BRAIN. 



467 



more as we ascend in the scale of Vertebrates, and become 

 more and more the seat of intelligence. Except in a few 

 cases, the prosencephalon is divided into two parts- 

 trie cerebral hemispheres which contain cavities known as 

 the lateral ventricles. The two hemispheres are united by 

 bridges or commissures, which have considerable classifica- 

 tory importance. With the anterior region of the hemi- 

 spheres olfactory lobes are associated. 



In Cyclostomata, "Ganoids," and Teleosteans, the fore-brain has no 

 nervous roof, but is covered by an epithelial pallium homologous with 

 what is called the choroid plexus of the third ventricle in higher Verte- 



y.c. 



FlG. 226. Longitudinal section of brain of young dog-fish 

 (diagrammatic). After Gaskell. 



C./i., Cerebral hemispheres; o.th., optic thalami; 3 y., third 

 ventricle ; /., infundibulum ; //./>., pituitary body ; a./., optic 

 lobes; cl>., cerebellum; AI.O.. medulla oblongata ; 4 V., 

 fourth ventricle ; S.C., spinal cord. 



brates. This choroid plexus is a thin epithelium, with blood vessels in 

 it. But in Elasmobranchs, Dipnoi, and Amphibians the basal parts of 

 the fore-brain have grown upwards to form a nervous roof, and this 

 persists in higher Vertebrates. 



The optic thalami (thalamencephalon or tween-brain) 

 form the second region of the adult brain. Hence arise 

 the optic outgrowths, which form the optic nerves and 

 some of the most essential parts of the eyes. The 

 original cavity persists as the third ventricle of the brain ; 

 the thin roof gives off the dorsal pineal outgrowth or epi- 

 physis, and, uniting with the pia mater, or vascular brain 

 membrane, forms a choroid plexus ; the lateral walls 



