THE CEREBRUM 463 



surface of the cerebrum the lateral fissure immediately 

 divides into three radiating branches. These are (i) the 

 ramus posterior ; (2) the ramus anterior horizontalis ; and 



(3) the ramus anterior ascendens. 



The posterior ramus (Fig. 186) is the longest and the most 

 important of the three. It extends posteriorly, with a slight 

 upward inclination, for a distance of three inches or more, 

 between the frontal and parietal lobes, which lie above it, and 

 the temporal lobe, which is placed below it. Finally, it 

 comes to an end by turning upwards into the parietal lobe in 

 the form of an ascending terminal piece (Fig. 186). 



The anterior horizontal limb (Fig. 186) extends anteriorly in 

 the frontal lobe, for a distance of about three-quarters of 

 an inch, immediately above and parallel to the posterior 

 part of the superciliary margin of the hemisphere. 



The anterior ascending limb (Fig. 186) proceeds upwards, 

 with a slight anterior inclination, into the lower part of the 

 lateral surface of the frontal lobe for a distance of about an 

 inch. In many cases the two anterior limbs spring from a 

 common stem of variable length (Fig. 186). 



Sulcus Circularis (O.T. Limiting Sulcus of Reil). If the 

 lips of the posterior branch of the lateral fissure are now 

 gently pulled asunder, the insula (O.T. island of Reil) will be 

 seen at the bottom. This is surrounded by a circular sulcus 

 which is separable into three parts, viz., an upper part 

 bounding the insula above, a lower part marking it off below, 

 and an anterior part limiting it anteriorly. The insula thus 

 mapped out is somewhat triangular in form. 



Opercula Insulae. The present is a good time to study 

 the manner in which the insula is shut off from the surface 

 of the hemisphere. When the lateral fissure is held widely 

 open, it will be noted that the insula is overlaid by portions 

 of cerebral cortex which appear as if they were undermined. 

 These by the approximation of their margins or lips form the 

 three branches of the lateral fissure, and are termed the 

 opercula insulce. The opercula are four in number, and are 

 named (i) temporal, (2) fronto- parietal, (3) frontal, and 



(4) orbital. They are easily distinguished. 



The temporal operculum extends upwards over the insula 

 from the temporal lobe ; it forms the lower lip of the posterior 

 branch of the lateral fissure. 



The fronto-parietal operculum is carried downwards over 



