250 THE ANATOMY OF THE HORSE. 



of the optic commissure, penetrated by numerous vessels for the corpus 

 striatum, which lies above the spot. 



The Fissure of Sylvius is a faint and ill-defined groove which 

 begins at the locus perforatus anticus, and extends outwards across the 

 hemisphere. 



The Great Longitudinal Fissure. In front of the optic chiasma the 

 cerebral mass is seen to be mesially divided by the great longitudinal 

 fissure. This fissure, as will be better seen when the brain is viewed 

 from above, is a great vertical mesial cleft extending the whole length 

 of the cerebrum, which it partially divides into right and left halves, or 

 hemispheres. 



The Olfactory Bulbs. The olfactory bulb is the white body situated 

 at the anterior end of the hemisphere. It occupies the olfactory fossa 

 at the forepart of the cranial cavity ; and, unless special care is taken in 

 the removal of the brain, the bulb is apt to be separated from the hemi- 

 sphere and left in that fossa. From the free surface of the bulb the 

 delicate filaments of the olfactory (1st cranial) nerve pass through the 

 cribriform plate of the ethmoid bone, and enter the nasal chamber. 

 The bulb is hollow, having a central cavity that is in communication 

 with the anterior cornu of the lateral ventricle. 



The Olfactory Peduncle is a short, thick, white cord immediately 

 behind the bulb, and in direct continuity with it. The hemisphere is 

 slightly depressed over the peduncle, the depression being termed the 

 olfactory fissure. The peduncle divides posteriorly into the olfactory 

 tracts. 



The Olfactory Tracts. These are two white diverging bands — an 

 inner and an outer — that continue the olfactory peduncle backwards. 

 The internal tract (inner olfactory root) is short, and passes backwards 

 and inwards to the edge of the great longitudinal fissure. The external 

 tract (outer olfactory root) is a much longer band which carves out- 

 wards and backwards across the fissure of Sylvius, and then encircles 

 outwardly the uncinate and hippocampal convolutions, to reach the 

 tentorial aspect of the hemisphere, on which it is lost. At the fissure 

 of Sylvius the tract seems to lose some of its fibres in front of the 

 uncinate convolution, and behind that point it becomes grey on its 

 surface. 



Behind the angle of divergence of the olfactory tract is a smooth 

 and slightly convex area — the quadrilateral space of Paul Broca. The 

 surface layer of this space consists of grey matter constituting the 

 middle or grey olfactory root ; and, according to Broca, it covers white 

 fibres that connect the olfactory bulb to the crus cerebri and to the 

 anterior cerebral commissure (anterior white commissure of 3rd 

 ventricle). 



If now the olfactory peduncle be raised from the olfactory fissure, it 



