THE FORE-BRAIN OR PROSENCEPHALON 



825 



The Insula (island of Reil; central lobe) (Fig. 731) lies deeply in the lateral or 

 Sylvian fissure, and can only be seen when the lips of that fissure are widely sep- 

 arated, since it is overlapped and hidden by the gyri which bound the fissure. 

 These gyri are termed the opercula of the insula ; they are separated from each other 

 by the three rami of the lateral fissure, and are named the orbital, frontal, fronto- 

 parietal, and temporal opercula. The orbital operculum lies below the anterior 

 horizontal ramus of the fissure, the frontal between this and the anterior ascending 

 ramus, the parietal between the anterior ascending ramus and the upturned end 

 of the posterior ramus, and the temporal below the posterior ramus. The frontal 

 operculum is of small size in those cases where the anterior horizontal and ascending 

 rami of the lateral fissure arise from a common stem. The insula is surrounded 

 by a deep circular sulcus which separates it from the frontal, parietal, and temporal 

 lobes. When the opercula have been removed, the insula is seen as a triangular 

 eminence, the apex of w T hich is directed toward the anterior perforated substance. 

 It is divided into a larger anterior and a smaller posterior part by a deep sulcus, 

 hich runs backward and upward from the apex of the insula. The anterior 

 art is subdivided by shallow sulci into three or four short gyri, while the posterior 

 part is formed by one long gyrus, which is often bifurcated at its upper end. The 

 cortical gray substance of the insula is continuous with that of the different opercula, 

 while its deep surface corresponds with the lentiform nucleus of the corpus striatum. 



FIQ. 731. The insula of the left side, exposed by removing the opercula. 



Limbic Lobe (Fig. 727). The term limbic lobe was introduced by Broca, and 

 under it he included the cingulate and hippocampal gyri, which together arch 

 around the corpus callosum and the hippocampal fissure. These he separated on 

 the morphological ground that they are w T ell-developed in animals possessing a 

 keen sense of smell (osmatic animals), such as the dog and fox. They were thus 

 regarded as a part of the rhinencephalon, but it is now recognized that they belong 

 to the neopallium; the cingulate gyrus is therefore sometimes described as a part 

 of the frontal lobe, and the hippocampal as a part of the temporal lobe. 



The cingulate gyrus (gyrus cinguli; callosal convolution) is an arch-shaped convo- 

 lution, lying in close relation to the superficial surface of the corpus callosum, 

 from which it is separated by a slit-like fissure, the callosal fissure. It commences 

 below the rostrum of the corpus callosum, curves around in front of the genu, 

 extends along the upper surface of the body, and finally turns downward behind 

 the splenium, where it is connected by a narrow isthmus \vith the hippocampal 



