THE NERVOUS SYSTEM 



357 



remains predominantly an olfactory center. Elongated olfactory tracts 

 are differentiated. A saccus vasculosus, which possibly functions as a 

 pressure organ, is appended to the infundibulum. The mid-brain has 

 lost its chorioid plexus and its roof has become thickened and wholly 

 nervous. With the increased importance of the lateral-Hne organs, their 



Nasal capsule 



/Olfactory nerve A'. I 

 / Rhinocoele 



— Olfactory bulh 



Nervus terminalis 



Olfactory tract - - ■ 



Cerebral hemisphere- 



Interventricular for 



Epiphysis 



Optic verve N. II 



Thalamu: 



Optic lobes 

 Trochlear nerve N. IV 



i — Cerebellum 



■Lobus lineae lateralis 

 ■ Facial nerve N. VII - 



,^^ -J Medulla oblongata 



Glossopharyngeal nerve N. IX 



— Acoustic nerve N. VIII 

 Tuberculiim acusiicum 



■Medial longitudinal fasc. 

 Visceral lobe- 



^._. 



Vagus nerve N.X 



Third ventricle 

 Diencephalon 



Mesoccele 

 Mesencephalon 



■ Spinal cord 



^Fourth ventricle 

 Metencephalon 



/ 



\Cerebellum 

 '^ {caudal part) 



/Fourth ventricle 

 I Myelencephalon 



Fig. 315. — The brain of the dogfish, 

 Squalus acanthias, dorsal view. (From 

 Ranson's "The Anatomy of the Nervous 

 System," courtesy of W. B. Saunders 

 Company.) 



Fig. 316. — The brain of the dogfish. 

 Squalus acanthias, with the ventricles 

 opened, dorsal view. (From Ranson's 

 "The Anatomy of the Nervous System," 

 courtesy W. B. Saunders Company.) 



centers in the lateral lobes of the medulla are more developed. Possibly 

 for the same reason, the cerebellum, which is a static center, is greatly 

 enlarged. 



In the spinal cord, dorsal and ventral columns of gray matter are 

 differentiated. The dorsal column, however, remains unpaired. 



The cranial nerves are identical with those of cyclostomes but rela- 

 tively enlarged both in correlation with the increased size of sense organs 



