and from its outer ends the optic tracts may 

 be followed backwards towards the optic lobes 

 by gently pressing aside the temporal lobes. 



e. The corpus mammillare, or corpus albicans, is a 



small rounded median elevation, immediately 

 behind the infundibulum. 



f. The pons Varolii is a stout band of transverse 



nerve-fibres, which crosses the ventral surface 

 of the medulla, opposite the posterior ends of the 

 hemispheres. It connects the two sides of the 

 cerebellum with each other. 



g. The crura cerebri are two large bands of nerve-fibres, 



immediately in front of the pons VaroUi, which 

 run forwards, diverging from each other, and 

 connect the medulla oblongata with the hemi- 

 spheres. 



h. The medulla oblongata, as seen from the ventral 

 surface, is broad in front and gradually narrows 

 behind. It is marked by a median ventral fissure, 

 bordered by two narrow bands, the ventral 

 pyramids. At the outer sides of the pyramids, 

 immediately behind the pons Varohi, are a pair 

 of oblong patches of transverse nerve-fibres, the 

 corpora trapezoidea. 



C. The Roots of the Cerebral Nerves. 



There are twelve pairs of cerebral or cranial nerves in the 

 rabbit, all of which, with the exception of the fourth pair, 

 arise from the ventral surface or from the sides of the brain. 



