148 



ZOOLOGY 



SECT. 



floor are two rounded bodies, the lobi inferiorcs (UL), which are 

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

 is a thin-walled sac foe pituitary l>ody or hypophysis cerebri (HS), 

 having a pair of thin- walled vascular lateral^ diverticula the 

 sacci vasculosi (S.v.), and having 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 (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 (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. The fourth 

 ventricle (F. rho) is a shallow space on 

 the dorsal aspect of the medulla ob- 

 longata covered over only by a thin 

 vascular membrane, the choroid plexus : 

 it is wide in front and gradually nar- 

 rows posteriorly. At the sides of the 

 anterior part of the fourth ventricle 

 are a pair of folded ear-shaped lobes, 

 the corpora resliformia. 



The fourth Ventricle (Fig. 776, 

 mcta.) is continuous behind with the 

 central canal of the spinal cord. In 

 front it is continuous with a narrow 

 passage, the iter (iter.), which opens 

 anteriorly into a wider space, the 

 diaccde or third ventricle (diet.) occupy- 

 ing the interior of the diencephalon. 

 From this opens in front a median 

 prosocosle, which gives off a pair ofpara- 

 ccelcs (para.) extending into the two 

 lateral portions of the prosencephalon. 

 From the anterior enlargements of 

 the olfactory lobes already mentioned 

 spring numerous fibres which consti- 

 tute the first pair of cranial nerves 

 and enter the olfactory capsules. 

 From the optic chiasma the two optic nerves (Fig. 775, //) run 

 outwards through the optic foramina into the orbits, eacu per- 

 forating the sclerotic of the corresponding eye and terminating 

 in the retina. The third, fourth, and sixth pairs of nerves have 



mela, 



FIG. TYBT^Chiloscyllium. The 



brain viewed from the 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- 

 coele is given off ; dia. diacoele, 

 pointing to the opening leading into 

 the infundibulum ; iter. iter or 

 passage between the diacoele and 

 the metacoele ; meta. metacrele ; opt. 

 optocoele ; para. paraco3le ; pros. 

 prosocosle ; rh, rhinocoele. 



