The Cerebrum. 437 



for the Sylvian fissure and terminating about three-quar- 

 ters of an inch behind and on the same level with the end 

 of the fissure of Sylvius. The middle tempero-sphenoidal 

 lies parallel with the latter line and about three-quarters 

 of an inch below it. 



Having located the lines for the fissures, the convolu- 

 tions lying between them can be readily appreciated. 



Operations. 7>/>/*wwg. The skull may be open- 

 ed by means of the trephine, the saw, or the osteotome, 

 and in turning down the flap for this purpose, it is advis- 

 able to bear in mind the general direction, situation and 

 landmarks for the supra-orbital, temporal and the occipi- 

 tal arteries, already referred to, so as to avoid, as far as 

 possible, interference with the circulation in the flap. 



Operation on the sigmoid sin us. The incision should 

 extend from the tip of the mastoid process over its promi- 

 nence to the beginning of the posterior root of the zy- 

 goma and the bone should be trephined three-quarters of 

 an inch behind the centre of the external auditory meatus 

 and along Reid's base line. Lateral sinus. One and a 

 quarter inches from the external meatus and about a quar- 

 ter of an inch above Reid's line. Tempero-sphenoidat 

 abscess. The pin of the trephine should be applied three- 

 quarters of an inch above Reid's line starting from the 

 posterior border of the meatus. For cerebellar abscess. 

 one and a half inches behind the centre of the meatus and 

 a quarter of an inch below the base line. Middle menin- 

 geal artery. Anterior branch, one and a half inches from 

 the external angular process and the same distance above 

 the zygoma. Posterior branch, one and three-quarter 

 inches from the external angular process and one-quarter 

 of an inch above the zygoma. Tapping the lateral rcntricle. 

 One and a half inches above the centre of the external 

 meatus. (Fig. 14). 



