THE TELENCEPHALON. II45 



tral sulci respectively. Above, the convolution is continuous with the pri-cvntral 

 lobule of the mesial surface between the terminations of the calloso-margin;il and tin- 

 Rolandic fissures. In its width and general oblique course across the' hemisphere 

 the postcentral convolution strongly resembles the precentral gyrus and with the 

 latter and the three associated sulci the precentral, central and postcentral forms 

 a conspicuous feature in the modelling of the external surface of the hemisphere and 

 affords a ready means of locating the Rolandic fissure. 



The superior parietal gyrus is the triangular tract lying between superior 

 postcentral sulcus, the horizontal limb of the interparietal sulcus and the supero- 

 mesial border of the hemisphere. Behind, it is limited by the overturned outer end 

 of the parieto-occipital fissure, around which, however, it is continuous with the 

 occipital lobe by means of the curved convolution, the arcus parieto-occipitalis. 

 Farther forward it is frequently deeply incised by an ascending branch from the inter- 

 parietal sulcus. It is connected with the postcentral gyrus around the upper end of 

 the superior postcentral sulcus and, in those cases in which the last-named sulcus fails 

 to unite with outer segments of the interparietal fissure, additionally joins the post- 

 central gyrus about the inferior postcentral sulcus. 



The inferior parietal gyrus is included between the curved interparietal 

 sulcus and the conventional lower boundary of the lobe. Since only the front end of 

 this boundary is defined by a groove, its greater part being the arbitrary line above 

 described, it follows that behind the Sylvian fissure the inferior parietal convolution 

 is continuous with the subjacent temporal gyri. The convolution is cut into from 

 below by the upturned end of the Sylvian fissure and the terminations of the first and 

 second temporal sulci and by these incisions is somewhat uncertainly subdivided into 

 three parts, the supramarginal, the angular and the postparietal gyri (Fig. 988). 

 The supramarginal gyrus arches around the upturned extremity of the Sylvian 

 fissure. It lies behind and below the front part of the r.nterparietal sulcus, around 

 whose lower end it joins the postcentral gyrus, whilst below it is continuous with the 

 superior temporal and behind with the angular gyrus. The angular gyrus surmounts 

 the upwardly directed end of the superior temporal sulcus and below is prolonged into 

 the superior and middle temporal convolutions. It is commonly imperfectly sepa- 

 rated from the postparietal gyrus by a shallow furrow. The postparietal gyrus 

 bends over the obliquely vertical extremity of the middle temporal sulcus and below 

 joins the middle and inferior temporal convolutions. It lies approximately opposite 

 the arcus parieto-occipitalis from which it is separated by the occipital branch of the 

 interparietal sulcus. 



The mesial surface of the parietal lobe includes an irregularly quadrate area ex- 

 tending from the internal limb of the parieto-occipital sulcus behind to the line of the 

 Rolandic fissure in front; below it is imperfectly defined from the limbic lobe by the 

 calloso-marginal sulcus, to a very slight extent, and its continuation, the post-limbic 

 furrow. By far the greater part of this surface is embraced by the quadrate lobule 

 or precuneus, an irregularly quadrilateral area (Fig. 987) limited in front by the 

 upturned terminal limb of the calloso-marginal and behind by the parieto-occipital 

 sulcus. The lobule, the mesial aspect of the superior parietal convolution, is usually 

 marked by one or more furrows, the precuneate sulci, which incise the upper margin 

 of the hemisphere and extend for a short distance onto the outer surface. 



The Occipital Lobe. The occipital lobe is pyramidal in form and includes 

 the occipital pole and the adjacent parts of the hemisphere. It is represented on all 

 of the aspects of the hemisphere and possesses, therefore, a lateral, a mesial and an 

 inferior or tentorial surface. A well-marked occipital lobe is found only in the brain 

 of man and of the anthropoid apes and is developed as a backward prolongation of 

 the parietal and temporal lobes, from which, therefore, it is but imperfectly sepa- 

 rated. On the mesial surface its extent is definitely limited by the internal parieto- 

 occipital sulcus, by which it is cut off from the quadrate lobule or precuneus of the 

 parietal lobe. On the lateral surface, on the contrary, it is continuous with the pari- 

 etal and temporal lobes, its anterior boundary being arbitrary and indicated by the 

 parieto-occipital line drawn from the overturned limit of the parieto-occipital sulcus 

 above to the preoccipital notch below. On the inferior or tentorial aspect its demar- 

 cation is even more uncertain, the occipital, limbic and temporal lobes being here 



