XIII PHYLUM CHORDATA 471 



splenium of the latter behind, but diverging from it anteriorly by 

 dipping down towards the base of the brain. In the angular space 

 between the corpus callosura above and the fornix below is the 

 septum lucidum with the " fifth ventricle." The taeniae hippocampi 

 are the posterior pillars of the fornix (p. fo.) ; the anterior pillars 

 (a.fo.) are a pair of vertical bands which pass from the anterior 

 end of the body of the fornix downwards to the corpus mammillare 

 (see 'below) at the base of the diencephalon. 



Lying immediately in front of the hippocampus major is a vas- 

 cular membrane, the choroid plexus (ch. pi.) ; this passes inwards to 

 join its fellow of the opposite side through a transverse passage, 

 the foramen of Monro (/. m.), which opsns behind into the diacoele. 

 The floor of the anterior cornu is formed of an eminence of grey 

 matter — the corpus striatum (cp. s.). The right and left corpora 

 striata are connected together by a narrow transverse band of 

 white fibres — the a7ite7'ior commisstcre (a. co.) — situated in front of 

 the anterior pillars of the fornix. 



The diacoele (v.^) is a laterally compressed cavity, the roof of 

 which is formed by a delicate vascular membrane, the velum inter- 

 positum (vl. ip.), in which there is a network of blood-vessels 

 (choroid plexus of the diacoele) continuous with the choroid plexuses 

 of the lateral ventricles. From the posterior part of the roof of 

 the diacoele arise the peduncles of the pineal body. The optic 

 thalami (o. th.) are large masses of mixed grey and white matter 

 forming the lateral portions of the diencephalon ; they are con- 

 nected together by a thick mass of grey matter, the middle or 

 soft commisstire (m. co.), not represented in lower Vertebrates, 

 passing across the diacoele. A rounded elevation near the anterior 

 end of the external surface of each thalamus is the corpus genicii- 

 latum (c. gn.). The anterior boundary of the diacoele is a thin 

 vertical lamina — the lamina terminalis (t.t) — of which the septum 

 lucidum is a mesial anterior prolongation. The floor of the 

 diencephalon is produced downward into a mesial rounded 

 process, the tuher cinereum or infundibidum (inf.), to which the 

 pituitary body is attached. In front of this, on the ventral aspect 

 of the brain, is a thick transverse band of nerve-fibres, the 

 united optic tracts, from the anterior border of which the optic 

 nerves are given off. Behind the tuber cinereum, and formed as 

 a thickening of its posterior wall, is a rounded elevation, the 

 corpus mammillare (c. ma.). 



In the mid-brain the dorsal part is remarkable for the fact 

 that each optic lobe is divided into two by a transverse furrow, so 

 that two pairs of lobes (o.l.'^, o.l.^), the corpora quadrigemina, are pro- 

 duced. Between the anterior lobes passes the delicate posterior 

 commissure (p.co.). On the ventral region of the mid-brain the 

 crura cerebri are far more prominent than in the lower groups. In 

 the hind'brain the cerebellum (Fig. 1094, cb'. cb".) is very large; it 



