In this way practically all the points of the brain which are wanted for operative purposes may be mapped out. Thus the quadrilateral space MDCA contains the Rolandic area. MA represents the præcentral sulcus, and if it be trisected in K and L, these points will correspond to the origins of the superior and inferior frontal sulci. The pentagon ABRPN corresponds to the temporal lobe. The apex of the temporal lobe extends a little in front of N. The supra-marginal convolution lies in the triangle HBC. The angular gyrus is at B. A is over the anterior branch of the middle meningeal artery, and the bifurcation of the lateral or Sylvian fissure; AC follows the horizontal limb of the lateral fissure. The transverse or lateral sinus at its highest point touches the line PS at R (Fig. 181). The _fissure of Rolando_ or _central sulcus_ may be marked out by taking a point half an inch behind the mid-point (M) (Fig. 181), and drawing a line downwards and forwards for a distance of about three and a half inches, at an angle of 67.5° with the line GO. The angle of 67.5° can be readily determined by folding a square piece of paper on itself so as to make a triangle. The angle at the fold equals 45°. By folding the paper again upon itself in the same direction, the right angle of the paper is divided into four angles of 22.5° each. Three of these angles taken together make up the 67.5°. If the straight edge of the paper be placed along the sagittal suture with the angle of folding over the upper end of the fissure of Rolando, the folded edge falls over the line of the fissure (Chiene). [Illustration: FIG. 182.--To illustrate the site of various operations on the skull.] LUMBAR PUNCTURE Quincke, in 1891, first suggested the withdrawal of cerebro-spinal fluid from the theca in the lumbar region, as a means of relieving excessive intra-cranial tension in tuberculous meningitis, and to obtain specimens of the fluid for diagnostic purposes. The scope of the procedure, both as a therapeutic and as a diagnostic measure, has since been widely extended.