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).
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.
Technique.—The puncture may be made with the patient either lying on his left side, the spine being fully flexed by approximating the knees and shoulders; or sitting on the table with the knees drawn up and the body bent forward. The upper edge of the fourth lumbar spine is identified by drawing a horizontal line across the back at the level of the highest part of the iliac crests ([Fig. 183]). The space between the fourth and fifth lumbar vertebræ being the widest, is that usually selected. The skin having been purified, an exploring needle, about three inches long, is introduced about half an inch below the fourth lumbar spine in the middle line, and passed for about two inches in a direction forwards and slightly upwards. The needle usually encounters some resistance as it pierces the interspinous ligament, and then enters the sub-arachnoid space. If bone is struck, the needle should be withdrawn and introduced at a different level. If the cerebro-spinal fluid does not escape at once, a stylet should be passed through the needle to clear it of blood-clot or shreds of tissue. When the intra-thecal tension is normal, the fluid trickles away drop by drop, but if it is increased, as, for example, in meningitis, intra-cranial tumour, hydrocephalus, or uræmia, it may escape in a jet.
Fig. 183.—Localisation of site for introduction of needle in Lumbar Puncture.