KII PHYLUM CHORDATA 377 



lateral region each half gives off a thick cord, which 

 dilates into a large mass of nervous matter, the olfactory lobe 

 (L. <?/.), closely applied to the posterior surface of the corre- 

 sponding olfactory capsule. The diencephalon (Zff) is com- 

 paratively small ; its roof is very thin, while the side walls and 

 floor are composed of two thickish masses the optic thalami. 

 Attached to the roof is a slender tube, the epiphysis cerebri 

 or pineal body (Gp.\ which runs forwards and terminates in 

 a slightly dilated extremity fixed to the membranous part of 

 the roof of the skull. Projecting downwards from its floor 

 are two rounded bodies, the lobi inferiores, which are dilated 

 portions of the infundibulum ; and attached to this, behind, 

 is a thin-walled sac \hzpituitary body or hypophysis cerebri^. 

 having a pair of thin-walled vascular lateral diverticula the 

 sacci vasculosi, and bearing on its ventral surface a median 

 tubular body attached at its posterior end to the floor of the 

 skull. In front of the infundibulum, and also on the lower 

 surface of the diencephalon, is the optic chiasma, formed by 

 the decussation of the fibres of the two optic nerves. The 

 mid-brain (MH) consists of a pair of oval optic lobes 

 dorsally, and ventrally of a band of longitudinal nerve-fibres, 

 corresponding to the crura cerebri of the higher vertebrate 

 brain. The cerebellum (ffff) is elongated in the antero- 

 posterior direction, its anterior portion overlapping the 

 optic lobes, and its posterior the medulla oblongata. Its 

 surface is marked with a few fine grooves. The medulla 

 oblongata (Nff), broad in front, narrows posteriorly to pass 

 into the spinal cord. The fourth ventricle (F. rko.) is a 

 shallow space on the dorsal aspect of the medulla oblongata 

 covered over only by a thin vascular membrane, the choroid 

 plexus : it is wide in front and gradually narrows posteriorly. 

 At the sides of the anterior part of the fourth ventricle are a 

 pair of folded ear-shaped lobes, the corpora restiformia. 



