924 THE NER VE SYSTEM 



The principal fissures marking its surface consist of a group of integral segments 

 showin^ various degrees of confluence in different individuals and formerly 

 known in the aggregate as the intraparietal sulcus of Turner (sulcus intrapariefalis). 

 Two of the fissural segments present much the same parallelism to the central 

 fissure which was noted for the precentral group, and hence these are termed 

 the postcentral fissural complex. 



The postcentral fissural complex (sulcus posicentralis) comprises a longer mesal 

 and a shorter lateral (and ventral) segment, which are confluent in about 75 per 

 cent, of brains and then very much resemble in length, continuity, and course 

 the central fissure. This appearance has given rise to reports of alleged duplica- 

 tion of the central; an analysis of the relations of the dorsal ends of the fissures 

 in question with the caudal limb of the paracentral removes all doubt. 



The postcentral fissure (proper) is the longer mesal (and dorsal) segment. Its 

 dorsal end is frequently bifurcated and sometimes embraces the dorsal extension 

 of the caudal limb of the paracentral. The subcentral fissure constitutes the shorter 

 lateroventral segment. 



The parietal fissure is usually a slightly arched fissure inclining mesocaudad, 

 sometimes independent but more often confluent, with one or both of the post- 

 central segments just described. It demarcates the parietal gyre from the sub- 

 parietal district. 



The paroccipital fissure, in whole or in part, probably represents a part of the 

 simian exoccipital or "Affenspalte," isolated by the upgrowth of gyral protons 

 which are totally submerged in the ape brain, but rose 'to the surface concomitant 

 with the rise in functional dignity of cortical areas so important in the human 

 brain. The fissure is almost invariably of zygal shape, its stem directed sagittally, 

 its ends bifurcated. Its confluence with the parietal fissure seems to be subject 

 to some morphologic law; continuity is the rule on the left side (77 per cent.), and 

 occurs less often on the right. The combination of continuity on the right and 

 separation on the left is a rare one (6 per cent.). 



Less constant fissures are the transparietal, in the parietal lobe, and the inter- 

 medial (Fig. 674). In the subparietal district terminate the upturned ends of 

 the sylvian (i. e., episylvian ramus) of the supertemporal and the meditemporal 

 fissures. 



2. The mesal surface of the parietal lobe is equivalent to the quadrangular 

 precuneus, limited by the paracentral and occipital fissures, while ventrad it is 

 imperfectly separated from the callosal gyre by the precuneal fissure (postlimbic 

 sulcus), usually of zygal or triradiate form and occasionally confluent with the 

 paracentral. 



GYRES OF THE PARIETAL LOBE. 1. Lateral Surface. The postcentral gyre 

 (gyrus centralis posterior) is one of the chief somesthetic areas of the cortex. It 

 is a long, more or less sinuous convolution extending obliquely from the dorsimesal 

 border to the sylvian fissure and demarcated by the central and the postcentral + 

 subcentral fissures. 



The parietal gyre (gyrus parietalis) lies between the dorsimesal border and the 

 parietal fissure, bounded cephalad by the postcentral, caudad partly by the 

 occipital fissure, the transition to the occipital lobe being maintained by the 

 arched paroccipital gyre. 



The subparietal district or lobule (lobulus parietalis inferior) is divided into 

 three convolutions which arch around the upturned ends of the sylvian. super- 

 temporal, and meditemporal, and merge insensibly with the adjacent temporo- 

 occipital gyres. The marginal (supramarginal) gyre arches over the extremity of 

 the episylvian ramus and is connected frontad with the postcentral gyre, ventrad 

 with the supertemporal gyre. The angular gyre (gyrus angularis) arches over the 

 upturned extremity of the supertemporal fissure, and its limbs fuse with the 



