22 A MANUAL OF ANATOMY. 



The External Portion of the Parieto-occipital Fis- 

 sure. This is five-twelfths to five-eighths of an inch in front 

 of the lambda, or one and seven-eighths inches behind the 

 central extremity of the fissure of Rolando, and extends 

 about an inch outward and slightly forward on either side 

 of the median line. 



The Principal Sulci. 



The Frontal Lobe. The superior frontal sulcus is 

 indicated by a line starting from the supra-orbital notch (at 

 the inner and middle thirds of the supra-orbital arch), drawn 

 backward parallel with the median line until within an inch 

 of the fissure of Rolando. 



The inferior frontal sulcus coincides with the frontal part 

 of the temporal ridge (the bony not the muscular ridge). 

 It begins at the E. A. P., and follows the curve of the ridge 

 upward and backward until within an inch of the Rolandic 

 fissure. This sulcus is parallel with and about an inch 

 from the preceding one. 



The superior, middle, and inferior convolutions are 

 thus indicated, and the region in front of the fissure of Ro- 

 lando and an inch in width will correspond to the ascend- 

 ing frontal convolution. 



The Parietal Lobe. The intraparietal sulcus is shown 

 by drawing a line from a point one inch behind the fissure 

 of Rolando and the same distance above the horizontal 

 limb of the Sylvian fissure upward, one inch from and par- 

 allel with the first fissure, until at the junction of its middle 

 and upper thirds, where the line turns backward to run 

 parallel with the middle line, and about an inch and a quar- 

 ter from it, until it terminates a little behind the parieto- 

 occipital fissure in the occipital region. The space behind 

 the fissure of Rolando and in front of the first part of the 



