THE TELENCEPHALON. 



1 143 



The Parietal Lobe. — This division includes a considerable part of the hemi- 

 sphere and presents two surfaces, an external and a mesial. The external surface, 

 much the more extensive and irregularly quadrilateral in outHne, is bounded above, 

 in front and partially below by well marked fissures, but behind and postero-infe- 

 riorly its limits from the occipital and temporal lobes are defined for the most part 

 by imaginary lines. Its upper boundary corresponds with the supero-mesial border 

 of the hemisphere ; its anterior boundary is the central fissure, by which the pari- 

 etal lobe is completely separated from the frontal except below, where the postcen- 

 tral gyrus is continuous with the precentral by the bridge closing the lower end of 

 the Rolandic fissure. Its posterior boundary, which separates the parietal from the 

 occipital lobe, is largely conventional and indicated by a line drawn from the point 

 where the parieto-occipital fissure cuts the upper margin of the hemisphere to an in- 

 dentation, the preoccipital notch (page 1134), which grooves the infero-lateral border 

 of the hemisphere at a point from 3.5-4 cm. in front of the occipital pole. Its 

 inferior border, between the parietal and the temporal lobes, is definite where formed 

 by the posterior limb of the Sylvian fissure. Beyond the upturned end of the latter, 



Fig. 987. 



Infero-mesial aspect of left cerebral hemisphere; cm., calloso-marginal fissure; ros., rostral; r., overturned 

 end of Rolandic ; p. I., post-lirpbic ; i. p-o., internal parieto-occipital ; p. cal., a. cal., posterior and anterior calcarine; 

 p. col., a. col., posterior and anterior collateral ; i. t., incisura temporalis or rhinial ; o-t., occipito-temporal. 



the parietal and the temporal lobes are continuous and their separation is conven- 

 tionally assumed to be made by an arbitrary line prolonged backward in the direc- 

 tion of the posterior limb of the Sylvian fissure until it meets the parieto-occipital 

 line previously described. • 



The external surface of the parietal lobe is subdivided by a composite fissure, 

 the interparietal sulcus, into three general tracts, the postcentral, the superior pari- 

 etal and the. inferior parietal gyrus. 



The interparietal sulcus, especially described by Turner, starts in the antero- 

 inferior angle of the lobe a short distance above the Sylvian fissure, with which it is 

 rarely continuous, ascends for about an inch parallel with the central fissure, and 

 then sweeps backward and slightly upward across the parietal into the occipital lobe. 

 The interparietal sulcus is developed as four originally distinct parts, which in the 

 fully formed brain, notwithstanding their usual fusion, are recognized as the inferior 

 and the superior postcentral sulcus and the horizontal and occipital limbs (Cun- 

 ningham). 



The inferior postcentral sulcus lies behind and parallel with the lower part 

 of the central fissure. Although in most cases continuous with either the superior 

 postcentral sulcus (in 72 per cent, according to Retzius'), or with the horizontal limb 



' Biologische Untersuchungen, VIII., 1898. 



