648 



THE NERVOUS SYSTEM. 



fissure behind and the collateral fissure in front belongs to the temporal lobe. The 

 remainder of the mesial and tentorial surfaces of the hemisphere constitute what 

 Broca termed the lirnbic lobe, which is subsequently referred to (page 652). 



The surface of the hemisphere has thus been divided into its different parts, 

 viz. : the frontal, the parietal, the occipital, the temporal, the limbic, the 

 olfactory lobes, and the island of Reil. Each of these lobes is further subdivided 

 into convolutions or gyri by smaller fissures, which, though less constant in their 

 arrangement than the fissures already described, have a fairly definite course. 



1. The Frontal Lobe. On its external surface the frontal lobe presents three 

 sulci, which divide it into four convolutions (Fig. 346). The precentral sulcus 

 runs upward through this lobe, parallel to the lower half of the fissure of 



End of calloso- 

 margiual fissure. 



P 4 



*/, 



Parieto- 

 occipital 



fissure. 



^? 



Ascending fissure 

 of fiylvim. 



Fissure 

 of Sylvius. 



FIG. 346. Convolutions and sulci on the external surface of the cerebral hemisphere. 



Rolando. It is frequently broken or interrupted by annectant gyri. It limits 

 a convolution, which lies 'between it and the fissure of Rolando, and which is 

 called the ascending frontal convolution. From it two sulci, the superior and 

 inferior frontal, run forward and downward, and divide the remainder of the 

 outer surface of the lobe into three parallel principal convolutions, named respec- 

 tively the superior, middle, and inferior frontal convolutions. 



The ascending frontal convolution is a simple convolution, bounded in front by 

 the precentral sulcus, behind by the fissure of Rolando, and extending from the 

 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 longi- 

 tudinal fissure and the superior frontal sulcus. It extends above on to the inner 

 aspect of the hemisphere, forming the greater part of the marginal convolution, 

 and in front on to the orbital surface, forming the internal orbital convolution. 

 It is usually more or less completely subdivided into two by an antero-postenor 



