484 STRUCTURE OF V&RTEBRATA. 



process is completed, certain portions of the anterior region 

 of the medullary canal grow more rapidly than others, and 

 form the three primary brain vesicles. By further processes 

 of growth and constriction, these three form the five regions 

 of the adult brain. 



When first formed the brain vesicles lie 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. 



CH 



'OLF 



FIG. 258. Partial section of a Vertebrate brain (diagrammatic). 



OLF., Olfactory lobe ; CH. t cerebral hemispheres ; C., wall of cerebrum 

 cut to show ventricle, behind this the figure is that of a median sec- 

 tion ; PA., parietal organ arising from thalamencephalon ; /Y., 

 pineal organ ; INF. , infundibulum descending from thalamen- 

 cephalon ; H. t hypophysis; OL., optic lobes; CB., cerebellum; 

 CPL., choroid plexus on roof of fourth ventricle ; MO., floor of the 

 medulla oblongata ; CC., central canal of spinal cord. 



We must now follow the metamorphosis of the primary 

 brain vesicles. 



The first vesicle gives rise anteriorly to the cerebral hemi- 

 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 



