THE PARIETAL LOBE 



861 



by an arbitrary line drawn transversely around the convex surface of the hemi- 

 sphere from the superior extremity of this fissure to the infero-lateral border; 

 and it is separated from the temporal lobe below by the horizontal part of the 

 posterior ramus of the lateral fissure, and by a line drawn in continuity with this 

 horizontal part to intersect the transverse line drawn to limit it from the occipital 

 lobe. 



The preoccipital notch. — In situ, the infero-lateral border of the posterior portion of the 

 hemisphere rests over a smaU portion of the parieto-mastoid suture of the cranium, and upon 

 this structure occurs a fold or vertical thickening of the dura mater, which slightly indents the 

 infero-lateral border. This indentation occurs about 4 cm. from the occipital pole, and is con- 

 sidered one of the points of limitation of the parietal from the occipital lobe, and is therefore 

 called the preoccipital notch. While during late foetal life and early childhood it is well marked, 

 it is usually very sUght in the adult brain, and is, as a rule, evident only in brains hardened 



Fig. 678. — Convex Surface of the Cerebral Hemispheres as Viewed from Above. 

 (After Toldt, "Atlas of Human Anatomy," Rebman, London and New York.) 



Frontal pole 



Supero-mesial border 



Longitudinal fissure 



Superior frontal 

 sulcus 



Precentral sulcus 



Central sulcus 



Interparietal sulcus 



-occipital fissure 

 ";. Superior occipital sulci 



Occipital pole 



in situ. When it is visible, the arbitrary transverse line from the superior extremity of the 

 parieto-occipital fissure, used as a boundary between the convex surfaces of the parietal and 

 occipital lobes, should be so drawn as to bisect the preoccipital notch. 



The convex surface of the parietal lobe comprises the following gyri and sulci : — 

 The posterior central gyrus (ascending parietal) extends obliquely across the 

 hemisphere parallel with the anterior central gyrus of the frontal lobe, from which 

 it is separated by the central sulcus. Its inferior end is bounded by the posterior 

 ramus of the lateral fissure, and constitutes the posterior or parietal portion of the 

 fronto-parietal operculum. Its upper end takes part in the supero-mesial 

 border of the hemisphere, and is bounded posteriorly by the tip end of the 

 marginal portion of the sulcus cinguli. Its postero-lateral boundary consists 

 of the two more or less vertical rami or factors of the interparietal sulcus, viz., the 

 inferior and superior portions of the postcentral sulcus, either continuous with each 

 other or detached. 



The interparietal sulcus (intraparietal) is often the most complicated sulcus 

 of the pallium. Its development usually begins as four different furrows in the 

 foetal brain, and the difficulty with which it is traced in the adult brain depends 



