252 



AN INTRODUCTION TO ZOOLOGY 



ventricle, and it is roofed by a thin membrane. This is composed 

 of a part of the ependyma, or epithelium lining the ventricles, and 

 the pia mater. On each side at the front end this dips down into 

 the third ventricle and passes forward into the lateral ventricle, 

 where it becomes highly vascularised and constitutes the choroid 

 plexus of the lateral ventricle. A short distance behind this it 

 again dips inwards in the form of a noticeable transverse fold, the 

 velum transversum, which hangs down into the cavity and marks 

 the actual dorsal line of demarcation between telencephalon and 



o.c O.L 



CR 



o.c 



C.R. 



I 



FIG. 82. -Brain of Scyllium. A, dorsal view ; B, ventral view. 



A.C., anterior choroid plexus; A. P., anterior lobe of pituitary body; C., cerebrum; Cb., 

 cerebellum; C.R., restiform bodies; L.I., lobus inferior; M.. medulla oblongata ; O.C., olfactory 

 capsule; O.F., olfactory peduncle ; O.L., olfactory lobe ; Op., optic lobe; O.X., optic chiasma ; 

 P., pineal stalk; P.C., posterior choroid plexus'; P.P., posterior lobe of pituitary body; S.V., 

 saccus vasculosus; T., lhalamencephalon ; II.-X., cranial nerves. 



thaiamencephalon. At its posterior end the roof again becomes 

 nervous and bears two ganglionic masses, the habenular ganglia, 

 almost meeting in the middle line, but joined by a band of transverse 

 fibres, the superior commissure. Just behind this in the middle line 

 arises a small strand which runs forward as the pineal stalk, and 

 terminates in swollen enlargement, the pineal body or epipnysis 

 cerebri, attached to the membrane covering the anterior cranial 

 fontanelle. This is a vestigial structure, being the remnant of a 

 pair of eye-like organs present in ancestral forms. Immediately 



