ANATOMY AND PHYSIOLOGY OF THE BRAIN 361 



than behind. Its inferior surface is flattened and 

 overlaps the midbrain and cerebellum, from which it 

 is separated by the tentorium cerebelli. The outer 

 surface, including the fissures, is composed of gray 

 matter, the cortical substance, while the interior is 

 of white matter. 



LOBES OF THE CEREBRUM. The lobes are the 

 frontal, the parietal, the occipital, the temporal, and 

 the central lobe or Island of Reil. 



The Frontal Lobe. The lateral surface is separated 

 behind from the parietal lobe by the central fissure or 

 the fissure of Rolando, and below, from the temporal 

 lobe by the Sylvian fissure, in part, and rests on the 

 orbital plate of the frontal bone. 



The Parietal Lobe. The lateral surface is bounded 

 in front by the central fissure, below by the Sylvian 

 fissure, above by the back part of the internal border; 

 it is only partially separated from the occipital lobe 

 by the occipital fissure, merging gradually into the 

 temporal lobe. 



The Occipital Lobe. The lateral surface is bounded 

 anteriorly by the occipital fissure, which partially 

 separates it from the parietal lobe, also the para- 

 occipital and exoccipital fissures are seen extending 

 into the lobe. 



The Temporal Lobe. The lateral surface is bounded 

 by the basisylvian and Sylvian fissures and by the 

 ventrolateral border; posteriorly, it merges into the 

 adjacent parietal and occipital lobes. 



The Island of Reil (central lobe or insula). This is 

 seen after separating the lips of the sylvian cleft, 

 after raising the frontal lobe; it is overlapped by the 

 opercula; the latter removed, the island of Reil is 

 seen as a tetrahedral-shaped mass with its apex 

 directed forward and upward. 



The Rhinencephalon or Olfactory Lobe. This con- 

 stitutes the central olfactory structures, as distin- 



