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- 

 tinuous anteriorly 

 with the paired cere- 

 bral hemispheres 

 (159, Fig. 49): 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. 

 ol) each connected 

 with the prosen- 

 cephalon by a short, 

 stout stalk {Tro) and 

 applied distally to 

 the corresponding ol- 

 factory capsule. The 

 prosencephalon contains paired lateral ventricles, which 



-F.rho 



mi 



Fig. 115. — Dorsal view of the brain of Scylliuvi 



caniculd. 

 The posterior division of the brain is the medulla 

 oblongata (JV/f), enclosing the fourth ventricle 

 i^F.rho). The large cerebellum {HH') nearly 

 covers the optic lobes (^MH ) The diencephalon 

 (JZH') shows in the middle the third ventricle, 

 and the place of attachment of the pineal stalk 

 (G/>). The prosencephalon {VH') gives off the 

 olfactory lobes (JFro. L.ot). The origins of 

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

 trochlear (/K), trigeminal (F), facial (F//), 

 auditory (^///), glossopharyngeal {IX), and 

 vagus Cx). (From Wiedersheim's Anatomy.) 



