This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The conformation of the various fissures and convolutions varies so much within normal limits that it is not possible to outline them on the surface of the scalp or skull with absolute exactness. The various lines which are laid out to indicate their course are, therefore, only approximate, but they are sufficiently accurate for operative purposes. To allow for variations, the openings made are usually large, and the motor areas are sometimes identified by the application of an electrode.
To indicate the course of the Sylvian fissure, a line is drawn from the external angular process of the frontal bone through a point 2 cm. (3/4 in.) below the most prominent part of the parietal eminence and ending 1.5 cm. (5/8 in.) above the lambda. The main portion of the Sylvian fissure begins 2 cm. (3/4 in.) behind the angular process; 2 cm. farther back or 42 mm. (1 5/8 in.) behind the angular process is the Sylvian point, where the anterior horizontal and anterior ascending limbs are given off. From this point the posterior horizontal limb passes backward to 2 cm. ( 3/4 in.) below the highest point of the parietal eminence and then curves upward and backward for a distance of 1.25 cm. to 2 cm. (1/2 to 3/4 in.).
The line of the central fissure begins at the upper Rolandic point, 1.5 cm. (5/8 in.) behind the middle of a sagittal line passing from the glabella to the inion. It then passes down and forward at an angle of approximately 700 (67 1/2, Chiene) toward the middle of the zygoma (Le Fort) to end at the lower Rolandic point, where it intersects the Sylvian line. It is about 9 cm. (3 1/2 in.) long. The central fissure stops 1 cm. above the Sylvian line or fissure.
The position of this fissure is quite variable, an average being 1.5 cm. (5/8 in.) above the lambda, and extending 1.25 cm. (1/2 in.) out from the median line. It is about 6 cm. (2 1/4 in.) above the inion and on or below the line of the Sylvian fissure.