THE BRAIN AND ITS MENINGES 229 



brain obliquely and separates the frontal from the parietal 

 lobe. It begins at the supero-median border and runs down- 

 wards, forwards, and laterally to end a little above the posterior 

 ramus of the lateral fissure (of Sylvius). The Anterior and 

 Posterior Central (Pre- and Post-Central) Gyri are two parallel 

 and nearly vertical convolutions, which lie one on each side of 

 the central sulcus. The motor area is situated in the grey matter 

 of the anterior central gyrus and in the anterior wall of the 

 central sulcus (of Rolando), but it does not extend on to the 

 surface of the posterior central gyrus. In front of the lower end 

 of the sulcus, the motor centre for the face is situated, and above 

 lie the centres for the neck, upper limb, trunk, and lower 

 limb, in that order. The last-named extends from the supero- 

 median border on to the upper part of the medial surface of the 

 hemisphere (Fig. 69). 



The upper end of the central sulcus (of Rolando) corresponds 

 to a point half an inch behind the mid-point of the line joining 

 the nasion to the external occipital protuberance. The sulcus 

 itself can be mapped out by drawing a line, inclined at an angle 

 of 67! to the sagittal suture, downwards and forwards from 

 this point for 3! inches. The direction of this line can be in- 

 dicated on the head by placing the palm upon the scalp with the 

 index-finger along the median line and the thumb extended so 

 as to point just behind the apex of the chin. When the apex 

 of the angle between the thumb and the index-finger corresponds 

 to the commencement of the sulcus, the thumb itself indicates 

 the situation of the sulcus on the surface. The motor area 

 extends forwards for half an inch in front of this line. It is 

 crossed by the superior temporal line at or near the lower part 

 of the upper limb centre, and, therefore, that part of the skull 

 which overlies the face centre is itself covered by the temporal 

 muscle and fascia. In the child, owing to the smaller size of 

 the muscle, the temporal lines lie entirely below the level of the 

 motor area. 



The Lateral Fissure of Sylvius breaks up into three rami as soon as it 

 appears on the lateral surface of the brain. The point at which this takes 

 place lies ij inches vertically above the mid-point of the zygomatic arch. It 

 is known as the Sylvian point and is situated opposite the Pterion, the region 

 where the great wing of the sphenoid articulates with the sphenoidal (antero- 

 inferior) angle of the parietal bone. The anterior horizontal and anterior 

 ascending rami extend on to the inferior frontal gyrus, and the convolutions 

 which surround them are known as Broca's Area. This area is said to contain 

 the motor speech centre, which is only developed on the left side of the brain. 

 It lies immediately above the Sylvian point. 



156 



