XIII 



PHYLUM CHOKDATA 



457 



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

 between the corpus callosum above and the fornix below is the 

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

 are the posterior pillars of the fornix (Fig. 1106, p.fo.) ; the anterior 

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

 anterior end of the body downwards to the corpus mammillare 

 (see below) at the base of the diencephalon. 



Lying immediately in front of the hippocampus is a vascular 

 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 (f. m.), which opens 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 

 anterior commis- ty 



sure (a. co.) situ- 

 ated in front of 

 the anterior pillars 

 of the fornix. 



The diacoele^. 3 ) 

 is a laterally com- J' m ' 

 pressed cavity, the 

 roof of which is 

 formed by a deli- 

 cate vascular 

 membrane, the 

 velum interpositum 

 (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 

 diaccele rise 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 connected 

 together by a thick mass of grey matter, the middle or soft commis- 

 sure (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 geniculatum (c. gn.). 

 The anterior boundary of the diacoele is a thin vertical lamina 

 the lamina terminal-is of which the septum lucidum is a mesial 

 anterior prolongation. The floor of the diencephalon is produced 

 downward into a mesial rounded process, the tuber cinereum or 

 infundibulum (inf.), to which the pituitary body is attached. In 

 front of this, on the ventral aspect of the brain, is a thick transverse 



VOL. II F F 



olf. 



O.CO 



1 o.ch.\ in f t 



a.fo. 



FIG. 1107. Lepus cuniculus. 



frma^ 



c.c. \ 

 m.co. pvcL. 



r.vn 



Longitudina Ivertical section of 



the brain (nat. size). Letters as in preceding figure ; in addi- 

 tion cb. cerebellum, showing arbor vite ; c. c. crus cerobri ; 

 c. h 1 . parencephalou ; c. h' 2 . temporal lobe ; c. ma. corpus 

 mammillare ; /. in. foramen of Monro ; inf. infundibulum ; 

 ly. psalterium or lyra ; m. o. medulla pblongata ; o. ch. optic 

 chiasma ; olf. olfactory bulb ; ply. pituitary body ; p. va. puns 

 Varolii; vl. if. velum iuterpositum ; v. vn. valve of Vieussi-ns; 

 //, optic nerve. (From Parker's Zootomy.) 



