728 THE XERVOrS SYSTEM 



surfact'S of the orbital plate of the frontal lioneand the small wing- of the sphenoid; 

 it is therefore in close relation with the upper wall of the or))ital cavity. The pos- 

 terior part, behind the Sylvian fissure, is formed by the anterior- portion of the 

 temporal lobe, including its apex. The apex itself projects against the orbital 

 plate of the great wing of the sphenoid bone, and it is in relationship with the 

 posterior part of the outer wall of the orbit. The lower surface of the hemisphere, 

 behind the apex of the temporal lobe, is in contact with the upper surfaces of the 

 great wing of the si)henoid and the petrous part of the temporal l)one. 



The external surfaces of the cerebral hemisplieres have the most extensive rela- 

 tionshii)s with the cranial wall, and it is more especially to these surfaces that the 

 practical surgeon turns his attention. The general area in which the outer surface 

 of each cerebral hemisphere is in relation with the skull bones is readily indicated 

 by a series of lines wdiich correspond Avith the positions of its supraciliary, the 

 infero-lateral, and the supero-mesial liorders. 



The line marking the supraciliary margin of the hemisphere commences at the 

 nasion (the mid-point of the fronto-nasal suture); it passes outwards above the 

 su]:)raciliary ridge, crosses the temporal ridge, then, turning backwards in the 

 tem})oral fossa, it reaches the parieto-sphenoidal suture, along which it continues 

 V)ack wards to its posterior extremity. 



The line n;iarking out the infero-lateral l)order conunences at tlie posterior end 

 of the parieto-sphenoidal suture, whence it passes downwards, in front of the 

 spheno-squamous suture, to the pterygoid ridge (infra-temporal crest) ; there it 

 turns l^ackwards; running parallel with and internal to the zygomatic arch, it 

 crosses the root of the zygoma, and ascending slightly it passes above the external 

 auditory meatus; continuing backwards with an inclination upwards it reaches a 

 ]M_)int innnediately above the asterion; thence it descends, and, crossing the lower 

 2)art of the lambdoid suture and the superior curved line, it passes inwards to the 

 lower part of the external occii)ital protu Iterance. 



The supero-mesial 1)order is defined b}' a line which rims from the nasion to the 

 inion; this line should be drawn about 5 mm. to the outer side of the sagittal 

 suture, because the mesial area is occupied by the superior longitudinal sinus, and 

 it should be further away from the middle line on the right than on the left side 

 because the sinus tends to lie more to the right side. 



The area of the skull wall enclosed by the three linQS which mark the positions 

 of the supraciliary, infero-lateral, and the supero-mesial borders of the cerebral 

 hemisphere is formed by the vertical plate of the frontal Ijone, the parietal bone, 

 the great wing of the sphenoid, the squamous part of the temporal, and the ui)per 

 section of the supra-occipital segment of tlie occipital bone; it covers the outer sur- 

 faces of the frontal, parietal, temj^oral, and occipital lobes and the fissures and 

 sulci which ])ound and mark them. 



The frontal bone covers the superior, middle, and inferior frontal convolutions, 

 except their posterior extremities, which are beneath the parietal bone (fig. 483). 

 The ascending limb of the fissure of Sylvius, which cuts into the posterior part of 

 the inferior frontal convolution, runs ]iarallel with and undercover of the lower part 

 of the coronal suture or innnediately in front of it, and the anterior horizontal limb 

 is parallel with and beneath the up])er margin of the great wing of the sphenoid. 

 The parietal ])one is in relation with the outer surfaces of four lobes of the l)i'ain. 

 Speaking very generally, it may l)e said that the anterior third covers the posterior 

 part of the frontal lobe, including the ascending frontal convolution and the i)os- 

 terior ends of the superior, middle, and inferior frontal convolutions and the u])])er 

 and lower precentral sulci, whilst the posterior two-thirds are superficial to the 

 j>arietal lobe, the posterior part of the temporal lobe, the anterior part of the t)cci- 

 l)ital lobe, the posterior part of the horizontal liml) of the Sylvian fissure, tlieu]>])<'r 

 and lower ])Ost-central sulci, the intraparietal sulcus, the })osterior sections of the 

 first and second tem])oral sulci, and the external ]iarieto-occipital fissure. The 

 fissure of Rolando is beneath the ])ariet;d l)one at the junction of its middle and 

 anterior thirds (fig. 483). 



The upper end of the fissure of Rolando lies 55 ]ier cent, of the whole length of 

 the naso-inionic line behind the nasion; it is 55 mm. from the coronal suture in 

 dolichocephalic, and 54-4 nun. in brachycephalic heads. The lower end of the 



