Henry Gray (1825–1861). Anatomy of the Human Body. 1918.

only be regarded as approximately correct, yet they are all sufficiently accurate for surgical purposes. The longitudinal fissure corresponds to the medial line of the scalp between the nasion and inion. In order to mark out the lateral cerebral (Sylvian) fissure a point, termed the Sylvian point, which practically corresponds to the pterion, is defined 35 mm. behind and 12 mm. above the level of the frontozygomatic suture; this point marks the spot where the lateral fissure divides. Another method of defining the Sylvian point is to divide the distance between the nasion and inion into four equal parts; from the junction of the third and fourth parts (reckoning from the front) draw a line to the frontozygomatic suture; from the junction of the first and second parts a line to the auricular point. These two lines intersect at the Sylvian point and the portion of the first line behind this point overlies the posterior ramus of the lateral cerebral fissure. The position of the posterior ramus can otherwise be obtained by joining the Sylvian point to a point 2 cm. below the summit of the parietal eminence. The anterior ascending ramus can be marked out by drawing a line upward at right angles to the line of the posterior ramus for 2 cm. and the anterior horizontal ramus by a line of the same length drawn horizontally forward—both from the Sylvian point. To define the central sulcus (fissure of Rolando) two points are taken; one is situated 1.25 cm. behind the center of the line joining the nasion and inion; the second is at the intersection of the line of the posterior ramus of the lateral cerebral fissure with a line through the preauricular point at right angles to Reid’s base line. The upper 9 cm. of the line joining these two points overlies the central sulcus and forms an angle, opening forward, of about 70° with the middle line of the scalp. An alternative method is to draw two perpendicular lines from Reid’s base line to the top of the head; one from the preauricular point and the other from the posterior