THE BRAIN AND ITS MENINGES 227 



theory Stiles treats the condition by ligature of the common 

 carotid arteries (p. 119). 



Acquired Hydrocephalus may be caused by obstruction to 

 the outflow into the subarachnoid space (p. 226) by adhesions fol- 

 lowing meningitis, or it may be produced by pressure on the great 

 cerebral vein (of Galen). In the latter case the veins of the 

 choroid plexus become conges ted, and increased transudation 

 takes place through the ependyma into the interior of the ven- 

 tricles. In the former case the condition may be cured by mak- 

 ing a fresh opening in the roof of the fourth ventricle so as to 

 re-establish the communication between the ventricular system 

 and the subarachnoid space. In this operation an anchor- 

 shaped incision is employed. The curved base of the anchor 

 lies along the superior nuchal (curved) lines of the occipital bone, 

 and from its centre a vertical incision passes downwards along 

 the ligamentum nuchae. The knife is carried down to the bone 

 and the two flaps are reflected downwards and laterally. The 

 skull is opened below the level of the transverse sinus (p. 224) 

 and at the thinnest part of the occipital bone, to one side of the 

 median line. The opening is enlarged by removing the lower 

 portion of the squamous part of the occipital bone piecemeal, 

 including the posterior margin of the foramen magnum as far 

 forwards as the posterior edges of the condyles. During the 

 process considerable oozing occurs from injured diploic veins. 

 The occipital sinus is ligatured above and below, and the dura 

 mater is then opened along one side of the sinus. Horizontal 

 incisions are made at each end of this cut, and two flaps of dura 

 mater, one of which contains the ligated portion of the occipital 

 sinus, are turned laterally. This step exposes the postero- 

 inferior aspect of the cerebellum, covered by the arachnoid and 

 the pia mater. The cerebellum is pressed upwards, away from 

 the posterior aspect of the medulla oblongata (Fig. 69), and a 

 large subarachnoid cistern (cerebello- medullary) is brought 

 into view. After the cistern is opened, the lower part of the 

 bulging roof of the fourth ventricle, which consists of ependyma 

 and pia mater, is exposed, and the communication between the 

 ventricular system and the subarachnoid space may be re- 

 established. 



The operation of suboccipital decompression, as practised by 

 Gushing, follows the same course as far as the incision of the 

 dura mater. 



In evacuating a cerebellar abscess the same route is adopted. 



15 a 



