THE CKANIUM. 



1363 



fourth ventricle may be opened up by making a somewhat larger trephine opening 

 and separating the posterior extremities of the ton sillar lobes of the cerebellum. 



To expose a hemisphere of the cerebellum, trephine over the centre of a line drawn 

 from the tip of the mastoid process to the external occipital protuberance. 



To expose both hemispheres of the cerebellum a flap is turned downwards by 

 carrying a curved incision between the bases of the mastoid processes, the centre of 

 the incilion reaching upwards to a little above the inion. The occipital arteries are 



FIG. 1069. CRANIO-CEREBRAL TOPOGRAPHY. 



Guiding lines (Chiene's), deep black ; sutures, fine black ; meningeal arteries, red ; sulci, blue. 

 G. Glabella. C. Mid-point of AB. 



0. Extemal occipital protuberance (inion). CD is drawn parallel to AM. 



Mid-point between G and 0. Z. Post-auricular point. 



Mid-point between M and 0. VW. Guide to anterior limit of transverse sinus. 



Mid-point between T and 0. 

 Zygoraatic process of frontal. 

 Root of zygoma (pre- auricular point). 

 Mid- point of EP. 

 Mid-point of PS. 



Y. Tympanic antrum. 



X 1 . Site at which subarachnoid space may be opened. 



X 2 . Site for draining lateral ventricle (Kocher). 



X 3 . Site for draining lateral ventricle (Keen). 



divided, but the anterior extremities of the incision should, if possible, be kept 

 behind the mastoid emissary veins. As the flap contains the suboccipital muscles 

 the bone itself may be removed. If more room is required, the opening in the 

 bone may reach above the level of the transverse sinuses without wounding them 

 as they can be displaced along with the dura. The occipital sinus is divided 

 between two ligatures. After dividing the dura a hemisphere of the cerebellum 

 may be displaced towards the median plane to enable the finger to be passed 



