THE NERVOUS SYSTEM 1505 



The foramen magnum, through which the medulla oblongata is 

 continuous with the spinal cord, hes midway between the mastoid 

 processes. Its posterior margin is 2 inches from the inion in a 

 downward and forward direction. 



The tentorium cerebelli and superior surface of the cerebellum 

 practically coincide with the level of the transverse sinus, as indi- 

 cated by a line, slightly curved upwards, connecting the inion and 

 asterion. 



The lower level of the cerebral hemisphere may be indicated by 

 the following line: Commencing at a point J inch external to the 

 nasion, the line passes outwards in an arched manner, with the 

 convexity upwards, lying about ^ inch above the centre of the 

 supra-orbital arch. It then inclines downwards and crosses the 

 temporal ridge of the frontal bone about i inch above the fronto- 

 malar suture, the guide to which is a point just above the outer 

 canthus of the eyelids. Thereafter the line passes backwards and 

 shghtly downwards to the Sylvian point, and thence to the upper 

 border of the posterior part of the zygomatic arch. From this point 

 the line passes backwards, Ipng about ^ inch above the upper 

 margin of the orifice of the meatus auditorius extemus. It then 

 crosses the supramastoid crest (posterior root of the zygoma), and 

 passes to the asterion. Thereafter it nearly follows the line of the 

 transverse sinus from the asterion to a point a little above, and 

 external, to the inion. In other words, speaking generally, the 

 cerebral hemisphere extends as low as the superior cur\-ed line of the 

 occipital bone posteriorly, the upper border of the zygomatic arch 

 laterally, and the upper part of the e3'ebrow anteriorly. The frontal 

 lobe of the cerebral hemisphere is to a large extent in contact with 

 the frontal portion of the frontal bone, but it is also related to the 

 anterior part of the parietal bone as far back as the Rolandic line. 



The parietal lobe is related to the part of the parietal bone which 

 lies behind the Rolandic line. The lobe extends as far back as the 

 parieto-occipital fissure, which is usually situated opposite the 

 lambda. 



The occipital lobe occupies the cerebral fossa of the tabular part 

 of the occipital bone, its limits being the level of the lambda 

 superiorly and the level of the inion inferiorly. 



The temporal lobe is under cover of the squamous portion of the 

 temporal bone and the postero-inferior part of the parietal bone. 

 The parallel sulcus, which separates the first and second temporal 

 convolutions, is indicated by a line dra\NTi from the lambda to the 

 marginal tubercle on the temporal or posterior border of the malar 

 bone, about ^ inch below the fronto-malar suture. 



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