THE NERVOUS SYSTEM 



487 



Elasmobranchs. The nervous system of elasmobranchs shows some 

 advances above that of cyclostomes. The roof or pallium of the telen- 

 cephalon has thickened and expanded. To the corpus striatum is added 

 an epistriatum, both connected with olfactory fibers. The telencephalon 

 remains predominantly an olfactory center. Elongated olfactory tracts 



Nasal capsule 



/Olfactory rrerve N. I 

 ' / Rhinoccele 



Lateral ventricle 



" Olfactory bulb 

 Nervus terminalis 

 ->-- Olfactory tract- 



~ Cerebral hemisphere- 



I ntervenlricular for. 



Y Epiphysis 



Optic nerve N. II 

 ' -- - Thalamus 



-Optic lobes 



*^ Trochlear nerve N. I\' 



-Cerebellum 



■Lcbus linecB lateralis. 

 Facial nerve N. VII 



Acoustic nerve N. VI IT.., 



Tuberculum acusticum 



Medulla oblongata 



Glossopharyngeal nerve N. IX 

 ■—Medial longitudinal fasc. ... 

 -Visceral lobe 



Vagus nerve N. X 



Spinal cord 



Telencephalon 



\ Third ventricle 

 > Diencephalon 



'-Mesoccde 

 ^Mesencephalon 



y ^Fourth ventricle 



Metencephalon 



'.^■Cerebellum 



(caudal part) 



4 Fourth ventricle 

 \ Myelencephalon 

 > 



Fig. 402. — 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. 403. — 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.) 



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

 pressure organ, is appended to the infundibulum. The midbrain has 

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

 nervous. With the increased importance of the lateral line organs, their 

 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. 



