834 THE NERVOUS SYSTEM 



lobe. In their functional significance, the frontal and parietal lobes, especially, 

 overlap each other. 



The temporal lobe. This is the first lobe whose demarcation is indicated. 

 During; the second month of intra-uterine life there appears a slight depression on the 

 lateral aspect of the then smooth hemisphere. As the pallium further grows, this 

 depression deepens into a well-marked fossa with a relatively broad floor. This 

 fossa marks the beginning of the lateral cerebral fissure or fissure of Sylvius, and is, 

 therefore, known as the Sylvian fossa. As the pallium continues to project out- 

 wards, the folds which form the margins of the Sylvian fossa increase in size and 

 height and begin to overlap and conceal its broad floor. The overlapping folds thus 

 become the opercula, and as their lips approach each other, there results the deep 

 fissure of Sylvius, which marks off anteriorly a ventro-lateral limb of the pallium, 

 termed by position the temporal lobe. As growth proceeds further, the temporal lobe 

 thickens, the temporal pole extends further forwards and becomes a free projection, 

 thus lengthening the fissure of Sylvius and resulting in the inferior extension or stem 

 of this fissure, which runs between the temporal pole and the frontal lobe and curves 

 under so as to appear on the basal surface of the hemisphere. Finally the cortex of 

 the lobe itself is thrown into folds or gyri. Its posterior end is never marked off from 



FIG. 621. DIAGRAM OF THE CONVEX SURFACE OF THE LEFT CEREBRAL HEMISPHERE SHOWING 



THE FIVE PRINCIPAL LOBES OF THE PALLIUM. 



The opercular regions of the frontal, parietal, and temporal lobes are removed to show the cen- 

 tral lobe or island of Keil. 



CENTRAL SULCUS (ROLAXD1) 



PARIETAL LOBE 



OCCIPITAL LOBE 

 FRONTAL LOBE 



CEREBELLUM 



CENTRAL LOBE (INSULA) 



TEMPORAL LOBE 



CENTRAL SULCUS OF INSULA 



the lobes above and behind, except by arbitrary lines which will be mentioned in 

 connection with those lobes. 



The temporal lobe forms part of the lateral convex and tentorial surfaces of the 

 hemisphere, and its anterior portion is adapted to the surface of the middle cranial 

 fossa. It thus has a superior and lateral surface and a basal and tentorial surface. 

 In these surfaces are the following gyri with their intervening and bounding sulci: 



The superior temporal gyrus is bounded by the posterior ramus of the lateral 

 fissure, and extends from the temporal pole backwards into the supra-marginal region 

 of the parietal lobe above. The upper margin of this gyrus constitutes the tem- 

 poral operculum, in that it aids in overlapping and enclosing the insula in the floor 

 of the lateral fissure. This margin is for the most part smooth, being occasionally 

 interrupted by a few weak twigs of the lateral fissure. It is separated from the gyrus 

 below by the superior temporal sulcus, which is parallel with the posterior ramus 

 of the lateral fissure and is frequently called the parallel sulcus. The posterior ex- 

 tremity of this sulcus divides the angular gyrus of the parietal lobe, and its anterior 

 end disappears in the temporal pole, sometimes as a continuous groove, sometimes 

 in isolated pieces. 



The middle temporal gyrus likewise begins in the temporal pole and is con- 

 tinuous backwards into the angular gyrus of the parietal lobe. 



