1088 SURGICAL AXD TOPOGRAPHICAL ANATOMY 



an inch above the supraorbital niar<iin; next, crossing the temporal crest about half 

 an inch above the external angular process, it passes not quite horizontally, but 

 descending slightly to a point in the temporal fossa just below the tip of the great 

 wing of the sphenoid (pterion), an inch behind the external angular process. 

 From this point the line of the level of the brain, now convex forwards and corre- 

 sponding to the anterior extremity of the temporo-sphenoidal lobe, would dip down, 

 still within the great wing of the sphenoid, to about the centre of the zygoma. 

 Thence the line would travel along the ujiper border of this process about a 

 quarter of an inch above the roof of the external auditory meatus, and thence just 

 above the base of the mastoid and the posterior inferior angle of the parietal, and 

 so along the superior curved line, and corresponding to that of the tentorium and 

 horizontal part of the lateral sinus, to the external occipital protuberance. 



The upper margin of each hemisphere would be represented by a line drawn 

 from just below the glabella, suthciently to one side of the middle line to allow for 

 the falx and superior longitudinal sinus, to one immediately a))ove the superior 

 external occipital protuberance and ini<»n (p. 1091 ). 



B. Relation of the chief fissures and convolutions to the skull. Local- 

 ization of the chief sulci and convolutions. These headings will be taken 

 together. 



It will be well to first indicate the position of the chief sutures which mark 

 off the parietal bone, under which lies that part of the brain which is most impor- 

 tant to the surgeon — the motor area. The upper limit of the bone will be indicated 

 by the line already spoken of as giving the upper margin of the hemisphere — the 

 sagittal line, or suture (p. 1091). The anterior limit of the parietal bone, formed 

 by the coronal suture, may be traced thus: The point where it leaves the sagittal 

 suture (the bregma) will be found by drawing a line from a point just in front of 

 the external auditory meatus (the preauricular point) (p. 1091, figs. 659 and 665) 

 straight upwards on to the vertex; from this point a line drawn downwards and 

 forwards to the middle of the zygomatic arch would indicate that of the coronal 

 suture. Under this suture lie the posterior extremities of the three lateral frontal 

 convolutions; for the frontal lobe lies not only under the frontal bone, but extends 

 backwards under the anterior part of the parietal, the fissure of Rolando, which 

 separates the frontal from the parietal lobe, lying from one and a half to two inches 

 l:)ehind the coronal suture. 



The squarnoso-parietal suture, which marks the lower border of the anterior 

 two-thirds of the parietal bone, is not so easy to define, owing to the irregularity 

 and variations of its curve. Its highest point is usually one and three-quarter 

 inches above the zygoma. 



The lambdoid suture, which forms the posterior boundary of the parietal 

 bone, will be marked out by a line which starts from a point (lambda) two and 

 three-quarter inches above the external occipital protuberance, and runs downwards 

 and forwards to a point on a level with the z3'goma, one and three-quarter inches 

 behind the meatus. 



The })osition of the chief sulci will now be given: 



Sylvian fissure (figs. 664 and 665). — The point of appearance of this, on the 

 outer side of the brain, practically corresponds to the i)terion (fig. 659) — a point 

 which lies in the temporal fossa, about an inch and a half behind the external an- 

 gular process and about the same distance above the zygoma. From this point the 

 Sylvian fissure, which here separates the frontal and parietal from the temporo- 

 sphenoidal lobe, runs backwards and upwards, ascending gently, at first in the line 

 of the squamo-parietal suture, then crossing this suture about its centre, and thence, 

 ascending more rapidly, it climbs up to the temporal ridge, to end three-quarters of 

 an inch l)elow the parietal eminence. Its termination is surrounded by the supra- 

 marginal convolution, to which the parietal eminence corresponds with sufficient 

 accuracy. Such being the surface-marking of the chief or ])osteri()r borizontal limb 

 of the fissure of Sylvius (s'^, fig. 664), it remains to indicate briefly the tAvo 

 shorter limbs which bound the inferior frontal convolution, which, on the left side, 

 contains the centre for speech (Broca's convolution). Of these, the anterior hori- 

 zontal (s^, fig. 664) runs forwards across the termination of the coronal, just 

 above the line of the spheno-parietal suture. The ascending limb (s^, fig. 



