THE BRAIX AXD ITS MEMBRANES. 



791 



localizing the fissure with data for representing its length and direction. 1 Professor Thane 

 gives the lower end of the furrow as "close to the posterior limb, and about half an inch 

 behind the bifurcation of the fissure of Sylvius." So that, according to this anatomist, a line 

 drawn from a point half an inch behind the mid-point between the glabella and external occipi- 

 tal protuberance to this spot would mark out the fissure of Rolando. Dr. Reid adopts a differ- 

 ent method (Fig. 476). He first indicates, on the surface the longitudinal fissure and the hori- 

 zontal limb of the fissure of Sylvius (as above). He then^draws two perpendicular lines from 







. 



FIG. 47f>. Relations of the principal fissures and convolutions of the cerebrum to the outer surface of 

 scalp. (Reid.) 





his "base-line" (that is, a line from the lowest part of the infra-orbital margin through the 

 middle of the external auditory meatus to the back of the head) to the top of the cranium, one 

 (D E, Fig. 476) from the depression 'in front of the external auditory meatus, and the. other 

 (F o, Fig. 476) from the posterior border of the mastoid process at its root. He has thus 

 described on the surface of the head a four-sided figure (p D G E, Fig. 476), and a diagonal line 

 from the posterior superior angle to the anterior perpendicular line where it is crossed by the 

 fissure of Sylvius will represent the furrow. 



The parieto-occipital fissure on the upper surface of the cerebrum runs outward at right 

 angles to the great longitudinal fissure for about an inch, from a point one-fifth of an inch in 

 front) of the lambda (posterior fontanelle). Reid states that if the horizontal limb of the fissure 

 of Sylvius be continued onward to the sagittal suture, the last inch of this line will indicate the 

 position of the suleus. 



The precentral suJciis lies in a line drawn vertically downward from the point of junction of 

 the sagittal and coronal sutures. It begins four-fifths of an inch in front of the middle of the 

 fissure of Rolando, and extends nearly, but not quite, to the horizontal limb of the fissure of 

 Sylvius. 



The superior frontal fissure runs backward from the supra-orbital notch, parallel with the 

 line of the longitudinal fissure to two-fifths of an inch in front of the line indicating the position 

 of the fissure of Rolando. 



The inferior frontal fissure follows the course of the superior temporal ridge on the frontal 

 bone. 



The infra parietal fissure begins on a level with the junction of the middle and lower third 

 of the fissure of Rolando, on a line carried across the head from the back of the root of one 

 auricle to that of the other. After passing upward it curves backward, lying parallel to the 

 longitudinal fissure, midway between it and the parietal eminence ; it then curves downward to 

 en 1 midway between the posterior fontanelle and the parietal eminence. 



1 Lancet, vol. i., 1888, p. 408. 



