107 



Plaice, us it is eft'octcd some distauec in fioni of the 

 pituitary body, and is not hidden by tlie olfactory nerves 

 on the ventral surface. It is also ([iiite clear that they 

 merely cross and do not exchang-e fibres, whilst their 

 ]>laited nature is at once revealed l)y a little simple dissec- 

 tion. On removing the optic nerves the two small and 

 asymmetrical olfactory bulbs are Avell seen lying largel}^ 

 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. 



llegarding 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 houi side to side. It soon 

 ojjcns 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 

 IxM'omes completeh' looted over b}- the tuberculum 

 acustieum, and at the same time is reduced to a very small 

 size. Opposite tlie junction of the medulla and cere- 

 b(>lhim it again expands, but does not communicate with a 

 (•(Mchcllar cavity (metacoele), the eerel)ellum being solid. 

 Ill front of the body of the cerebelluin it passes into the 

 a([ueductus Sylvii (mesocoele — iter a tertio ad quartum 

 v(>ntiicuhim), 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 ehoroid 

 roof. The third ventricle is prolonged downwards and 



