PHYLUM CHORD AT A. 



159 



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 lohi ivferiores (UZ), which 

 are dilated portions of the infundihulum. Behind these give 

 off a thin-walled vascular outgrowth — the saccus vasculosus (Sv.) 

 Attached to the infundihulum and extending backwards from it 

 is a thin- walled sac — the pihiitary body or hypophysis cerebri {ItS), 

 having on its ventral surface a median tubular body attached at 

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

 dihulum, 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 {MB) 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 {US) 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. The 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 firont 

 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 resti- 

 fovmia. 



The fourth ventricle or mttacmle 

 (Fig. 822, meta.) is continuous behind 

 with the central canal of the' spinal 

 cord. It gives oft' an epicoele above, 

 and in front is continuous with a 

 narrow passage, the iter or mesocmle 

 {iter.), which opens anteriorly into a 

 wider space, the diaccele or third ven- 

 tricle {dia), occupying the interior of 

 the diencephalon. From this opens 

 in front a median prosocmle, which 

 gives off a pair of paracodes {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 6nter the olfactory capsules. Between the 



meltx 



iG. 822.— Hemiscyllium. The 



brain viewed from the dursal side, 

 tiie roofs of the various ventriples 

 removed so as to show the relations 

 of the cavities (semi-diagrammatic)., 

 cer, dilatation from which the epi- 

 ccele is given oflf ; dia, diacoile, 

 pointing to the opening loading into 

 the infundihulum ; iter, iter or 

 mesoccele ; meta. metacoile ; opt. 

 optoca3le ; ficird. puracoile ; jn os. 

 prosoc(ele ; rh. rhinoco3le. 



