862 



THE NERVOUS SYSTEM 



upon the extent to which these four factors become continuous in the later de- 

 velopment. When continuity of the furrows is well established, the entire sulcus 

 may be described as consisting of a convex horizontal ramus, which gives off a few 

 short collateral t^^ngs and whose either end is in the form of an irregular, reclining 

 H . The transverse bar of the anterior end arises fron two of the four factors of 

 the entire sulcus: — (1) an inferior furrow, the inferior posfce7itral sulcus, commenc- 

 ing above the posterior ramus of the lateral fissure and ascending as the boundary 

 of the lower half of the posterior central gyrus, and (2) a superior furrow, the 

 superior postcentral sulcus, lying behind the upper portion of the posterior central 

 gyrus, and which, upon approaching the supero-mesial border, may turn back- 

 ward a short distance parallel with the horizontal ramus, as in fig. 674. When 

 confluent, these two factors form together a continuous postcentral sulcus. 

 In the adult the inferior of the two is always continuous with the horizontal 

 ramus; when confluent, the two figures join so as to form the transverse bar of 

 the anterior end of this ramus. The horizontal ramus, which represents the 



Fig. 679. — Outline Drawing op Mesial Surface op Left Cerebral Hemisphere. 

 (After Toldt, "Atlas of Human Anatomy," Rebman, London and New York.) 



Central sulcus (Rolandi) 



Sulcus cingtHi (marginal portion) 



'\ 

 Subparietal sulcus 



Parieto-occipital fissure 



Calcarine fissure 



Massa intermedia 



Sulcus cinguli (subfrontal portion) 



Sulcus corporis caUosi 



Mesencephalon 



Tuber cinereum 



Genu of corpus callosunt 

 '\ Rostum of corpus callosum 

 \ \ \ Anterior parolfactory sulcus 

 \ \ Parolfactory area (Broca's area) 

 \ Posterior parolfactory sulcus 

 Hypophysis Sub-callosal gyrus (peduncle of 

 corpus callosum) 

 Infundibulum 



third of the primary furrows, is continued backward past the superior extremity 

 of the parieto-occipital fissure into the occipital lobe, where it usually joins the 

 occipital ramus, the fourth of the primary furrows. This ramus divides shortly 

 into two branches which run at right angles to the stem, forming the transverse 

 occipital sulcus, and thus arises the transverse bar of the posterior end of the inter- 

 parietal sulcus. The occipital ramus may, however, consist of little more than 

 the transverse bar, which may or may not be joined by the horizontal ramus. 

 The occipital ramus is more frequently separate from the horizontal than is the 

 postcentral sulcus. In their development the inferior postcentral sulcus appears 

 first (during the latter part of the sixth month), the occipital ramus second, the 

 horizontal ramus third, and last, the superior postcentral sulcus. 



The superior parietal lobule (gyrus) is the area of the supero-mesial border 

 of the parietal lobe. It is limited in front by the superior postcentral sulcus, 

 below by the horizontal ramus of the int(>rparictal sulcus, and posteriorly it is 

 continuous around the superior end of the i)arieto-occi])ital fissure into the cortex 

 of the occipital lobe. It is a r(>latively wide area (lobuk;), always invaded by 

 collateral twigs of its limiting sulci, and usually contains a few short, isolated 

 furrows. When the parieto-occipital fissure is considerably prolonged over the 

 supero-mesial border (external parieto-occipital fissure), the continuation of the 



