776 



THE NERVOUS SYSTEM. 



upper margin of the hemisphere above to a little behind the bifurcation of the 



fissure of Sylvius below. 



The superior frontal convolution 

 is situated between the margin of 

 the longitudinal fissure and the su- 

 perior frontal sulcus. It extends 

 above on to the inner aspect of the 

 hemisphere, forming the marginal 

 convolution, and in front and below 

 on to the orbital surface, forming 

 the internal orbital convolution. It 

 is much divided by secondary sulci. 

 The middle frontal convolution 

 is situated between the superior and 

 inferior frontal sulci, and extends 

 from the precentral sulcus to the 

 lower margin of the lobe, where it 

 forms the anterior orbital convolu- 

 tion. 



The inferior frontal convolution 

 is situated below the inferior frontal 

 sulcus, and extends from the lower 

 part of the precentral sulcus, circling 

 round the ascending and anterior 

 limbs of the fissure of Sylvius, to 

 the under surface of the lobe, where 

 it forms the posterior orbital convo- 

 lution. 



The parietal lobe is situated be- 

 tween the frontal and occipital lobes, 

 and is not much more than half the 

 size of the former. It is bounded in 

 front by the fissure of Rolando ; be- 

 hind, by the external parieto-occip- 

 ital fissure and a line drawn in continuation of that sulcus over the hemisphere ; 

 and below, by the horizontal limb of the fissure of Sylvius and a line connecting 

 this with the lower end of the superior occipital sulcus. Above, it extends to the 

 longitudinal fissure. It presents for examination two sulci and three convolutions. 

 The intraparietal sulcus commences close to the horizontal limb of the fissure 

 of Sylvius, about midway between the fissure of Rolando arid the upturned 

 extremity 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 some- 

 times becomes continuous with the superior occipital sulcus. The ascending por- 

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

 between it and the fissure of Rolando, whilst the horizontal portion divides the 

 remainder 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 intraparietal 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, intraparietal and post central, somewhat differ- 

 ently. He regards them as both belonging to the intraparietal sulcns, which he divides into three 

 parts: the ascending portion of the intraparietal, as described above, he terms the ramus vertical is infe- 

 rior; the horizontal portion as the ramus horizontal is ; while the post-central sulcus he denominates 

 the ramus vertically superior. He states that considerable variability is exhibited in the relation to each 



FIG. 467. Convolutions and fissures of the under surface 

 of the anterior lobe. 



