TELEOST OR POLYPTERUS ACIPENSER 



Figure 13-5. Cross sections of cerebral hemispheres of different vertebrates with suggested path- 

 ways of evolution. 1, hippocampus; 2, general pallium (neopallium); 3, pyriformts; 4, lateral olfac- 

 tory nucleus (striate body); 5 tuberculum olfactorium (polaeopollium); 6, septal nuclei; ventricular 

 layer of nuclei cross hatched. (Moinly after Rudebeck, 1945) 



CHIMAERID 



fore the cerebral lobes. Basically the cerebral lobes are ol- 

 factory lobes, but have incipient higher centers. 



The diencephalon has a pineal evagination which generally 

 lacks an eye at its tip; a pineal nerve is lacking. In the frog a 

 pineal eye is present and the pineal sac has in part lost con- 

 nection with the brain (Figure 13-7). Anterior to the pos- 

 terior commissure is a saccus dorsalis which in front culmi- 

 nates in a paraphysis. Paraphysis and roof are much 

 thickened as a chorioid plexus from which fingers of tissue 

 extend down into the third ventricle and through the 

 foramina of Monroe into the telencoels. A distinct velum 

 transversum is lacking. 



The optic nerves decussate in entering the brain and some 

 fibers of each optic tract synapse at a lateral geniculate 

 nucleus with neurons leading to the cerebrum. The reduced 

 eyes o{ Necturus are reflected by the relatively smaller optic 

 lobes, as compared with the frog. 



The amphibian brain is peculiar in having a greatly re- 

 duced cerebellum which is no more than a transverse band 

 of tissue anterior to the chorioid plexus in the roof of the 

 medulla. 



The cranial nerves of the amphibian resemble those of the 

 preceding groups (Figure 13-8). The seventh has a super- 

 ficial ophthalmic branch in salamanders but not in anu- 



THE CONDUCTING AND INTEGRATING SYSTEM • 389 



