780 



THE NERVOUS SYSTEM. 



lobe which begins just in front of the anterior perforated space at the base of the 

 brain : it passes forward below the rostrum, and then ascends in front of the genu 

 of the corpus callosum, and runs backward along the upper surface of this body 

 to its posterior extremity, around which it bends to join the hippocampal convo- 

 lution by a constriction, the isthmus. It is bounded below, in front, and above, 

 in the greater part of its extent, by the calloso-marginal fissure, which separates 

 it from the marginal convolution ; above and behind its bend it is separated by 

 the post-limbic fissure from the quadrate lobe. Between it and the corpus callosum 

 is the eallosal sulcus. 



The marginal convolution is situated parallel with the anterior portion of the 

 preceding, and has received its name from its position along the border of the 

 hemisphere. It commences in front of the anterior perforated space, runs along 

 the margin of the longitudinal fissure on the under surface of the orbital lobe, 

 being subdivided by the sulcus for the olfactory tract ; it then turns upward to the 



FIG. 470. Convolutions and fissures of the inner surface of the cerebral hemisphere. 



upper surface of the hemisphere and runs backward, forming the marginal convo- 

 lution, on the inner surface, to the point where the calloso-marginal fissure turns 

 upward to reach the superior border of the hemisphere. At this point, together 

 with the upper extremities of the ascending frontal and parietal gyri, which are 

 bent over on the inner surface of the hemisphere, it forms the paracentral lobule. 

 This convolution is regarded as being on the mesial aspect of the frontal lobe. 



The quadrate lobe (precuneus) is the "marginal" convolution of the longitu- 

 dinal fissure behind the posterior portion (paracentral fissure) of the calloso-marginal 

 sulcus, lying between this fissure in front and the internal parieto-occipital behind. 

 It is separated by the post-limbic fissure from the gyrus fornicatus below, and is 

 continuous above with the superior parietal convolution. 



The cuneate or occipital lobule is triangular in shape, being situated between 

 the internal parieto-occipital and calcarine fissures, which, as above mentioned, 

 meet behind the isthmus of the gyrus fornicatus. 



The infracalcarine (fifth temporal) convolution extends from the posterior ex- 

 tremity of the temporal lobe to join the hippocampal gyrus, being bounded above 

 by the calcarine and its anterior prolongation, after its junction with the parieto- 

 occipital fissure, and separated below from the fourth temporal convolution by the 

 collateral fissure. The back part of this convolution that is, the part below the 



