THE BRAIN. 435 



The Brain. 



At an early stage, even before the closing-in 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 overlapping 

 of parts so obvious in the adult. 



We must now follow the metamorphoses of the primary 

 brain vesicles. 



The first vesicle gives rise anteriorly to the cerebral hemi- 



FIG. 138. Longitudinal section of brain of young Dogfish 

 (diagrammatic). (After GASKELL.) 



CJt., Cerebral hemispheres; o.tk., optic thalami ; 3-F., third ven- 

 tricle ; /., infundibulum ; pt.b., pituitary body; o.L, optic lobes; 

 cb., cerebellum; M.O., medulla oblongata ; 4. K., fourth ventricle; 

 S.C., spinal cord. 



spheres, while the remainder forms the region of the optic 

 thalami or thalamencephalon. 



The cerebral hemispheres, also called 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 home of thought. Except 

 in a few instances, the prosencephalon is divided into two 

 parts the cerebral hemispheres which contain cavities 

 known as the lateral ventricles. The two hemispheres are 



