Cerebral Convolutions. 133 



In the Cercopithecus Sabaeus the great Median Fissure divides the 

 Brain into two Hemispheres ; each Hemisphere having an external 

 surface, and an internal surface. The external surface comprehends 

 all that can be seen on taking a profile view of the Brain : the 

 internal, all that is seen on looking at the surface exposed by severing 

 the two Hemispheres from each other by a perpendicular section 

 through the great Median Fissure and Corpus Callosum. The line 

 formed by the junction of these two surfaces above, is the superior 

 border : that formed by their junction below, is the inferior border. 

 The most prominent point of the Frontal lobe, where the two borders 

 meet anteriorly, is the anterior or Frontal extremity : the point 

 where they meet posteriorly, is the posterior or Occipital extremity. 



A. External Surface, 



The lobes on the External Surface are, 



1. Frontal. 



2. Parietal. 



3. Temporo-sphenoidal. 



4. Occipital. 



The Central lobe, or * Island of Eeil ' is hidden within the lips of 

 the Fissure of Sylvius, and corresponds with the inferior aspect of 

 the Corpus Striatum. 



The External Perpendicular Fissure divides the External Surface 

 into an anterior and a posterior part: the posterior comprehending 

 the Occipital lobe : the anterior being divided again into two by 

 the Fissure of Sylvius. The portion above this fissure- embraces the 

 Parietal and Frontal lobes, that below it the Temporo-sphenoidal 

 lobe. 



1. The Frontal lobe is limited posteriorly by the ascending branch 

 of the Fissure of Sylvius, or, as Mr. Huxley names it, the Antero-parie- 

 tal Sulcus. It is divided into an Orbital lobule and a Frontal lobule, 

 the former corresponding to that part which rests on the orbital plate 

 of the Frontal bone. The convolutions of the Orbital lobule are in- 

 constant: those of the Frontal lobule are three in number and all 

 longitudinal in direction, viz. : 



a. Superior Frontal. h. Middle Frontal. e. Inferior Frontal, 

 or Supra-ciliary. 



2. Parietal Lohe. This Lobe is bounded anteriorly by the Antero- 

 parietal Sulcus, posteriorly by the External Perpendicular Fissure. 



