198 ANATOMY OF THE CENTRAL NEEVOUS SYSTEM. 



Posteriorly, it turns upward to the edge of the hemisphere and there ends 

 in a small incision behind the posterior central gyrus. 



The sulcus einguli — which also bears the names of sulcus calloso-marginalis, 

 marginal fissure, flssura limMca, and fissura splenialis— consists, properly speak- 

 ing, of three parts lying one behind the other. Not infrequently these are really 

 separate sulci. 



That which lies above and in front of this sulcus einguli is regarded 

 as belonging to the superior frontal convolution; the convolution lying be- 

 tween it and the corpus callosum is called the gyrus fornicatus. A glance af 

 a specimen or the illustration shows that the gyrus fornicatus widens out 

 superiorly in its posterior part and passes directly over the edge of the hemi- 

 sphere into the lobulus parietalis superior. This widened portion is called 

 the prcecuneus. Directly in front of the praacuneus lies a part of the cortex 

 which adjoins both central gyri without and connects these with one an- 

 other. It is called the paracentral lohule. 



The prsecuneus is terminated posteriorly by a deeply-incised fissure,— 

 the fissura parieto-occipitalis, — which always extends for some distance over 

 upon the outer surface of the hemisphere. This parieto-occipital fissure 

 frequently passes very far Ijeyond the inner surface and runs out over the 

 hemisphere externally as a deep vertical fissure, the fissura perpendicularis 

 ext. This is very frequently the case in the brains of idiots. In almost 

 all simian brains a broad fissure begins in the parieto-occipital fissure (or just 

 behind it, Ziehen and KiikenthaT), which passes downward over the greater 

 part of the lateral surface of the hemisphere and in a very striking manner 

 separates the parietal from the temporal lobe. It is called the simian fissure, 

 or "Affenspalte." 



The fissura calcarina joins the parieto-occipital fissure at an acute 

 angle. This fissure lies exactly in the outer wall of the posterior horn of the 

 lateral ventricle. The wall of the brain infolded by it is indicated within the 

 posterior horn by au elongated prominence known as the calcar avis. The 

 triangular cortical area inclosed by the fissura parieto-occipitalis and the 

 fissura calcarina is called the cuneus. If the vertex of this triangle is now 

 located, several small annectant gyri, superficially or deeply situated, are 

 found connecting it with the end of the gyrus fornicatus, which passes by 

 in front of the vertex of the cuneus. Notice this comparatively narrow part 

 of the gyrus fornicatus: the hilus. It is seen to continue as a rapidly widen- 

 ing convolution to the apex of the terajDoral lobe, where it ends in a hook- 

 like process: the uncus, or gyrus uncinatus. 



This convolution consequently surrounds the entire margin of the 

 hemisphere. In fact, it is called the marginal convolution; in which case 

 only the anterior portion receives the name of gyrus fornicatus, while the 



