THE SULCI AND GYEI OF THE CEREBEAL HEMISPHERES. 655 



Opercula Insulae. The overlapping portions of the cerebral substance which 

 cover over the insula are termed the insular opercula, and they form, by the apposi- 

 tion of their margins, the three rami of the lateral fissure. The rami of the 

 fissure extend from the exposed surface of the hemisphere to the submerged surface 

 of the insula, and, in this manner, separate the opercula from each other. 



The temporal opercuhim (pars temporalis) extends upwards over the insula from 

 the temporal region, and its superior margin forms the inferior lip of the posterior 

 ramus of the lateral fissure. 



The superior opercuhim is carried downwards from the parietal (pars parietalis) 

 and frontal (pars frontalis) regions over the insula, and its inferior margin, meeting 

 the temporal operculum, forms the superior lip of the posterior ramus of the lateral 

 fissure. 



The small triangular piece of cerebral substance which sometimes intervenes 

 between the ascending and horizontal anterior rami of the lateral fissure is 

 formed by the bending downwards of the front part of the upper operculum. It 



Superior opercuhim 



Gyri breves insulte 



Sulcus centralis insulse 



Gyrus longus insulse 



Gyrus temporalis transversus 

 anterior 



Tractus olfactorius 



Area piriformis | Limen insulse 

 Line of obliterated rhinal fissure 



Area acustica extending on to 

 the superior temporal gyrus 



FIG. 582. PART OF A LEFT CEREBRAL HEMISPHERE WITH THE OPERCULA OF THE INSULA WIDELY 

 SEPA HATED TO EXPOSE THE INSULA AND THE SUPERIOR SURFACE OF THE TEMPORAL OPERCULUM. 



The area acustica is coloured a uniform blue, the area intermedia with large blue spots and the 

 area circumambiens with fine blue dots. 



covers over a small part of the anterior portion of the insula, and is sometimes 

 termed the pars triangularis. 



The orbital operculum is, for the most part, on the inferior surface of the hemi- 

 sphere. It lies below and to the medial side of the horizontal anterior ramus of the 

 lateral fissure, and proceeds backwards from the orbital aspect of the frontal lobe 

 over the anterior part of the insula. 



Development of the Lateral Fissure and of the Insular District of the Cerebral Hemi- 

 sphere. It is only during the latter half of the intra-uterine period of development that the 

 opercula take shape and grow over the insula, so as to shut it out from the surface. In its 

 early condition the insula presents the form of a depressed area on the side of the cerebral 

 hemisphere, surrounded by a distinct boundary wall formed by the surrounding more elevated 

 surface of the hemisphere (Fig. 583, A). After a time this depressed area, which is called the 

 fossa lateralis, assumes a triangular outline, and then the bounding wall is observed to be com- 

 posed of three distinct parts, viz., a superior or fronto- parietal, an inferior or temporal, and 

 an anterior or orbital part (Fig. 583, B). The angle formed by the meeting of the superior 

 and anterior portions of the boundary, may become flattened, and a short oblique part of the 

 limiting wall develop into a small triangular frontal operculum (Fig. 583, F). Each of these 

 portions of the bounding wall of the fossa becomes a line of growth, from which an operculum 



