THE NEURAL TUBE AND ITS DERIVATIVES 



consider the medulla oblongata as extending from the spinal cord to the mesen- 

 cephalon. It forms the ventral and lateral walls of the fourth ventricle; and 

 when the roof of this cavity has been removed these walls are seen to surround 

 a long and rather broad depression the fossa rhomboidea or floor of the fourth 

 ventricle which tapers caudally like the point of a pen (Fig. 9). 



The cerebellum forms an elongated mass the rostral end of which overhangs 

 the optic lobes, while the caudal extremity projects over the medulla oblongata 



/'Nasal capsule 



---O'factory bulb 



/ -Nervus terminalis 



..Olfactory tract 



/ Olfactory nerve N. I 

 ' / Rhinoccele 



, Lateral ventricle 



! ' 



Cerebral hemisphere. 



Interventricular for 

 Epiphysis 

 -Optic nerve N. II 



Thalamus- 



Optic lobes- 



-Trochlear nerve N. Ill 



-Cerebellum 



Lobus linecR lateralis 



Facial nerve N. VIL 



Acoustic nerve N. VIII. 



Tuberculum acusticunt 



Medulla oblongata- 



Glossopharyngeal nerve N. IX 



Medial longitudinal fasc 



Visceral lobe.__ 



-Vagtts nerve N. X. 



-Spinal cord.- 



&? Telencephalon 



-- - - -;- Third ventricle 

 ) Diencephalon 



< 

 ---- '-Mesoccele 



> Mesencephalon 



f Metencephalon 

 -j- Cerebellum 



(caudal part) 



i^ -I- Rhomboid fossa 

 V Myelencephalon 



Fig. 8. The brain of the dogfish, 

 Squalus acanthias, dorsal view. 



Fig. 9. The brain of the dogfish, 

 Squalus acanthias, with the ventricles 

 opened, dorsal view. 



(Fig. 8). Its dorsal surface is grooved by a pair of sulci arranged in the form 

 of a cross. It contains a cavity, a part of the original rhombencephalic vesicle, 

 which communicates with the fourth ventricle proper through a rather wide 

 opening (Fig. 11). Behind the cerebellum the fourth ventricle possesses a thin 

 membranous roof which was torn away in the preparation from which Fig. 8 

 was drawn. 



