DEVELOPMENT OF THE BRAIN 539 



Nervous system. — This includes ^ — (a) the central 

 nervous system, consisting of brain and spinal cord ; {b) the 

 peripheral system, consisting of spinal and cranial nerves ; 

 and (c) the sympathetic nervous system. 



The central nervous system first appears as a superficial 

 groove along the mid-dorsal fine of the embryo. The sides 

 of this ectodermic groove meet, and, uniting, convert the 

 medullary groove into the medullary canal. The greater 

 part of this canal forms the spinal cord ; the anterior 

 portion of it is specialised as the brain. There is at first 

 a posterior connection between the neural canal and the 

 primitive gut of the embryo ; when this is lost the cavity of 

 the neural tube still persists as a little ciliated canal in the 

 centre of the cord, and as the internal cavity of the brain. 



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 pro- 

 cesses of growth and constriction, these three form the five 

 regions of the adult brain. 



When first formed the brain vesicles He in a straight hne, 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. Cvclostomata, 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- 

 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. 



