1 144 



HUMAN ANATOMY. 



(66 per cent.), or with both (55 per cent.), the inferior limb may remain ununited 

 (17 per cent.). When joined, the two limbs together form a continuous postcentral 

 sulcus which parallels the fissure of Rolando and bounds the postcentral convolution 

 behind. In rare instances the inferior postcentral sulcus opens below into the 

 Sylvian fissure. 



The superior postcentral sulcus lies behind and parallel with the upper part 

 of the fissure of Rolando, gaining the superior margin of the hemisphere between the 

 incisions of the Rolandic fissure and the upturned end of the calloso-marginal sulcus. 

 Although in 59 per cent, of the brains studied by Retzius the fissure was confluent 

 with the horizontal limb, in 24 per cent, it remained isolated. 



The horizontal limb passes backward and slightly upward and separates the 

 superior and inferior parietal convolutions from each other. It is usually continuous 

 in front with one or the other or with both postcentral sulci and behind with the 



FIG. 



Lateral aspect of left side of brain. LF, longitudinal fissure; >-., ;., r.. Rolandic fissure; i- PC., s. f>c., inferior and 

 superior precentral; sf., if., superior and inferior frontal; Sj>, S. asc., posterior and ascending limbs of Sylvian 

 fissure; /', p*, A 1 ./ 4 , inferior, superior, horizontal and occipital limbs of interparietal ; p-o. parieto-occipital ; /. o., 

 I. o., transverse and lateral occipital ; /', t l asc., superior temporal and its upturned limb; <*, t-asc., middle temporal 

 and its upturned limb. 



posterior or occipital limb. As a rule it joins a continuous postcentral sulcus, in 

 which case the three furrows form a | shaped fissure, which subdivides the parietal 

 lobe into its three main convolutions. 



The occipital limb is usually attached to the horizontal one and then directly 

 prolongs the interparietal sulcus into the occipital lobe. Sometimes, however, it 

 retains its original independence and is separated from the ramus horizontalis by a 

 deep annectant gyrus. It is irregularly curved and marks the lower boundary of the 

 gyrus, the arcus parieto-occipitalis, which receives the outer end of the parieto- 

 occipital fissure. Beyond the line of this furrow, the sulcus lies in the occipital lobe 

 and behind the arcus parieto-occipitalis ends by bifurcating into two widely divergent 

 arms, which constitute the transverse occipital sulcus. 



The chief con-solutions on the external surface of the parietal lobe are three the 

 postcentral, the superior parietal and the inferior parietal. 



The postcentral gyrus, also called the ascending -parietal, forms the posterior 

 wall of the fissure of Rolando, and itself is bounded behind by the postcentral sulcus, 

 either l>y the continuous fissure or by its two divisions. The lower end of the gyrus 

 is connected with the precentral convolution in front and with the inferior parietal 

 one behind by the annectant ^yri closing the lower ends of the central and postcen- 



