24-1- THE ANATOMY OF THE HORSE. 



cerebral mass is scon to be mesially divided by the gi-eat 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) nen^e pass through the 

 criliriform plate of the ethmoid bone, and enter the nasal chamber. The 

 bulb is hollow, having a centi'al 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 olfiictory 

 tracts. 



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

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

 The intenml 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 curves 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 unci- 

 nate convolution, and behind that point it becomes grey on its surface. 



Behind the angle of divergence of the olfactory tracts 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 

 greT/ olfactory root; and, according to Broca, it covers white fibres that 

 connect the olfactory bulb to the crus cerebri and to the anterior cere- 

 bral commissure (anterior white commissure of 3rd ventricle). 



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

 will be seen to be connected to the frontal lobe of the hemisphere by 

 a lamina termed by Broca the sujyei^ior olfactory root. This lamina is 

 grey on its surface and white beneath, and if it be ruptured the cavity 

 of the bulb and the communication between that cavity and the anterior 

 cornu of the lateral ventricle will be brought into view. 



External to the outer olfactory tract, each hemisphere shows numerous 

 winding worm-like ridges, termed convolutions, and internal to the pos- 

 terior half of the same tract there is seen a thick ridge — the hippo- 



