1382 A MANUAL OF ANATOMY 



portion in front of this fissure is known as the orbital area, and is 

 concave, in adaptation to the convexity of the orbital plate of the 

 frontal bone, upon which it rests. The extensive portion behind 

 the stem of the Sylvian fissure is known as the tentorial area, and is 

 prominent and arched. Its anterior portion is received into the 

 lateral division of the middle cranial fossa, and its posterior portion 

 rests upon the tentorium cerebelli. 



The borders of each hemisphere are four m number, namely, 

 supero-mesial, infero -lateral, superciliary, and internal occipital. 

 The supero-mesial border separates the external from the internal 

 surface. The infer o-lateral border separates the external surface 

 from the tentorial area of the inferior surface. The superciliary 

 border separates the front part of the external surface from the 

 orbital area of the inferior surface. The internal occipital border 

 separates the internal surface from the tentorial area of the inferior 

 surface, and it extends from the occipital pole to the splenium of 

 the corpus callosum. 



The exterior of each hemisphere is broken up into tortuous 

 eminences, called gyri or convolutions, and these are separated from 

 each other by clefts, called sulci or fissures. The exterior is com- 

 posed of grey matter, which is spoken of as the cerebral cortex, 

 and the interior is occupied by white matter, which forms the 

 medullary centre. The breaking up of the hemispheres into gyri, 

 with the intervening sulci, greatly increases the amount of cerebral 

 cortex and, to a proportionate extent, of pia mater. 



The fissures are of two kinds — complete and incomplete. The complete 

 fissures are produced by infoldings of the entire wall of each cerebral hemi- 

 sphere, and corresponding to each of these fissures there is a projection on 

 the wall of the lateral ventricle. The complete fissures are as follows : (i) The 

 fissure of Sylvius, giving rise to the corpus striatum; (2) the choroidal fissure, 

 giving rise to the choroid plexus of the lateral ventricle; (3) the hippocampal 

 fissure, giving rise to the hippocampus major; (4) the calcarine fissure (anterior 

 part), giving rise to the calcar avis or liippocampus minor; and (5) the collateral 

 fissure (middle part), giving rise to the eminentia collateralis. 



The parieto-occipital fissure, though it gives rise to no internal projection, 

 may be regarded as a complete fissure. 



The incomplete fissures are merely furrows on the suriace of the hemisphere, 

 formed in association with the growth of the gyri. 



Each hemisphere presents six principal fissures, called interlobar 

 fissures, and by means of these it is divided into six lobes. The 

 fissures and lobes are as follows : 



Interlobar Fissures. Lobes. 



1. Fissure of Sylvius. 1. Frontal. 



2. Fissure of Rolando. 2. Parietal. 



3. Parieto-occipital. 3. Occipital. 



4. Calloso-marginal. 4. Temporal. 



5. Collateral. 5. Island of Reil or Insula. 



6. Circular or Limiting. 6. Limbic. 



Interlobar Fissures. — ^The fissure of Sylvius, which is the first 

 fissure to appear in the course of development, commences on the 

 inferior surface of the hemisphere at the locus perforatus anticus in 

 a depression, called the vallecula Sylvii. From this point it passes 



