BRAIN 



211 



olf.l 

 c-.h 



prs 



In Sharks, especially in Scymnus and the Notidanidse, 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 teloccele 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 mfundibulum, as in Elasmobranchs. 



The optic lobes are well-marked in most 

 Ganoids. The large cerebellum gives rise to 

 a mlvuhi 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. 



opt. I 



cbl 



7TL.O 



FIG. 159. BRAIN OF 

 Lepidosteus. Dorsal 

 view. (After Balfour 

 and Parker.) 



rW, cerebellum ; c.h, cere- 

 bral hemispheres ; ill, 

 diencephalon ; m.o, 

 medulla oblongata ; 

 olf. /, olfactory lobes ; 

 opt. I, 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 (ianoids, as well as in the Placoderms, there was a parietal foramen 

 (p. 103). 



p 2 



