188 A MANUAL OF ANATOMY. 



is opposite and external to the corpus striatum, and which, 

 not keeping up in growth with the adjoining portions of the 

 cortex cerebri, becomes covered in by those portions. The 

 island is traversed by three or four shallow sulci which 

 radiate from the anterior portion of the lobe in a direction 

 upward and backward. One of these fissures, the central 

 sulcus, is deeper than the others, and divides the lobe into 

 a large anterior portion and a smaller posterior part. The 

 island is surrounded by a well-marked depression, the cir- 

 cular sulcus. 



The Occipital Lobe. Figs. 23, 24, 26. 



This constitutes that portion of the cerebrum posterior 

 to the parieto-occipital fissure, and the prolongation of the 

 course of this fissure across the external surface of the 

 cerebrum. 



The sulci and fissures are usually very poorly marked 

 off. 



The sulci may be determined as follows : The superior 

 occipital sulcus is the posterior extremity of the intra- 

 parietal sulcus, or at least of a sulcus continuing the direc- 

 tion of this fissure backward ; similarly the inferior occipital 

 sulcus is part of, or a backward extension of, the middle 

 temporal sulcus. By these two irregular and indistinct 

 grooves three convolutions are determined, the superior, 

 middle, and inferior, or the first, second, and third occipital 

 convolutions. 



The superior convolution is continued into the cuneate 

 convolution on the internal surface of the occipital lobe. 



The Inner or Mesial Surface of the hemisphere taken as 



a whole. Figs. 27, 38. 



Upon the internal surface of the hemisphere are found 

 four of the primary fissures of the cerebrum, viz. : The 



