THE BRAIN AND SPINAL COED. 



193 



Divisions of the Neural Canal (Fig. 159).— At the end 

 ' the 3rd week the neural tube is divided into four parts. They 

 e: 



(1) An anterior dilatation, the fore-brain, which forms the 

 rd and lateral ventricles and their walls. 



(2) The mid-brain, which becomes transformed into the aque- 

 lct of Sylvius and crura cerebri. 



(3) The hind-brain, the basis of the 4th ventricle, pons, cere- 

 ilium and medulla. 



(4) The central canal and spinal cord. 



3rd vent, 

 aqueduct Sylvius 



4th vent — 



central canal 



^f-fore-brain 

 optic uesic. 



mid-brain 



-hind brain 



-spinal cord. 



Fig! 159.— Diagram of the Four Primary Divisions of the Neural Tube. 



Cerebro-spinal fluid fills the canal. We do not know how it 

 secreted or absorbed, but under certain abnormal conditions it 

 y collect and give rise to a cystic condition of the neural tube. 

 3 expansion may involve the whole tube or only a part of it. 



N 



