HEMISPHERES OF THE BRAIN. 



647 



of the hemisphere, a short distance behind the superior extremity of the fissure 

 of Rolando. 



The internal parieto-occipital extends in an oblique direction downward and 

 forward to join the calcarine fissure, on a level with the hinder end of the corpus 

 callosum. 



The calcarine fissure commences, usually by two branches, close to the posterior 

 extremity of the hemisphere. These soon unite, and the fissure runs nearly hori- 

 zontally forward, and is joined by the parieto-occipital fissure, and continues as 

 far as the posterior extremity of the corpus callosum, a little below the level of 

 which it terminates. Its anterior part causes the prominence in the interior of 

 the brain, known as the hippocampus minor or calcar avis. 



FIG. 345. Fissures and lobes on the internal surface of the cerebral hemispheres. 



The collateral fissure is situated on the tentorial surface, below and external 

 to the preceding, being separated from it by the sub-collateral or uncinate gyrus. 

 It runs forward, from the posterior extremity of the brain, nearly as far as the tip 

 of the temporal lobe. It lies below the posterior and descending horns of the 

 lateral ventricle, and its middle part causes the prominence in the interior of the 

 brain, known as the eminentia collateralis. 



The dentate or hippocampal fissure commences immediately behind the posterior 

 extremity of the corpus callosum, and runs forward to terminate at the recurved 

 part of the hippocampal gyrus. It causes the prominence of the hippocampus 

 major in the descending horn of the lateral ventricle. In addition to these fissures, 

 which are constant, there is frequently an irregular broken fissure, which appears 

 to be a continuation backward of the posterior part of the calloso-marginal fissure, 

 before it ascends to reach the upper edge of the hemisphere. This has been 

 termed the post-limUc fissure. These fissures map off portions of the internal and 

 tentorial surfaces of the hemispheres, which form parts of the lobes found on the 

 external surface. That portion which lies in front and above the calloso-marginal 

 fissure belongs almost entirely to the frontal lobe ; its posterior extremity, which 

 extends for a short distance behind the upper end of the fissure of Rolando, forms 

 a small part of the parietal lobe ; that portion which lies above the internal parieto- 

 occipital fissure and behind the calloso-marginal fissure forms a part of the parietal 

 lobe; that between the parieto-occipital fissure above and the calcarine 

 below is a portion of the occipital lobe ; and all the region below the calcarine 



