THE BASE OF THE FORE-BRAIN. 89 



callosum and the gyrus cinguli. It follows the convexity of the 

 corpus callosum and was formerly called the ventricle of it (Fig. 

 28). The callosal sulcus, behind the corpus callosum, is continu- 

 ous with the hippocampal fissure. 



The occipito-parietal sulcus (Figs. 27, 28 and 20), the inter- 

 nal part, extends downward from the supero-medial border to 

 the middle of the calcarine fissure. The two form a lambda- 

 shaped fissure ^ (Fig. 28); the lambda being tilted toward the 

 frontal pole has one anterior and two posterior rami. The anterior 

 ramus and the lower of the posterior rami constitute the calcarine 

 fissure; the posterior superior ramus, is the occipito-parietal 

 sulcus. This latter sulcus cuts the supero-medial border at the 

 junction of the posterior one-sixth with the anterior five-sixths 

 of that border; it is situated about two inches above the occipital 

 pole, and lies one-sixth of an inch anterior to the point in the 

 skull called the lambda. It separates the parietal lobe from the 

 cuneus of the occipital lobe. The occipito-parietal sulcus is a 

 deep one. In the embryo the primary occipito-parietal fissure 

 produces an eminence in the posterior horn of the lateral ven- 

 tricle (Cunningham). It is then a true fissure. But that prim- 

 itive fissure and the ventricular eminence entirely disappear, 

 and the adult sulcus is a secondary and superficial furrow, 

 hence it is properly called a sulcus and not a fissure. At the 

 inferior end of the occipito-parietal sulcus a buried annectant 

 gyrus, the gyrus cunei, separates the occipito-parietal sulcus 

 from the calcarine fissure, with which superficially it is contin- 

 uous. 



The calcarine fissure begins a quarter of an inch below the 

 posterior end of the corpus callosum and runs backward and 

 slightly upward to the low.er end of the occipito-parietal sulcus; 

 and, then, curves downward to a point near the occipital pole; 

 where it ends bifid (Figs. 28 and 26). It is thus divided by the 

 sulcus occipito-parietalis into an anterior calcarine and a posterior 

 calcarine fissure. These three furrows are continuous with one 

 another superficially in the human brain; but buried annectant 

 gyri actually separate them from each other: the gyrus cunei 

 separates the occipito-parietal sulcus from the calcarine fissure 



