THE EXTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES 



2 37 



aspect. In this case a small semilunar furrow develops around it on the lateral 

 surface and is known as the sulcus lunatus (Fig. 168). This sulcus, called by 

 Riidinger the "Affenspalte," forms a conspicuous feature of the lateral surface of 

 the cerebral hemisphere in the lower Old World apes (Ingalls, 1914). 



The Insula. The part of the cortex which overlies the corpus striatum lags 

 behind in its development and becomes overlapped by the surrounding pallium. 

 The cortex, which thus becomes hidden from view at the bottom of the lateral 

 fissure, forms in the adult a somewhat conical mass called the insula or island of 

 Reil (Fig. 169). Its base is surrounded by a limiting furrow, the circular sulcus, 

 which is, however, more triangular than circular, and in which we may recognize 

 three portions: superior, inferior, and anterior. The apex of this conical lobe 



Parietal lobe 



,/ Central sulcus of insula 

 Circular sulcus 

 , Frontal lobe 



Occipital lobe x 



V?- Short gyri of insula 



Temporal lobe i ong gyrus O f i nsu la 



Fig. 169. Lateral view of the human cerebral hemisphere with the insula exposed by removal of 



the opercula. (Sobotta-McMurrich.) 



is known as the limen insulce; and the remainder is subdivided by an oblique 

 groove (sulcus centralis insulae) into the long gyrus of the insula and a more 

 rostral portion, which is again subdivided into short gyri. 



The Operculum. As the adjacent portions of the pallium close over the 

 insula (Fig. 164) they form by the approximation of their margins the three 

 rami of the lateral fissure. These folds constitute the opercula of the insula. 

 Each of the three surrounding lobes takes part in this process; and we may 

 accordingly recognize a, frontal, a temporal, and a parietal operculum (Fig. 166). 



At this point it will be instructive to examine the lateral surface of the cerebral 

 hemisphere of the sheep. It will be seen that the region which corresponds to 

 the insula is on a level with the general surface of the hemisphere; no opercula 

 have developed, and the lateral sulcus is only a shallow groove (Fig. 173). 



