254 AN INTRODUCTION TO ZOOLOGY 



myelencephalon, is thickened and gives off a large hollow outgrowth, 

 the cerebellum. This is an oval structure with pointed ends, and 

 marked by a slight median furrow ; in front it projects freely over the 

 optic lobes and behind over the roof of the fourth ventricle. Its 

 internal cerebellar ventricle or metaccele opens into the front end of 

 this ventricle. It is connected to the medulla by a solid tract of 

 nervous tissue on each side, the cerebellar peduncle. The succeeding 

 part of the brain is the myelencephalon or medulla oblongata, and, 

 like the thalamencephalon, its roof is composed of ependymal 

 epithelium with which the pia mater is closely associated, and the 

 joint membrane so formed is richly supplied with blood-vessels 

 forming the choroid plexus of the fourth ventricle. The ventral 



OL 



c.a 



LL 



L.V 



SV 



FIG. 83. -Longitudinal section of brain of Scyllium slightly out of median 



plane. 



A.C., anterior choroid plexus ; A. P., anterior lobe of pituitary body ; C., cerebrum; CB., cere- 

 bellum ; Cr., crus ; F.M., foramen of Munro ; I., infundibulum ; It., iter; L.L., lobus lineae 

 lateralis ; L.T., part of lamina lerminalis ; L.V., lobus visceralis ; M., medulla oblongata ; O., 

 opening to cavity of lobus inferior; O.L., olfactory lobe; Op., optic lobe; O.X., optic chiasma ; 

 P., pineal stalk ; P.B., pineal body ; P.C., posterior choroid plexus ; P. Co., posterior commissure ; 

 P.O., pre-optic recess; P.P., posterior lobe of pineal body; S.C., superior commissure; S.V., 

 saccus vasculosus ; T., thalamencephalon ; T.A., tuberculum acusticum ; V.T., velum transversum ; 

 III. and IV., third and fourth ventricles. 



and lateral walls of the ventricle are greatly thickened, and contain 

 a large number of ganglion cells which are related to the 5th, 7th, 

 8th, Qth and loth cranial nerves. The antero-lateral walls are 

 continued on each side to form the characteristic wing-shaped 

 corpora res tif or mia orrestiform bodies, passing forward just beneath 

 the hinder end of the cerebellum. In the inside walls of the ventricle 

 are three well-marked projections, the tuber acusticum, containing 

 centres related to the auditory fibres ; above it, the lobus lineae 

 lateralis, related to the nerves of the lateral line, and below it the 

 lobus visceralis. These two ridges are readily seen if the brain is cut 

 in median longitudinal section. The medulla passes over imper- 

 ceptibly into the spinal cord. 



The cerebrum in Scyllium, as in all lower Chordata, is mainly 



