THE SPINAL MEDULLA 537 



herniated dorsal wall or roof of the fourth ventricle. When the 

 tumour occurs in the line of the frontal (coronal) suture it 

 contains a diverticulum from the lateral ventricle. 



The sac of the tumour is covered by skin, normal or thinned, 

 and it consists of arachnoid, save in simple meningoceles, where 

 the dura mater may be present. The sac is opened in order to 

 ascertain whether it contains any brain tissue (encephalocele or 

 hydr-encephalocele) or merely cerebro-spinal fluid (meningocele). 

 If brain tissue is found and cannot be reduced within the skull, 

 it may be ligated and removed. In suturing the cut edges of 

 the membranes care must be exercised to prevent subsequent 

 leakage of the cerebro-spinal fluid. 



