GENERAL MORPHOLOGY OF THE NERVOUS SYSTEM. 15 



broad thin membranous roof of the central canal. This canal 

 at the same time becomes enormously enlarged and is known 



N 



Bulbus olf. 

 Tractus olf. 



Metencephalon 



My elenceph alon 



Medulla spinalis -f* 





FIG. 2. The brain of a selachian, Heptanchus. A, dorsal view; B, lateral view. 

 The roots of the nerves are drawn in black as they appear after painting with 

 osmic acid. The left half of the roof of the fourth ventricle is removed. 



as the fourth ventricle of the brain. Its roof is called the plexus 

 chorioideus of the fourth ventricle. 



If the plexus be removed and the brain examined from within 



