The surgeon who is called upon to carry out operations on the skull and brain must possess an accurate knowledge of the anatomy of the parts involved. Added to this, he must have at his command some simple method of depicting on the surface of the skull the more important structures.

The more complicated systems of cranio-cerebral topography are of little practical value to the surgeon. Simplicity is essential, and the following outline will be found to furnish an adequate practical guide.

Firstly, the skull can be divided into two lateral halves by the surface-marking of the superior longitudinal venous sinus.

The superior longitudinal sinus.

This sinus originates at the crista galli and, passing backwards along the attached margin of the falx cerebri, terminates at the internal occipital protuberance. It may be represented by a line drawn from the base of the nose (the nasion), over the vertex of the skull, to the external occipital protuberance (the inion)—this line corresponding in its course to the occasionally persistent metopic suture between the two halves of the frontal bone, to the sagittal suture between the parietal bones, and to the middle line of the upper or tabular portion of the occipital bone.

Secondly, each lateral half of the skull can be subdivided into supra- and infratentorial regions by a line which marks the external attachment of the tentorium cerebelli; in other words, by the line of the lateral sinus.

The lateral sinus.

This sinus is represented by a line presenting a slight upward convexity, which is drawn from the external occipital protuberance to the upper and posterior part of the mastoid process of the temporal bone.

The infratentorial region.

The cerebellum lies wholly beneath the tentorium cerebelli, and it is obvious that, in operations carried out over this portion of the brain, the surgeon is limited in his field of exposure, above by the line of the lateral sinus, and on either side by the posterior border of the mastoid process. The division between the halves of the cerebellum may be represented by a line drawn vertically downwards from the external occipital protuberance to the nuchal region. This line also represents the surface-marking of the occipital sinus and falx cerebelli.

The supratentorial region.

Brief allusion must be made to certain landmarks that aid in the representation of structures situated in this region:—

(a) The external angular frontal process.

The suture between the external angular frontal process and the corresponding process of the malar bone lies immediately above the central point of the outer border of the orbital cavity.

(b) The malar tubercle.

A slight prominence on the posterior border of the frontal process of the malar bone, about ¹/₄ inch below the external angular frontal process.

(c) The temporal crest.

A prominent ridge that is directed upwards and backwards from the external angular frontal process. The crest cuts across the lower portion of the parietal bone, passing below the parietal prominence, and curves downwards towards the upper and posterior portion of the mastoid process. It terminates by becoming continuous with the upper root of the zygomatic process. The crest consists of two parts, the upper and lower temporal crests. To the upper is attached the temporal fascia, to the lower the temporal muscle. The lower crest is almost invariably the more prominent.

(d) The parietal prominence.

The central and most prominent part of the parietal bone. It indicates the point at which ossification commenced, and lies about ³/₄ inch above the termination of the posterior horizontal limb of the fissure of Sylvius.

(e) The zygoma.

When traced in the backward direction, the zygoma is found to divide immediately in front of the ear into three roots, of which the anterior, merging into the eminentia articularis, and the middle, aiding in the formation of the post-glenoid process, are of no practical utility in surface-marking. The upper or posterior root sweeps backwards above the external auditory meatus to become continuous with the suprameatal and supramastoid crests, the former of which forms the upper boundary of Macewen’s suprameatal triangle, a triangular depression at the upper and posterior border of the external auditory meatus. This triangle may be taken as representing the opening of the mastoid antrum into the middle ear.

These landmarks having been determined, the following structures may be mapped out on the surface of the skull.

The middle meningeal artery.

This artery is given off from the internal maxillary; after a short extra-cranial course it enters the skull through the foramen spinosum, and soon divides into two main terminal branches. The site of division corresponds to a point situated just above the centre of the zygoma.

The anterior branch passes at first in a forward and upward direction towards the anterior inferior angle of the parietal bone, and then turns upwards and backwards towards the vertex of the skull. The main ‘danger zone’ in the course of this vessel may be mapped out by taking points which lie respectively 1, 1¹/₂, and 2 inches behind the external angular frontal process and an equal distance above the upper border of the zygoma. A line uniting these three points represents that part of the anterior division of the middle meningeal artery which is most liable to injury and which therefore most frequently requires exposure.

The uppermost point may, however, be regarded as the ‘site of election’ for exposure of the artery, as, in trephining over either of the two lower points, difficulty may be experienced in the removal of the disk of bone, since the posterior border of the great wing of the sphenoid tails off on to the anterior inferior angle of the parietal bone in such a manner that to effect a clean removal of the disk is often impossible. Another disadvantage to trephining low down lies in the fact that in about 30 to 40 per cent. of cases the artery occupies, in that situation, a canal in the bone.

The posterior branch passes almost horizontally backwards, parallel to the zygoma and supramastoid crest, towards the posterior inferior angle of the parietal bone. The vessel can readily be exposed by trephining over the point at which a line drawn backwards from the upper border of the orbit, parallel to Reid’s base-line,[1] cuts another line directed vertically upwards from the posterior border of the mastoid process.

Both branches of the middle meningeal artery possess important relations to the cortex cerebri, the anterior branch passing upwards in relation to the precentral or motor area, traversing, from below upwards, the motor speech area (on the left side of the head), the centres for the movements of the face, upper extremity, trunk, and lower extremity. The posterior branch, on the other hand, passes backwards in relation to the temporo-sphenoidal lobe, one of the so-called ‘silent areas’ of the brain. Throughout their course the middle meningeal vessels lie between the dura mater and the bone.

The lower limit of the cerebrum.

The lower limit of the cerebrum can be mapped out in the following manner: From a point situated about ¹/₂ inch above the nasion a line is drawn outwards which follows the curve of the upper border of the orbit as far as the external angular frontal process, thence curving upwards and backwards to the Sylvian point (see below). The temporo-sphenoidal lobe sweeps forwards to the posterior border of the malar bone, and its lower limit lies practically flush with the upper margin of the zygoma. At and behind the ear the lower limit of the cerebrum corresponds to the suprameatal and supramastoid crests, subsequently following the curve of the lateral sinus from the mastoid process to the external occipital protuberance.

The Sylvian point and fissure.

The Sylvian point represents the site of divergence of the three limbs of the Sylvian fissure. It lies 1¹/₄ inches behind the malar tubercle and 1¹/₂ inches above the upper border of the zygoma. The main posterior horizontal limb passes backwards and upwards to a second point situated ³/₄ inch below the parietal prominence.

The vertical limb is directed upwards for about 1 inch, whilst the anterior horizontal limb passes forwards for about the same distance.

The fissure of Rolando.

This, from a surgical point of view, the most important fissure of the brain, is represented as follows: A point is taken in the median antero-posterior line which lies ¹/₂ inch behind the mid-point between nasion and inion, and from this point a line is drawn, for 3¹/₂ to 4 inches, towards the mid-point of the zygoma. This line is inclined to the median antero-posterior line at an angle of 67¹/₂° (three-quarters of a right angle).

The parieto-occipital and first temporo-sphenoidal fissures.

In the representation of these two fissures, two points require to be determined—the malar tubercle and the lambda. Allusion has already been made to the former; the latter is usually readily located as the point of intersection of the sagittal and lambdoid sutures. A line uniting these two points corresponds in its middle third to the temporo-sphenoidal fissure, and in its posterior inch or so to the external parieto-occipital sulcus, a fissure separating the occipital and parietal lobes of the brain.

The cortical motor and sensory areas.

The researches of Sherrington and GrÜnbaum,[2] Campbell,[3] and others tend to show that the cortical motor areas are situated entirely anterior to the central fissure or fissure of Rolando, extending above well over on to the mesial aspect of the brain, though not so far as the calloso-marginal fissure, and in the downward direction to within a short distance of the fissure of Sylvius. In the posterior direction the motor area includes the anterior boundary and part of the bottom of the fissure of Rolando, whilst in front it spreads, by means of gyri annectantes, on to that part of the brain which lies anterior to the precentral sulcus. The anterior termination is indefinite, but the motor strip is, on an average, not more than ³/₄ inch in breadth.

The genua of the Rolandic fissure are said to bear a more or less definite relation to the motor areas. In my experience, however, they are too variable, and any importance which might obtain is further diminished by the fact that the surgeon should never rest content with that limited exposure of the brain which was so much in vogue till recent years. The suspected region is widely exposed by osteoplastic flap, and if doubt exists as to the particular part of cortex exposed the question is accurately settled by faradization.

This motor strip corresponds, from above downwards, to the movements of the contralateral lower extremity (toe to hip), trunk, upper extremity (shoulder to fingers), neck and face. It is a point of some general utility to bear in mind that the temporal crest intervenes approximately between the regions responsible for the movements of the upper extremity (above the crest) and those for the movements of the head and face (below the crest).

On the left side of the head—in normal right-handed individuals—the motor speech area of Broca corresponds to the third left frontal convolution, in the angle between the anterior and posterior horizontal limbs of the Sylvian fissure.

The ‘primary registration’ of ‘common sensation’ occurs in the post-central gyrus, immediately posterior to the fissure of Rolando. This tactile area occupies a position behind the fissure of Rolando similar in extent to that occupied by the motor area in front. It commences at the bottom of the fissure of Rolando and extends backwards over rather more than half the exposed area of the post-central convolution. It reaches down to near the Sylvian fissure and extends over on to the mesial aspect of the brain. Furthermore, it is probable that sensation in any given part lies on more or less the same level as the corresponding motor area.

Immediately posterior to the tactile area and occupying the posterior and upper part of the post-central convolution, the area responsible for muscle-sense is situated.

Stereognosis—memory pictures, object perception, &c.—is referred to the superior parietal lobe.

Primary visual impressions are received in the occipital lobe, more especially on the mesial aspect thereof.

Finally, the four areas concerned in speech—motor speech, writing, reading, and hearing—are anatomically separated from one another and yet closely associated, so much so that one can hardly be involved without the other. The motor speech centre of Broca has already been mentioned as occupying—in right-handed individuals—the posterior part of the third left frontal convolution. Writing lies immediately above and in front, in the posterior part of the middle frontal gyrus, auditory impressions are received in the posterior and upper part of the first temporo-sphenoidal lobe, whilst the power of reading is dependent on the integrity of the supramarginal and angular gyri (see also Fig. 57).

Smell and Taste lie in close relation to the anterior pole of the temporo-sphenoidal lobes.