XIII 



PHYLUM CHORDATV 



l.VJ 



terininatos in aslightlydibitcd extremity fixed to the iiieiiihranous 

 part of the roof of the skull. Projecting downwards iV(jiu its 

 floor are two rounded bodies, the lohi ivferim'es (UL), which 

 are dilated portions of the infundihuhtm. Behind these give 

 off a ihin-walled vascular outgrowth — the mcrufi vascidoaus (Sv.) 

 Attached to the infundibuhua and extending backwards from it 

 is a thin-walled sac— the jJiiuitnry hody or hyj)i)phyds cerebri {IfS), 

 having on its ventral surface a median tubular body attached at 

 its posterior end to the Hoor of the skull. In front of the infun- 

 dibulum, and also on the lower surface of the diencephalon, is the 

 optic cltiasma, formed by the decussation of the fibres of the two 

 optic nerves. The mid-brain {MB) consists of a pair of oval optic 

 lobes dorsally, and ventral ly of a band of longitudinal nerve-fibres 

 corresponding to the crura cerebri of the higher vertebrate b)-ain. 

 The cerebellum {HH) 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 {NH), 

 broad in front, narrows posteriorly to 

 pass into the spinal cord. T\\e fourth 

 ventricle or fossa rhomboidalis (F. rho. ) 

 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 rcsti- 

 formia. 



The fourth ventricle or metaccelc 

 (Fig. 822, mcta.) is continuous behind 

 with the central canal of the spinal 

 cord. It gives off an epicoele abov«, 

 and in front is continuous with a 

 narrow passage, the iter or mesocmle 

 (iter.), which opens anteriorly into a 

 wider space, the diacoele or tliird ven- 

 tricle (dice), occupying the interior of 

 the diencephalon. From this opens 

 in front a median prosoccele, which 

 gives off a pair of paracceles {para.) 

 extending into the two lateral portions 

 of the prosencephalon. 



From the anterior enlargements of the olfactory bulbs already 

 mentioned spring numerous fibres which constitute the first pair 

 of cerebral nerves and enter the olfactory capsules. Between the 



Fig. 822.— Hemiscyllium. The 



brain viewed fmni tlie dorsal side, 

 the roofs of the various ventricles 

 removed so as to show the relations 

 of the cavities (semi-diagrammatic). 

 cer, dilatation from which the epi- 

 ccBle is given off ; dia, diacfele, 

 pointing to the opening leading into 

 the infundibnlum ; iter, iter or 

 niesoccele ; nieta. metaccele ; opt. 

 optocoele ; para, paracrele ; prox. 

 prosocojle ; rh. rhinoccele. 



