CRANIAL CAVITY. 



Island of Reil or Insula. This is a triangular area, apex 

 downward, lying at the bottom of the Sylvian fissure ; at the 

 apex (limen insulse) , where the limiting sulcus of Reil is absent, 

 its gray matter is continuous with the locus perforatus anticus. 

 Radiating sulci divide it into diverging gyri; the sulcus cen- 

 tralis insulse, in the same plane and direction as the fissure of 

 Rolando, divides it into a frontal and a parieto-limbic por- 

 tion. [567] 



Iambic Lobe. This curves, on the mesial surface, around 

 the corpus callosum ; the olfactory tract roots connect its ends 

 at the locus perforatus anticus; its upper part is the callosal 

 gyrus, and its lower the hippocampal gyms. The callosal con- 

 volution runs from the locus perforatus anticus, upward 

 around the genu, backward on the upper surface, and down- 

 ward around the splenium of the corpus callosum; a portion 

 (isthmus) which is narrowed by the calcarine fissure joins it 

 to the hippocampal gyrus. The calloso-marginal sulcus sepa- 

 rates it from the marginal gyrus, the postlimbic from the 

 prsecuneus, and the callosal from the corpus callosum. The 

 hippocampal convolution runs forward to the crus cerebri, then 

 folds back on itself and ends in a recurved hook-like tip 

 (uncus). It is bounded externally by the collateral fissure 

 and incisura temporalis, and internally by the dentate fissure; 

 posteriorly the calcarine fissure separates the isthmus from a 

 portion joining the gyrus lingualis. A white layer (substantia 

 reticularis alba) covers it. [568] 



The dentate fissure, continuous with the collateral, begins 

 behind the splenium, separates the hippocampal gyrus from 

 the gyrus dentatus, and ends in the uncus. It produces the 

 hippocampus major; hidden in it are the following structures. 

 The gyrus dentatus is a free notched edge of gray matter ex- 

 tending from near the splenium into the cleft of the uncus, 

 whence it emerges as a delicate band (frenulum Giacomini) 

 crossing the uncus ; the fimbrio-dentate sulcus separates this 

 gyrus from the fimbria, lying deeper. [569] 



[41] 



