SEA-FISHERIES LABORATORY. 25 1 



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 froni of the body of the cerebellum it passes into the 

 aqued.uctus Syivii (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 



