442 ZOOLOGY SECT. 



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

 cular membrane, the choroid plexus (ch.pt.}] this passes inwards to 

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

 the foramen of Monro (f. in.), which opens behind into the diacoele. 

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

 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 commissure (a. co.) situated in front of 

 the anterior pillars of the fornix. 



The diaccele (v 3 .) is a laterally compressed cavity, the roof of 

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

 positum (vl.ip.). On the upper surface of this is a network of 

 blood-vessels continuous with the choroid plexuses of the lateral 

 ventricle. From the posterior part of the roof of the diacoele arise 

 the peduncles of the pineal body, and just behind their point of 

 origin is the posterior commissure (p. co.), a delicate transverse 

 band of fibres connecting together the posterior parts of the optic 

 thalami. The latter (p. th.) are large masses of mixed gray and 

 white matter forming the lateral portions of the diencephalon ; 

 they are connected together by a thick mass of gray matter, the 

 middle or soft commissure (m. co.) passing across the diacoele. A 

 rounded elevation near the anterior end of the external surface of 

 each thalamus is the corpus geniculatum (c. gn.). Between the 

 optic thalamus and the corpus striatum is a thin band of white 

 matter the tcenia semicircular is (t.s.). The anterior boundary of the 

 diaccele 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 downwards 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 curved 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 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. 1 , o.l?), the corpora quadrigemina are pro- 

 duced. 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. 1031, cV. cb".) is very large ; it consists of a cen- 

 tral lobe or vermis and two lateral lobes, divided by very numerous 

 fissures or sulci into a large number of small convolutions. Each 

 lateral lobe bears an irregularly shaped prominence, the flocculus. 

 On section (Fig.l033,c&.) the cerebellum exhibits a tree-like pattern 

 (arbor mice) brought about by the arrangement of the white and 

 gray matter. On the ventral aspect of the hind r brain a flat band 

 of transverse fibres the pons Varolii connects together the 



