460 



THE DOGFISH 



CHAP. 



Apart from the large size of the cerebellum, the most 

 marked difference between the brain of the dogfish and 

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



frog, the dience- 

 phalon is continu- 

 ous anteriorly with 

 the paired cerebral 

 hemispheres ; in the 

 dogfish there is in 

 this region a rela- 

 tively smaller, 

 unpaired portion of 

 the brain, marked 

 in front by a slight 

 groove, and known 

 as the prosence- 

 phalon (Pros) , which 

 represents the cere- 

 bral hemispheres of 

 the higher Verte- 

 brates but which 

 does not become sub- 

 divided externally 

 into paired lobes. 

 Anteriorly it gives 

 off, right and left, a 

 large, oval olfactory 

 lobe (olf. /), each 

 connected with the 

 prosencephalon by 

 a short, stout stalk 



and applied distally to the corresponding olfactory cap- 

 sule. The prosencephalon contains paired lateral ventricles, 

 which communicate posteriorly with the third ventricle 

 and anteriorly are continued into the olfactory lobes. 



FIG. 124. Dorsal view of the brain of Sycllium 



canicula, (x ij.) 



The posterior division of the brain is the medulla 

 oblongata (Med. obi.), enclosing the fourth 

 ventricle (i). The large cerebellum (Cb) 

 nearly covers the optic lobes (Opt. I). The 

 diencephalon (Di) shows, in the middle, the 

 third ventricle, and the place of attachment of 

 the pineal stalk (pin). The prosencephalon 

 (Pros) gives off the olfactory lobes (olf. I). 

 The origins of the following nerves are 

 shown : optic (//), trochlear (IV), trigeminal 

 (V), facial (VII), auditory (VIII), glosso- 

 pharyngeal (IX), and vagus (X). (From 

 Wiedersheim's Comp. Anatomy.) 



