SCYLLIUM 41 



of the under surface of the snout just in front of the mouth, 

 and connected with it by grooves which run to its corners. 

 Inside the pits, the sensory olfactory epithelium is thrown into 

 a number of folds. 



Nervous System. — As in the Cyclostome, the brain is 

 divisible into fore-, mid-, and hind-regions. Further, the 

 fore- and hind-brains can also be divided into two for facilitat- 

 ing description. There are therefore five sections of the 

 brain, whose names from front to rear are : telencephalon, 

 diencephalon (also called thalamencephalon), mesencephalon, 

 metencephalon, and myelencephalon. The first two divisions 

 together form the forebrain or prosencephalon, the last two 

 form the hindbrain or rhombencephalon. The sides of the 

 telencephalon (or end-brain) are greatly expanded, and bear 

 the olfactory bulbs. On the floor is the optic chiasma, where 

 the optic nerves cross over from one side to the other. In 

 front of this is the lamina terminalis ; the thickened lower 

 portions of the side walls are the corpora striata. 



A transverse fold in the roof, the velum transversum, marks 

 the beginning of the diencephalon (or between-brain). The 

 sides are thickened and known as the optic thalami, the floor 

 is depressed to form the infundibulum to which the pituitary is 

 attached. The roof bears a projection : the epiphysis, vestige 

 of the pineal eye. 



The floor, sides, and roof of the mesencephalon are 

 thickened, so that its cavity is reduced and is known as the 

 aqueduct of Sylvius. The roof forms the paired optic lobes. 



The roof of the metencephalon is thick and forms the 

 cerebellum, that of the myelencephalon is thin. To the sides 

 of and behind the cerebellum are the restiform bodies. 



The cavity of the forebrain is called the 3rd ventricle ; 

 that of the hindbrain the 4th ventricle. The brain is sur- 

 rounded by a membrane carrying blood-vessels (the pia mater), 

 and this dips down in folds from the roof of the 3rd and of the 

 4th ventricles to form a choroid plexus. Connecting one side 

 of the brain with the other there are tracts of fibres called 

 commissures. Of these, the habenular and the posterior are 

 in the roof of the between-brain and midbrain respectively 



