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Plaice, as it is effected some distance in front of the 
pituitary body, and is not hidden by the olfactory nerves 
on the ventral surface. It is also quite clear that they 
merely cross and do not exchange fibres, whilst their 
plaited nature is at once revealed by a little simple dissec- 
tion. On removing the optic nerves the two small and 
asymmetrical olfactory bulbs are well seen lying largely 
under the anterior extremities of the two striata. In the 
medulla the ventral fissure of the spinal cord is continued 
as far forwards as the base of the lobi inferiores, where it 
slightly expands. 
Regarding the ventricles of the brain, the central 
canal of the spinal cord appears in the sections as a pin 
hole. It begins to widen rapidly into the fourth ventricle 
(myelocoele) at about the posterior region of the auditory 
organ. The ventricle is at first very deep from above 
downwards and very narrow from side to side. It soon 
opens above, and is only closed in by the choroid roof. 
The peculiarity of the roof of this ventricle has been 
already mentioned. In front of the expanded portion it 
becomes completely roofed over by the tuberculum 
acusticum, and at the same time is reduced to a very small 
size. Opposite the junction of the medulla and cere- 
bellum it again expands, but does not communicate with a 
cerebellar cavity (metacoele), the cerebellum being solid. 
In front of the body of the cerebellum it passes into the 
aqueductus Sylvii (mesocoele—iter a tertio ad quartum 
ventriculum), roofed over behind by the valvula cerebelli 
and communicating on each side and in front with the 
large space enclosed by the tectum opticum (optocoele). 
In front, the latter opens below into the third ventricle 
(thalamocoele), bounded laterally and below by the 
thalamus (optic thalami) and above in front by the choroid 
roof. The third ventricle is prolonged downwards and 
