THE BRAIN. 169 



This heaping up of cells goes on ; they rise higher and 

 higher and finally coalesce over the gutter, converting it 

 now into a tube. During this time the anterior half of 

 the tube spreads laterally and so becomes larger than the 

 posterior portion. The dilated portion is the forerunner of 

 the brain (cerebrum, cerebellum, pons, and medulla), while 

 the long, slender hinder portion eventually forms the spinal 

 cord. 



We will confine these remarks to the anterior portion of 

 the primitive cerebrospinal tube. 



The next change is the production of three distinct swell- 

 ings in the brain end of the tube, these swellings being pro- 

 duced by the dilatation of the central canal (internally) and 

 the heaping up of cells (externally) opposite the dilatations. 

 These dilatations are called vesicles, and are distinguished 

 as anterior, middle, and posterior. 



These three original vesicles are next increased to five 

 by an outgrowth from the anterior vesicle and a subdivision 

 of the posterior. 



The anterior extension enlarges forward, outward, up- 

 ward, and backward ; the outgrowth from the posterior vesi- 

 cle takes place upward, laterally, and backward. 



These five vesicles are now named the forebrain, inter- 

 brain (original anterior vesicle), the midbrain (middle vesi- 

 cle), hindbrain, and the afterbrain (both from the posterior 

 vesicle). From these vesicles all the complex structures of 

 the fully formed brain are developed by a process of thick- 

 ening here and there, thinning in other places, the pushing 

 out of portions here, the invagination there, and the flexions 

 or bending of the axis of the vesicles until the completed 

 brain is the result. 



The central canal of the primitive vesicles in the adult 

 brain constitutes the various ventricles. 



