SCYLL1UM 



43 



Attention may here be called to Reissner's fibre. It is a 

 wire-like structure of unknown function which runs from the 

 posterior commissure in the roof of the midbrain, through 

 the cavity of the nerve-tube right down to its hind end where it 

 is attached. Reissner's fibre is present in most chordates, 

 but not in Amphioxus nor in man. 



The pia mater has already been mentioned. It encircles 



Fig. 23. — Scyllium : ventral view of the brain showing the pituitary 



body. 



al, anterior lobe ; nil, neuro-intermediate lobe ; vl, ventral lobe of the 

 pituitary body ; il, floor of the diencephalon ; on, optic nerve ; sv, saccus 

 vasculosus. 



the whole nerve-tube, and corresponds to the choroid layer of 

 the eye. Outside it is a tougher membrane, the dura mater, 

 protective in function. It is applied to the inner wall of the 

 skull and corresponds to the sclerotic layer of the eye. 



Spinal Nerves. — In each segment on each side, behind the 

 head, the spinal cord gives off a ventral nerve, and a dorsal 

 nerve with a ganglion on it. These two nerves join to form a 

 mixed spinal nerve. Soon after joining, their components 



