Of importance to the surgeon is a knowledge of the location of the principal brain centres and their relation to the surface of the head. If two parallel lines be drawn from the nasion in front to a position ½ inch above the superior curved line of the occiput behind, one ½ inch to the right and the other ¼ inch to the left of the middle line, they will represent the inner margins of the right and left cerebral hemispheres, while the space between them is occupied by the superior longitudinal sinus, which enlarges as it passes back and lies rather to the right of the middle line, owing to the greater size of the left hemisphere. A line drawn from the nasion in front to the external angular process of the frontal with an upward convexity fully ¼ inch above the orbital margin, thence carried backwards along the upper border of the zygoma to the preauricular point, and from that to the external occipital protuberance, will roughly indicate the inferior margin of the cerebrum. A line drawn from the nasion to the inion (nasoiniac line), and divided into four equal parts, will represent the position of the parieto-occipital suture at the junction of its third and fourth quarters. The posterior horizontal limb of the Sylvian fissure will be represented by the anterior half of a line drawn from the pterion to the parietooccipital suture, while the short anterior horizontal (½ inch long) and vertical (¾ inch long) processes of the Sylvian fissure run from the pterion in the directions indicated by their names. The position of the fissure of Rolando is indicated by a line commencing ½ inch behind the mid-point of the naso-iniac line, and running down and forwards at an angle of 67½ degrees (three-quarters of a right angle) to meet the Sylvian line.
The frontal portion of the cerebrum is bounded centrally and interiorly by the lines already given, while posteriorly it is bounded by the Rolandic fissure.
It consists of superior, middle, and inferior convolutions, and also of a precentral convolution, which is about ¾ inch broad, and is bounded in front by the precentral sulci, which run parallel to the Rolandic fissure. The third or inferior left frontal convolution-Broca's lobe-contains the motor speech area, and would be indicated on the surface by a point nearly an inch in front of the Rolandic area, and slightly below the temporal ridge. It lies between the anterior and the ascending limbs of the Sylvian fissure. The precentral convolution is the most important motor area of the brain, and, according to Sherrington, contains all the centres pre-viously attributed to the posterior central convolution. This motor area occupies practically the whole length and breadth of the precentral convolution, and extends into the depth of the Rolandic fissure. Whether the posterior centrai convolution (belonging to the parietal region) is also involved is at present uncertain. The area for the limb occupies the region of the upper third of the Rolandic fissure, and also dips superiorly into the longitudinal fissure. The area for the upper limb occupies the region of the middle third of the Rolandic fissure, while the area for the face occupies the lower third.
The fissure of Sylvius separates the lower end of the Rolandic area from the temporal lobe, and as the latter is frequently affected by cerebral abscess, the pressure from which is directed chiefly upwards, it follows that the face centre is frequently affected in such cases (causing an incomplete paralysis of the face on the opposite side), whereas the arm is rarely, and the leg is practically never, affected. Further, it should be noted that the face is first affected, and the leg, if affected at all, last, the affection coming on gradually (compare with internal capsule).
The parietal lobe extends from the longitudinal fissure above to the Sylvian fissure below, and is bounded in front by the Rolandic fissure and behind by a line drawn from the position of the parieto-occipital fissure to the asterie point (parieto-mastoid line). It presents the posterior central convolution, bounded posteriorly by the posterior central sulci, which are parallel to and ¾ inch behind the Rolandic fissure. It also presents a superior parietal lobule and superior marginal and angular convolutions. The latter is supposed to contain the word-seeing centre, and lies just behind and above the posterior extremity of the Sylvian line.
The temporal lobe is limited above by the Sylvian fissure, below by the line indicating the lower level of the brain. It extends anteriorly to about ¾ inch from the outer margin of the orbit, while posteriorly it is bounded by the lower portion of the parieto-mastoid line. It presents superior, middle, and inferior convolutions, the two former separated by the parallel fissure, and the latter lying over the thin tegmina of the middle ear and mastoid, and being frequently infected by pyogenic invasion through these thin plates of bone. As the temporal lobe is limited anteriorly, posteriorly, and externally by bone, the pressure of an abscess must be directed upwards and inwards. The upward pressure affects the motor area of the face, while the inward pressure affects the third nerve, causing first irritation (contraction of pupil on same side), and later paralysis (dilatation of pupil on same side) of the nerve. The central portion of the superior temporal convolution is supposed to contain the word-hearing centre.
The occipital lobe lies posterior to the parieto-occipital fissure, but is not sharply demarcated from the parietal and temporal lobes, which merge with it. It presents superior, middle, and inferior convolutions. The line from the preauricular point to the external occipital protuberance indicates posteriorly the position of separation between cerebrum and cerebellum by the tentorium, and of the lateral sinus, which is formed by a splitting of the durai processes which form the tentorium.
The cerebellum occupies a strictly limited space between the tentorium and the containing bone, and hence, when affected by abscess, and even by tumour, is unable to accommodate the increased mass, the pain in such conditions being generally very intense, owing to pressure. Further, as there is no septum between the two halves of the cerebellum, it is generally difficult to determine the side occupied by a lesion by the symptoms alone, the pressure being diffused over both sides. Where the cerebellum is affected by abscess, this most often has arisen from the mastoid region, and the treatment necessitates a thorough opening up of the mastoid, exposing, and, if necessary, ablating the sigmoid sinus to prevent dissemination of the septic matter within it, and then, by cutting the bone still further backwards, exposing and opening the cerebellar fossa and evacuating the abscess. Tumour of the cerebellum may be reached by an incision under the external occipital protuberance, shelling the muscles (the parts generally bleeding freely), and then either trephining or cutting a bone flap, and finally opening the cerebellar dura.