THE NERVOUS SYSTEM ' 1389 



External Surface. — ^This surface presents the following sulci : 

 the intraparietal sulcus, composed of four parts ; and the ter- 

 minal portions of (a) the posterior limb of the fissure of Sylvius, 

 (b) the first temporal or parallel sulcus, and (c) the second temporal 

 sulcus. 



The intraparietal sulcus (Turner) is composed of four parts, 

 namely, inferior postcentral, superior postcentral, ramus hori- 

 zontalis, and ramus occipitalis. 



The inferior and superior postcentral sulci may be distinct, or 

 continuous with each other. They he behind the fissure of Rolando, 

 with which they are parallel, and from which they are separated by 

 the ascending parietal or postcentral gyrus. 



The ramus horizontalis passes backwards and slightly upwards 

 from the upper end of the inferior postcentral sulcus. It has the 

 superior parietal gyrus above it, and the inferior parietal gyms 

 below it. 



The ramus occipitalis lies behind the ramus horizontalis, with 

 which it is usually connected. It forms the outer boundary of 

 the gyrus which arches round the external part of the parieto- 

 occipital fissure, namely, the arcus parieto-occipitalis (first annectant 

 convolution). Posteriorly the ramus occipitalis divides into two 

 short sulci, which diverge, one ascending, and the other descend- 

 ing, in the occipital lobe. 



The terminal portions of (a) the posterior limb of the fissure of 

 Sylvius, (6) the first temporal or parallel sulcus, and (c) the second 

 temporal sulcus, are confined to the lower part of the external 

 surface of the parietal lobe, where they lie in the order named, from 

 before backwards. 



The gyri of the external surface are as follows : ascending parietal ; 

 superior parietal ; and inferior parietal, with its supramarginal, 

 angular, and postparietal gyri. 



The ascending parietal or postcentral gyrus is situated unmediately 

 behind the fissure of Rolando, which separates it from the ascending 

 frontal or precentral gyrus in front of that fissure. Posteriorly it 

 is limited by the superior and inferior postcentral sulci. It extends 

 from the supero-mesial border of the hemisphere to the posterior 

 horizontal limb of the fissure of Sylvius, and it lies parallel to 

 the ascending frontal or precentral gyrus, with which it is connected 

 below the fissure of Rolando. 



The superior parietal gyrus is situated between the ramus hori- 

 zontalis and the supero-mesial border of the hemisphere, where it 

 is continuous with the quadrate lobule, or praecuneus, of the internal 

 surface. Anteriorly it is limited by the superior postcentral 

 sulcus, round the upper end of which it is continuous with the 

 ascending parietal gyrus. Posteriorly it is bounded by the external 

 part of the parieto-occipital fissure, round the extremity of which 

 it is connected with the occipital lobe by the arcus parieto-occipitalis 

 (first annectant convolution). 



The inferior parietal gyrus is situated behind the inferior post- 



