Take a point 1/2 inch behind the centre of a line drawn across the vertex of the skull from the nasion to the inion, and from this point draw a line downwards and forwards for 3 1/2 to 4 inches, at an angle of 67 1/20; (three-quarters of a right-angle) to the median antero-posterior line. In front of this sulcus is the precentral or ascending frontal convolution, an area better known as the Rolandic or motor area. The main centres here situated correspond, from above downwards, to the movements of the lower extremity, upper extremity and face of the opposite side of the body. The superior temporal crest cuts across the Rolandic line at the junction of its lower and middle thirds. (Fig. I, 5.)
It may be regarded as the line of demarcation between the upper extremity area above and the face area below.
On the left side of the head, that part of the brain which is included in the obtuse angle between the anterior and posterior horizontal limbs of the Sylvian fissure is known as Broca's area (the motor speech centre). The middle meningeal artery, a branch of the internal maxillary, enters the skull through the foramen spinosum, and divides, after a short and variable course across the middle fossa of the skull, into two main trunks. The seat of bifurcation usually corresponds to a point just above the centre of the zygoma. (Fig. II, 3.)
The anterior branch is not only the larger of the two, (Fig. II, 4.) but it is also more liable to injury, since it is protected in the temporal region by a comparatively thin osseous barrier.
(Fig. II, 5.) The danger zone in the course of this branch may be mapped out by taking three points:
(1) 1 inch behind the external angular frontal process, and 1 inch above the zygoma.
(2) 1 1/2 inches behind the external angular frontal process, and 1 1/2 inches above the zygoma.
(3) 2 inches behind the external angular frontal process, and 2 inches above the zygoma.
A line uniting these three points indicates, therefore, that part of the anterior division of the middle meningeal artery which is most liable to injury.
The anterior division of the vessel will be exposed by trephining over any of these three points, but it is generally preferable to choose the highest point, as by such means the posterior border of the great wing of the sphenoid is avoided ;* and, as an additional reason, it should be added that, in the position of points 1 and 2, the artery frequently runs in an osseous canal. After trephining over the upper point, the bone can be chipped away in a downward and forward direction, if such an exposure of the artery is deemed necessary.
The posterior branch of the artery passes almost (Fig. II, 6.) horizontally backwards, parallel to the zygoma and to the supramastoid crest, and it can be exposed by trephining over a point where a vertical line drawn (Fig. II, 7.) upwards from the posterior border of the mastoid process cuts another line drawn backwards from the supra-orbital margin parallel to Reid's base line.
This point will be better understood by reference to the base of the skull.
The descending cornu of the lateral ventricle (Fig. II, 8.) may be tapped by trephining inches above Reid's base line and 1 1/4 inches behind the external auditory meatus. The trocar should be directed towards the summit of the opposite ear, the ventricle being reached within 2 inches from the surface (Keen).