THE FORE BRAIN 349 



THE PROSENCEPHALON: 



(a) The greatly enlarged cerebral hemisphere (hemisphaerium 

 cerebri), the dominant portion which correlates and co- 

 ordinates the activities of all the rest of the nervous system, 

 forms with its fellow of the opposite side the largest portion 

 of the brain. The two structures are separated by the 

 longitudinal cerebral fissure, but are connected with each 

 other by the commissures indicated below. Each hemi- 

 sphere has a superficial layer of grey matter, the cerebral 

 cortex, which in larger brains is thrown into numerous 

 folds but in the rabbit is practically smooth. 



(b) The olfactory bulb (bulbus olfactorius) is a small expansion 

 lying at the anterior end of each hemisphere. Its anterior 

 and ventral surfaces receive the fascicles of the first cranial 

 or olfactory nerve, which is not a compact structure but is 

 represented by numerous separate threads coming from 

 the mucous membrane of the nose. These may be found 

 in the skull, where they may be traced into the perforations 

 of the cribriform plate. 



(c) The olfactory bulb is the anterior portion of the olfactory 

 brain. When traced backward on the ventral surface of the 

 brain it is seen to be replaced by a white band of fibres, the 

 olfactory tract (tractus olfactorius) and a somewhat wider 

 strip of grey matter underlying the tract. This strip 

 expands caudally into a portion of the brain which, from 

 its shape, is known as the pyriform lobe (lobus piriformis) 

 and here the olfactory tract spreads out and most of its 

 fibres terminate. The lateral margin of the olfactory brain, 

 which includes the olfactory bulb and the related parts just 

 described is delimited superficially from the remaining 

 portions of the cerebral hemisphere by a longitudinal 

 furrow, the limbic fissure (fissura limbica). The anterior 

 portion of the furrow, known as the anterior rhinal fissure, 

 separates the grey matter underlying the olfactory tract 

 superficially from the narrow anterior end of the non- 

 olfactory part of the cerebral hemisphere. The correspond- 

 ing posterior portion of the furrow^ the posterior rhinal 

 fissure, marks off the pyriform lobe from the posterior, 



