BRAIN. 



439 



of growth and constriction, these three form the five regions 

 of the adult brain. 



When first formed the brain vesicles He in a straight line, but as a 

 consequence, probably, of their rapid and unequal growth, this condition 

 is soon lost, and a marked cranial flexure is produced. In the lower 

 forms, e.g. Cyclostomata, the flexure is slight, and is corrected later, 

 but in the higher types it is very distinct, and causes the marked over- 

 lapping of parts so obvious in the adult. 



We must now follow the metamorphosis of the primary 

 brain vesicles. 



The first vesicle gives rise anteriorly to the cerebral hemi- 



S0> 



NO 



Fig. 187. — Longitudinal section of brain of young dog-fish 

 (diagrammatic). — After Gaskell. 



C.h., Cerebral hemispheres; o.th., optic thalarai ; 3 V., third 

 ventricle; In., infundibulum : pt.b., pituitary body; o.l, optic 

 lobes; cb., cerebellum; M.O., medulla oblongata; 4 V., 

 fourth ventricle ; S.C., spinal cord. 



spheres, while the remainder forms the region of the optic 

 thalami or thalamencephalon. 



The cerebral hemispheres (prosencephalon or fore-brain) 

 are exceedingly important. They predominate more and 

 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 — 

 the 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 



