THE BRAIN IN THE SEPARATE CLASSES 



165 



length varies between wide limits. The epithelial roof of the 'twixt-brain is 

 wide and bears a pinealis which often penetrates the roof of the skull, but the 

 parietal organ is lacking. The hypophysis and infundibulum are provided 

 with large inferior lobes and a well-de- 

 veloped saccus vasculosus. The cerebel- 

 lum 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 side of the cerebellum, in front of 

 and lateral to the fossa rhomboidea. 



In most elasmobranchs the ventricular 

 system 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 epicoele extending upward from the 

 aqueduct into the optic lobes and a similar 

 cavity usually enters the cerebellum. 



TELEOSTOMES.— There is a wide 

 range of form in the brain of ganoids and 

 teleosts. It is usually small in proportion 

 to the size of the animal and is noticeable 

 for the small size of the telencephalon and 

 the usually non-nervous character of the 

 pallium, which in the teleosts is purely 

 epithelial. Consequently the cerebrum 

 consists largely of the corpora striata (fig. 

 173) and the intercerebral fissure is 

 slightly developed. The paired ventricles 

 are small, but they extend into the olfac- 

 tory lobes. The 'twixt-brain, at a lower 

 level than the rest, has a large infundib- 

 ulum, saccus vasculosus and inferior lobes. 

 On its roof is a large pinealis which 

 reaches the skull in a few ganoids. The 

 parietal organ appears in the embryo and 

 soon degenerates; the paraphysis is usu- 

 ally well developed. The optic lobes are 

 large and are usually divided into two 

 hemispheres by a median groove, but this 

 occasionally is scarcely noticeable. The cerebellum is large, much larger than 

 appears from the surface, since a considerable part, the valvula, projects into 

 the ventricle of the mid-brain. In the medullary region there is sometimes an 

 enormous development of tlu lobes of the vagus (fig. 173). 



Fig. 172. — Brain of Heptanchus, 

 after Gegenbaur. bo, bulbus olfac- 

 torius; c, cerebrum; cb, cerebellum; 

 em, eminentia teretes; i, infundibu- 

 lum; m, mesencephalon; 00, olfactory 

 organ; oi, olfactory tract; my, mye- 

 lencephalon; t, 'twixt-brain; II-X 

 cranial nerves. 



