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THE NERVOUS SYSTEM. 



tract ; and the portion internal to this is named the gyrus rectus, and is continuous 

 with the marginal gyrus, presently to be described. The mesial or internal surface 

 of the frontal lobe is occupied by a single curved convolution, which from its situa- 

 tion is termed the marginal gyrus (Fig. 348). It commences in front of the ante- 

 rior perforated space, runs along the margin of the longitudinal fissure on the 

 mesial surface of the orbital lobe, where it is continuous with the internal orbital 

 convolution ; it then ascends, and runs backward to the point where the calloso- 

 marginal fissure turns upward to reach the superior border of the hemisphere. 

 An oval portion at the posterior part of this convolution is sometimes marked off 

 by a vertical fissure, and is distinguished as the paracentral gyrus, because it is 

 continuous with the convolutions in front and behind the central fissure or fissure 

 of Rolando. 



2. The Parietal Lobe. On its external surface the parietal lobe presents for 

 examination two sulci and three convolutions. 



FIG. 348. Convolutions and sulei on the internal surface of cerebral hemispheres. 



The intra-parietal sulcus commences close to the horizontal limb of the fis.sure 

 of Sylvius, about midway between the fissure of Rolando and the upturned ex- 

 tremity of the fissure of Sylvius. It first runs upward parallel to and behind the 

 lower half of the fissure of Rolando, and then turns backward, extending nearly 

 to the termination of the external parieto-occipital fissure, where it sometimes 

 becomes continuous with the superior occipital sulcus. The ascending portion of 

 this sulcus separates off a convolution, the ascending parietal, which lies between 

 it and the fissure of Rolando, while the horizontal portion divides the remainder 

 of the external surface of the parietal lobe into two other convolutions, the superior 

 and inferior parietal. 



The post-central sulcus is a slightly marked groove, which is sometimes a branch 

 of the intra-parietal sulcus, being given off where the ascending portion of this 

 sulcus turns backward. It lies parallel to and behind the upper part of the fissure 

 of Rolando, and separates the ascending from the superior parietal convolution. 1 



1 Professor Cunningham describes these two sulci, the intra-parietal and post-central, somewhat 

 differently. He regards them as both belonging to the intra-parietal sulcus, which he divides into 

 three parts : the ascending portion of the intra-parietal, as described above, he terms the ramus 

 verticalis inferior ; the horizontal portion as the ramii-s horizontalis ; while the post-central sulcus he 

 denominates the ramus verticalis superior. He states that considerable variability is exhibited in the 

 relation to each other of these different parts of the intra-parietal sulcus, but that the one in which 

 the three parts of the sulcus are confluent is by far the most constant condition. Sometimes, however, 



