THE BRAIN 



gyms. The intermediate stria, which is not always present, 

 runs posteriorly across the trigonum olfactorium. 



The gyrus sub-callosus is a narrow cortical strip, of some morphological 

 importance, which lies on the medial surface of the hemisphere immediately 

 below the genu of the corpus callosum. 



The trigonum olfactorium is the little triangular field of 

 grey matter which occupies the interval between the medial 

 and lateral striae of the olfactory tract at the point where 

 they begin to diverge. 



Gyrus Fornicatus. This gyrus, sometimes called the 

 limbic lobe, is seen on the medial and tentorial surfaces of the 

 hemisphere. It is a ring-like convolution, the extremities of 

 which approach each other closely at the substantia perforata 

 anterior. 



The upper and anterior part of the gyrus fornicatus lies 

 in intimate relation to the extremities and upper surface of 

 the corpus callosum, and it is called the gyrus cinguli. The 

 lower portion of the gyrus fornicatus is termed the hippocampal 

 gyrus, and forms the medial part of the tentorial portion of 

 the lower surface of the hemisphere. The continuity between 

 the hippocampal gyrus and the gyrus cinguli is established, 

 below the posterior end of the corpus callosum, by a narrow 

 portion of the gyrus fornicatus called the isthmus. From this 

 point the hippocampal gyrus extends anteriorly towards the 

 temporal pole. Finally, at the side of the pedunculus cerebri, 

 the hippocampal gyrus is folded on itself, and ends in a 

 recurved hook-like extremity, termed the uncus. The uncus 

 does not reach the temporal pole. 



The gyrus cinguli (O.T. callosal convolution) begins below 

 the anterior end of the corpus callosum at the substantia 

 perforata anterior, and, winding round the genu of the 

 callosum, it is continued posteriorly on its upper surface to 

 the thickened posterior extremity or splenium. Finally, 

 curving round this, it becomes greatly narrowed through 

 the calcarine fissure cutting into it. This narrow part is 

 termed the isthmus, and constitutes the link of connection 

 between the gyrus cinguli and the hippocampal gyrus. 



The gyrus cinguli is separated from the superior frontal 

 gyrus and the paracentral lobule by the sulcus cinguli ; 

 from the praecuneus by the subparietal sulcus ; and from the 

 lingual gyrus by the calcarine fissure. It is separated from 

 the corpus callosum by the callosal sulcus. 



