THE BRAIN 159 



the lamprey, and the thalamencephalon cannot be seen from the 

 dorsal side owing to the larger size of the solid prosencephalon. 

 The mesocoele ends blindly in. front, the third ventricle being 

 almost completely obliterated. The cerebellum is relatively larger 

 than in Petromyzon, and no pallium has been recognised in the 

 prosencephalon of the adult. 



In Petromyzon the pineal apparatus is represented by two 

 vesicles, each connected with the dorsal surface of the thalamen- 

 cephalon (ganglion habenulse) and lying one above the other just 

 beneath the roof of the skull; the integument immediately above 

 these vesicles is pigmentless. The cells on the ventral side of the 

 dorsal vesicle (epiphysis) are arranged radially and contain pig- 

 ment, forming a kind of:' retina, but they show signs of degenera- 

 tion ; the lower vesicle (parietal organ, p. 155) is without pigment. 

 In Myxine there is only a single pigmentless vesicle. 



A saccus vasculosus (comp. pp. 154, 160, ct seq.) is present in 

 connection with the iniundibulum, to which a small pituitary 

 body is attached. 



Elasmobranchii and Holocephali. The brain of these 

 Fishes, like that of Cyclostomes, is in many respects of a specialised 

 form, characteristic of, and confined to, the group, though the par- 

 ticular regions are much more highly developed than in the 

 Cyclostomi. According to its external form two main types can 

 be distinguished. One of these, seen in Spinax, Scymnus, Noti- 

 danus and the Holocephali, is characterised by its very narrow and 

 elongated form, while in the rest of the Elasmobranchii the indi- 

 vidual parts are more closely compressed and approximated together 

 (Fig. 1 30). In almost all Sharks the prosencephalon is relatively 

 much larger than any of the other regions. The olfactory lobes 

 arise from the anterior or antero-laterrd ends of the prosencephalon, 

 and in some Elasmobranchs remain in close connection with the 

 latter : in others in which the olfactory capsules are situated further 

 forwards, they become drawn out into long olfactory tracts each 

 continuous anteriorly with an olfactory bulb from which the olfac- 

 tory nerves arise. 



A division of the prosencephalon into paired halves is hardly 

 indicated at all in Rays, and only slightly so in the commoner Dog- 

 fishes (e.g., Scyllium, Acanthias), in which, however, lateral and 

 olfactory ventricles are present. Only in Scymnus, and to seme 

 extent in the Notidanida?, is there a distinct separation of the 

 pallium into two hemispheres. In Rays there is only a small 

 single prosoccele, the prosencephalon consisting of a practically 

 solid mass, and the olfactory lobes are also solid. 



The thalamencephalon is roofed over by a choroid plexus, and 

 the tube-like epiphysis may reach such a length as to extend beyond 

 the anterior end of the brain for a considerable distance, and pass 

 distally into the roof of the skull : no indication can be seen of a 

 parietal organ. A pair of small lobes the lobi inferiores are 



