BRAIN 



211 



oljl 



jars 



In Sharks, especially in Scymnus and the Notidanidae, the 

 medulla oblongata is elongated and cylindrical, while in Rays it is 

 more compressed and triangular ; at its anterior end are a number 

 of elevations corresponding to origins of the nerves arising from 

 the gray matter of the floor of the fourth ventricle in this region. 

 In electric Rays a pair of electric lobes (p. 192) are present at this 

 point, and these enclose a mass of giant nerve-cells. 



Ganoids. The pallium covering the median telocoele consists 

 mainly or entirely of epithelial and connective tissue elements, much 

 as in Cyclostomes ; and the telencephalon, 

 which may be produced dorso-laterally into 

 lobes (Fig. 159), gives rise anteriorly to 

 cerebral hemispheres containing lateral ven- 

 tricles and continuous with the olfactory lobes. 



The well-developed diencephalon has a 

 marked ventral flexure, and from its roof 

 arises a strong pineal peduncle, the distal 

 end of which extends into a hollow in the 

 cranial roof, but undergoes atrophy in Amia, 

 becoming completely separated off from the 

 brain. 1 Well-marked lobi inferiores are pre- 

 sent, and the hypophysis and saccus vascu- 

 losus are voluminous : the latter consists 

 largely of glandular tubules which open into 

 the infundibulum, as in Elasmobranchs. 



The optic lobes are well-marked in most 

 Ganoids. The large cerebellum gives rise to 

 a valvula cerebelli (cf. Fig. 161) extending 

 forwards into the ventricle of the mid-brain. 



Except that only the median wall of the 

 pallium is epithelial, the brain of Amia on 

 the whole most nearly approaches that of the 

 Teleosts in structure. 



FIG. 159. BRAIN OF 

 Lepidosteus. Dorsal 

 view. (After Balfour 

 and Parker.) 



cbl, cerebellum ; c.h, cere- 

 bral hemispheres ; di, 

 diencephalon ; m.o, 

 medulla oblongata ; 

 olf.l, olfactory lobes; 

 opt.l, optic lobes ; prs, 

 lobes of telencephalon. 



Teleosts. As is the case in many other 

 Fishes, the brain in most Teleosts by no 

 means fills the cranial cavity, and it is separ- 

 ated from the roof of the skull by a greater or less amount of a fat- 

 like tissue and lymph : it never attains to so large a relative size 

 as does that of Elasmobranchs. Its form varies greatly, more by 

 far than in any other Vertebrate group, and only the following 

 essential points can be mentioned here. 



1 In Polypterus and Calamichthys the pineal body gives rise to a peculiar 

 and extremely large epithelial vesicle, and the hypophysis communicates with 

 the mouth-cavity by a hollow duct, even in the adult. The brain of these 

 forms presents other special characters, and requires further investigation. In 

 Devonian C4anoids, as well as in the Placoderms, there was a parietal foramen 

 (p. 103). 



P 2 



