504 THE BRAIN 



which are situated on the inner surface of the skull wall. They should 

 note especially, in relation to the bones and the arterial grooves, the 

 situations of the posterior horizontal limb of the lateral fissure, the central 

 sulcus and the occipital pole of each hemisphere, because : (i) in the lower 

 lip of the posterior horizontal limb of the lateral fissure lies the acoustic area ; 

 (2) the anterior central gyrus, which lies in front of the central sulcus, is the 

 general motor area of the brain; (3) the posterior central gyrus, which 

 lies behind the central sulcus, is the general sensory area ; and (4) the 

 occipital pole is in the region of the visual area (Figs. 153, 204). 



The anterior branch of the middle meningeal artery lies, as a rule, over 

 the region of the anterior central gyrus (Fig. 204), and the posterior branch, 

 which is, however, more irregular in position, frequently runs along the 

 ine of the first temporal gyrus, which is immediately below the posterior 

 horizontal limb of the lateral fissure, and consequently it crosses or lies 

 close to the acoustic area (Fig. 204). 



The general positions of the sulci and gyri are shown, in Figs. 135, 152, 

 and in Fig. 38, which is a reproduction of a radiograph of a head in which 

 metallic rods and pieces of metallic paste had been introduced, other 

 important areas have also been made visible. 



The exact positions of the various cerebral sulci vary, to a certain extent, 

 in heads of different shapes and sizes, but a sufficiently accurate estimation, 

 for practical purposes, can be made on any head or skull by the use of a 

 few easily remembered points and lines (Fig. 204). They are : 



(1) A base line extending from the lower margin of the orbit to the 

 upper border of the external acoustic meatus (5-5, black, Fig. 204). 



(2) A line, 30 mm. long, extending backwards from the lower end of 

 the zygomatic process of the frontal bone, parallel with the base line 

 (6, black, Fig. 204). 



(3) A line, 10 mm. long, projected upwards at right angles to (2) from its 

 posterior end (8, black, Fig. 204). The upper end of (3) marks the " Sylvian 

 point," which lies at or near the pterion, and it marks the position where 

 the lateral end of the stem of the lateral fissure divides into its three 

 terminal branches (9, black, Fig. 204). 



(4) A line projected from the lower end of the zygomatic process of the 

 frontal bone, through the " Sylvian point : ," to the median plane (2-2, black, 

 Fig. 204). This line usually strikes the median plane a short distance in 

 front of the lambda ; the first 75 mm. '(three inches) of it, behind the 

 " Sylvian point," mark the position of the straight part of the posterior 

 horizontal limb of the lateral fissure, and the last 18 mm. (about three- 

 quarters of an inch) indicate the position of the upper and lateral part of 

 the parieto-occipital fissure. 



(5) A line marking the general position of the central sulcus. It com- 

 mences 25 mm. (one inch) behind the " Sylvian point," immediately above 

 the lateral fissure, and extends upwards and backwards to a point in the 

 median plane situated 12.5 mm. (half an inch) behind the central point on 

 a line extending from the root of the nose (nasion) to the external occipital 

 protuberance (inion) (i, blue, Fig. 204). 



The occipital pole of the hemisphere lies immediately above and lateral 

 to the external occipital protuberance. 



