144 



THE DOGFISH 



marked difference between the brain of the dogfish and that 

 of the frog is seen in its anterior portion. In the frog, the 



diencephalon is con- 

 f-"^ tinuous anteriorly 



with the paired cere- 

 bral hemispheres 

 (159. Fig. 40): in the 

 dogfish there is in 

 this region a relatively 

 smaller, unpaired por- 

 tion of the brain, 

 marked in front by 

 a slight groove, and 

 known as the prosen- 

 cephalon ( FiY), which 

 represents the cere- 

 bral hemispheres of 

 the higher Verte- 

 brates but which does 

 not become subdi- 

 vided externally into 

 paired lobes. Ante- 

 riorly it gives off, 

 right and left, a large, 

 oval olfactory lobe (Z. 

 ('/ ) each connected 

 with the prosen- 

 cephalon by a short, 

 stout stalk {Tro) and 

 applied distally to 

 the corresponding ol- 

 factory capsule. The 

 paired lateral ventricles^ which 



P"lG. 115. — Dorsal view of the biaiii of ^cylliitiii 



canicula. 

 The posterior division of the brain is the medulla 

 oblongata (/V//), enclosing the fourth ventricle 

 (F. rho). The large cerebellum (////) nearly 

 covers the optic lobes (/T//-/ ) The diencephalon 

 (,?//) shows in the middle the third ventricle, 

 and the place of attachment of the pineal st.alk 

 {Gfi). The prosencephalon (^V/) gives off the 

 olfactory lobes {I'ro. L.o(). The origins of 

 the following nerves are shown; — optic (//), 

 trochlear (/T), trigeminal (F), facial {VII\ 

 auditory (K///), glossopharyngeal (/.^), and 

 vagus (A'). (From Wiedersheim's Anatotny.^ 



prosencephalon contains 



