28 



THE NERVOUS SYSTEM 



Mesencephalon. The optic lobes on the dorsal aspect of the mesencephalon 

 are a pair of rounded masses separated by a median sagittal sulcus. They 

 represent the bulging roof of the mesencephalic cavity and are accordingly 



Cerebellum. ... 

 Optic lobe 



Thalamus 



Cerebral hemisphere 



Olfactory bulb 



v 

 Vagus nerve N. X / 



Glossopharyngeal nerve N. IX ' 



Acoustic nerve N. VIII / 

 Abducens nerve N. VI 



Olfactory tract 



> ! Optic nerve N. II 



\ Inferior lobe 

 Oculomotor nerve N. Ill 

 Saccus vasculosus 



Trigeminal and facial nerves Nn. V, VII TroMear nene N _ IV 



Fig. 10. The brain of the dogfish, Squalus acanthias, lateral view. 



spoken of as the tectum mesencephali. Within this roof end the fibers which 

 come from the retinae through the optic nerves. The floor of the cavity is formed 

 by the ventral part of the mesencephalon. This appears like a direct continua- 

 tion of the medulla oblongata, and in the mammal bears the designation crura 



Paraphysis 



Cerebral hemisphere j | 

 Olfactory tract 

 Olfactory bulb 



Optic lobe 



Epiphysis . Mesoccde 



Cerebellum 



Metacode 



Tuberculum acusticum 

 . Tela chorioidea 



Fourth ventricle 

 Visceral lobe 



Telencephalon ( ! 



Preoptic recess 

 Velum transversum 



\ Metencephalon Myelencephalon 

 Saccus vasculosus 



Mesencephalon 

 Optic chiasma Third ventric i e 



Fig. 11. The brain of the dogfish, Squalus acanthias, medial sagittal section. 



cerebri. Emerging from the roof of the mesencephalon between the cerebellum 

 and optic lobe is the fourth or trochlear nerve, and from the ventral aspect of 

 this division of the brain arises the third or oculomotor nerve. 



The Diencephalon. The thin roof of the diencephalon, which can easily 



