NERVOUS SYSTEM. 



255 



cerebrum is double and gives off the olfactory lobes from the 

 dorsal surface anteriorly ; each half con- 

 tains a lateral ventricle, which is con- 

 tinued into the olfactory lobe. There 

 is no thin pallium, but each lateral 

 ventricle contains the usual choroid 

 plexus which projects into it from its 

 hind end, where its wall is continuous 

 with the thin roof of the thalamencepha- 

 lon (see below). Behind this the roof 

 dips inwards as a fold, the velum 

 transversum ; then follows a protrusion 

 of the roof with thin walls, behind 

 which is the superior commissure, the 

 attachment of the pineal stalk, and 

 the posterior commissure. The pineal 

 stalk is inclined forwards and ends in 

 the pineal body which lies on this 

 complex of structures. The roof of 

 the mid-brain is not bilobed externally, 

 and the cerebellum is small. On the 

 dorsal side of the 4th ventricle, there 

 is on each side a diverticulum of the 

 membranous labyrinth called the sac- 

 cus endolymphaticus, which gives off 

 diverticula. The infundibulum pre- 

 sents lobi inferiores and a succus 

 vasculosus. 



1)1 



Just in front of the veluni transversum 

 (Fig. 133) the roof of the brain is thin and 

 folded, and swollen up. In Lepidosiren 

 the velum is a paired structure, not ex- 

 tending across the middle line (Kerr). 



The first spinal nerve is the hypo- 

 glossal ; it perforates or notches the 

 " exoccipital." The second spinal nerve 

 either perforates the exoccipital {Pro- 

 topterus) or emerges between the ex- 

 occipital and the first distinct neural 

 arch. 



Fig. 132.— Dorsal view of the 

 brain of Protopierus (after 

 Burckhardt). 7 spinal cord ; 

 2 dorsal root of first spinal 

 nerve ; .3 diverticula of 4 

 the saccus endolymphaticus : .5 

 medulla oblongata ; H fourth 

 ventricle ; ; cerebellum ; s 

 mesencephalon ; ;* stalk of 

 pineal body ; ]<) tlialamen- 

 cephalon ; It velum trans- 

 versum ; rj pineal body ; 7.J 

 lobus hippocampi; N choroid 

 plexus ; ir> cerebrum ; 16 

 olfactory lobe. 



