854 



THE NERVOUS SYSTEM 



(6) Olfactory brain or rhinencephalon (including structures comprised in the 

 other lobes and often grouped under the two names olfactory lobe and limbic 

 lobe). 



This division of the cortex of the hemisphere is largely a merely topographical 

 one. With the exception of the temporal lobe and the rhinencephalon, it has 

 little of either morphological or functional value. The occipital lobe contains the 

 recognised visual area of the cortex, but this area, as such, does not involve all of 

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

 ally, 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 paUium 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 

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

 and height and begin to overlap and conceal its broad floor, which is the beginning 



Fig. 673. — 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 Reil. 



Central sulcus (Rolandi) 



Parietal lobe 



Frontal lobe 



Central lobe (insula) 



Occipital lobe 



Cerebellum 



Temporal lobe 



Central sulcus of insula 



of the insula. The overlapping folds thus become the opercula, and as their Hps 

 approach each other, there results the deep fissure of Sylvius, which marks off 

 anteriorly an infero-lateral limb of the pallium, termed by position the temporal 

 lobe. As growth proceeds further, the temporal lobe thickens, the temporal pole 

 extends further forward 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 the 

 lobes above and behind, except by arbitrary Unes which will be mentioned in con- 

 nection with those lobes. 



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

 hemis])liere, 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 (fig. 074) : — 



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

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

 regicm of tlx! parietal lobe above. The uj)per margin of this gyrus constitutes the 

 temporal 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 



