BRAIN. 



153 



aqueduct ending blindly in the mid-brain, in front of which is only the third ven- 

 tricle, completely cut off from the rest. The brain of Bdellostoma (fig. 153) 

 differs from this in several respects. 



ELASMOBRANCHS (figs. 154, 167) usu- 

 ally have the brain somewhat compact, but in 

 a few it is long and slender. The more strik- 

 ing features are the slight development of the 

 intercerebral fissure, the large hemispheres be- 

 ing lateral expansions just in front of the dien- 

 cephalon. The optic lobes are large and the 

 large cerebellum overlaps both lobes and the 

 fossa rhomboidea. The olfactory lobes arise 

 from the antero-lateral angle of each hemi- 

 sphere; their length varies between wide limits. 

 The epithelial roof of the 'twixt-brain is wide 

 and bears a pinealis which often reaches the roof 

 of the skull, but the parietal organ is lacking. 

 The hypophysis and infundibulum are pro- 

 vided with large inferior lobes and a well devel- 

 oped saccus vasculosus. The cerebellum has a 

 longitudinal groove and usually one or more 

 transverse grooves, dividing the upper surface 

 into paired lobes. The medulla differs in the 

 sharks and the skates, being very short in the 

 latter, much longer in the former. In both the 

 corpora restiformia are large folds on either _\x 

 side of the cerebellum, in front of and lateral 

 to the fossa rhomboidea. 



In most elasmobranchs-the ventricular sys- 

 tem is well developed, but in some the paired 

 and third ventricles are not well separated, 

 while in the Myliobatidae there is no cavity in 

 the cerebrum. There is a large epiccele ex- 

 tending upward from the aqueduct into the 

 optic lobes and a similar cavity usually enters 

 the cerebellum. 



TELEOSTOMES. There is a wide range FIG. 154. Brain of Heptanchus, 



of form in the brain of ganoids and teleosts. It afte r Gegenbaur. bo, bulbus olfac- 



. . .... tonus: c, cerebrum; cb. cerebellum; 



is usually small in proportion to the size of the em> em inentia teretes; /, infundibu- 

 animal and is noticeable for the small size of him; m, mesencephalon ; oo, olfactory 



the telencephalon and the usually non-nervous or Sf n j <*> olfactory tract; my myelen- 



cephalon; t. 'twixt-brain; II-X, era- 

 character of the pallium, which in the teleosts is ni a i nerves. 



purely epithelial. Consequently the cerebrum 



consists largely of the corpora striata and the intercerebral fissure is slightly de- 

 veloped. The paired ventricles are small, but they extend into the olfactory lobes. 

 The 'twixt-brain, at a lower level than the rest, has a large infundibulum, saccus 



