From this line (1-1) a vertical line, or one exactly perpendicular to the first, is drawn through the external auditory meatus. At the top of head this line—the auriculo-bregmatic line (A A)—indicates the bregma or true vertex, which important landmark is to be marked with carmine or aniline upon the scalp. Upon the top of the head an imaginary horizontal line (4-4), parallel with the alveolo-condyloid plane, is projected, and upon it we measure backward a distance of 50 mm. (2 inches), and then draw a second vertical line (B B). At the point where this line reaches the convexity of the head is the Rolandic point (R), under which, in average heads, lie the upper or posterior extremity of the fissure of Rolando, the upper ends of the pre- and postcentral gyri, and within the longitudinal fissure the paracentral lobe, which together constitute the cortical centre for the leg of the opposite side. The Rolandic point, thus determined, should be distinctly marked upon the shaven scalp.
A third horizontal line is next to be drawn from the external angular process of the frontal bone backward, parallel with Broca's plane. This line (2-2), which may be termed the fronto-lambdoid because its posterior extremity usually passes at or near the upper angle of the lambdoidal suture, serves for the determination of several important parts. In the first place, the line (2-2) passes at about 5 mm. above the upper border of the squamous suture, or through it, and under this line, mostly parallel to it, are the anterior two-thirds of the fissure of Sylvius. Secondly, at about 5 mm. above and behind the intersection of lines (A A) and (2-2) is the inferior extremity of the fissure of Rolando, bounded by the pre- and postcentral gyri. In the third place, upon this line (2-2), at a distance of 18 or 25 mm. behind the external angular process, is the folded part of the base of the third frontal gyrus or Broca's speech-centre (F 3).
FIG. 13.
Topographical Lines applied to Henle's Figure of the Skull. Location of the Rolandic point. Rolandic line, motor-centres, and the branches of the middle meningeal artery.
Having exactly determined and marked the situation of the Rolandic point and of the inferior termination of the fissure of Rolando above the ear, these two points are to be connected by a line, which is represented on the diagram by a black bar. This, the Rolandic line, is the guide for nearly all operations for the relief of spasm or paralysis of traumatic origin, or of such as may be caused by tumors, abscesses, etc. It is surrounded by the so-called motor centres for the face, arm, and leg (?) as demonstrated by experiments upon animals and by now numerous autopsies in human cases of injury and disease.
Other relations of interest are the apex of the temporal lobe, a little beneath the line (2-2) and at about 10 to 15 mm. posterior to the external angular process of the frontal bone; the situation of the occipito-parietal fissure, almost immediately under the posterior end of the line (2-2) at its junction with the lines (E E), which indicate the posterior extremity of the cerebrum; the anterior end of the brain being marked off by the line (D D).
Furthermore, for certain purposes the limits of the basal or central ganglia may be estimated as follows: Their superior limit is indicated by a horizontal line (3-3) drawn at 45 mm. below the vertex, their anterior limit corresponding to the head of the nucleus caudatus by the vertical line (C C), and their posterior limit, the hinder end of the thalamus, by the vertical line (B B).