2 THE NERVOUS SYSTEM 



and it subsequently gives rise to the whole of the nervous 

 system. 



From the lateral walls of the cephalic (or anterior) extremity 

 of the neural tube, which persists in the adult as the third 

 ventricle, two diverticula grow out, one on each side, and form 

 the cerebral hemispheres. The cavities of the diverticula persist 

 as the lateral ventricles and their connexions with the primi- 

 tive tube remain in the adult as the interventricular foramina 

 (of Monro) (Fig. 2). 



The fourth ventricle arises as a dilatation of the neural 

 tube, caudal to the lateral diverticula, and the part of the tube 

 immediately cephalad to this dilatation subsequently forms 



FIG. 2. Section through cephalad extremity of Neural Tube. 



A . Neural tube. B. Primitive interventricular foramen (of Monro). 



C. Developing lateral ventricle. 



the cerebral aqueduct (of Sylvius), which connects the fourth 

 ventricle with the third ventricle in the adult (p. 15). The 

 caudal (or posterior) portion of the tube persists as the central 

 canal of the spinal medulla (spinal cord). 



The ectodermal cells which line the neural tube undergo 

 specialisation. Some of them are converted into nerve-cells ; 

 others form the neuroglia, which constitutes the supporting 

 tissue of the nervous system ; while others form the ependyma, 

 which lines the whole of the interior of the ventricular system. 

 Over the caudal part of the roof of the fourth ventricle the 

 ectoderm gives rise only to ependyma, so that in this situation 

 the ependyma comes into direct contact with the overlying 

 pia mater (p. in). 



