840 THE NERVOUS SYSTEM 



which is the motor area for the muscles of the arm, and the development of this area 

 in man probably aids in producing it. (2) The inferior genu occurs below, is 

 concave backwards, and is probably in part a result of the superior genu the turn 

 of the sulcus in resuming its general course. The upper part of this genu also 

 accommodates a smaller (inferior) portion of the arm area. 



The central sulcus (Rolandi) appears in the pallium of the foetus during the latter part 

 of the fifth month. It then consists of a lower longer and an upper shorter part. Usually 



thesc two parts become continuous before birth; very rarely do they remain separate in the adult. 

 The point of their fusion is sometimes manifest within the depth of the sulcus. If the lips of the 



fused across the floor of the sulcus, so as to form a bridge of grey substance known as the deep 

 annectant gyrus. This interruption of the continuity of the sulcus, when present, represents the 

 point at which the two parts of the sulcus in the foetal brain joined each other without the con- 

 linuity becoming wholly completed in the adult. The genua of the adult sulcus may often be 

 due to the precedent parts not being in line at the time of their fusion. 



The parietal lobe. The parietal lobe occupies a somewhat smaller area of 

 the human telencephalon than either the frontal or the temporal lobe. It has a 

 convex and a mesial surface, but no basal surface. It is separated from the frontal lobe 

 in front by the central sulcus; from the occipital lobe behind by the parieto-occipital 

 fissure (fig. 626), and by an arbitrary line drawn transversely around the convex sur- 

 face of the hemisphere from the superior extremity of this fissure to the infero-lateral 

 border; and from the temporal lobe below by the horizontal part of the posterior 

 ramus of the lateral fissure, and by a line drawn in continuity with this horizontal 

 part to intersect the transverse line drawn to limit it from the occipital lobe. 



The preoccipital notch. In situ, the infero-lateral border of the posterior portion of the 

 hemisphere rests over a small portion of the parieto-mastoid suture of the cranium, and upon 

 this structure occurs a fold or vertical thickening of the dura mater, which slightly indents the 

 infero-lateral border. This indentation occurs about 4 cm. from the occipital pole, and is con- 

 sidered one of the points of limitation of the parietal from the occipital lobe, and is therefore 

 called the preoccipital notch. While during late foetal life and early childhood it is well marked, 

 it is usually very slight in the adult brain, and is, as a rule, evident only in brains hardened 

 in situ. When it is visible, the arbitrary transverse line from the superior extremity of I IK; 

 parieto-occipital fissure, used as a boundary between the convex surfaces of the parietal and 

 occipital lobes, should be so drawn as to bisect the preoccipital notch. 



The convex surface of the parietal lobe comprises the following gyri and sulci : 

 The posterior central gyrus (ascending parietal) extends obliquely across the 

 hemisphere parallel with the anterior central gyrus of the frontal lobe, from which it 

 is separated by the central sulcus. Its inferior end is bounded by the posterior ramus 

 of the lateral fissure, and constitutes the posterior or parietal portion of the fronto- 

 parietal operculum. Its upper end takes part in the supero-mesial border of the 

 hemisphere, and is bounded posteriorly by the tip end of the marginal portion of the 

 sulcus cinguli. Its postero-lateral boundary consists of the two more or less vertical 

 rami or factors of the interparietal sulcus, viz., the inferior and superior portions 

 of the postcentral sulcus, either continuous with each other or detached. 



The interparietal sulcus (intraparietal) is often the most complicated sulcus 

 of the pallium. Its development usually begins as four different furrows in the fu-tal 

 brain, and the difficulty with which it is traced in the adult brain depends upon the 

 extent to which these four factors become continuous in the later development. 

 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 twigs 

 and whose either end is in the form of an irregular, reclining I. The transverse 

 bar of the anterior end arises from two of the four factors of the entire sulcus: (1) 

 an inferior furrow, the inferior postcentral sulcus, commencing above the posterior 

 ramus of the lateral fissure and ascending as the boundary of the lower half of the pos- 

 terior central gyrus, and (2) a superior furrow, the superior postcentral sulcus, lying 

 behind the upper portion of the posterior central gyrus. and wjiich, upon approaching 

 the supero-mesial border, may turn backward a short distance parallel with the 

 horizontal ramus, as in fig. 622. When confluent, these two factors form together a 

 continuous postcentral sulcus. In the adult the inferior of the two is always con- 

 tinuous 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 



