1 146 



HUMAN ANATOMY. 



directly continuous, and depends upon the recognition of an arbitrary line which 

 may be drawn, as suggested by Cunningham, from the preoccipital notch on the 

 infero-lateral border to the isthmus of the limbic lobe, just below the splenium of 

 the corpus callosum. 



The external surface of the -occipital lobe is modelled by two well-defined fissures, 

 the transverse occipital and the lateral occipital, and by two somewhat uncertain 

 convolutions, the superior and the inferior occipital (Fig. 988). 



The transverse occipital sulcus is, as above pointed out, the widely diver- 

 gent terminal bifurcation of the interparietal fissure, whose last segment beyond the 

 outer end of the parieto-occipital sulcus enters the occipital lobe to end in the manner 



just indicated. 



Fig. 989. 



Inferior aspect of cerebral hemispheres, i.o., t.o., e.o., internal, transverse and external orbital fissures; 

 i.t., incisura temporalis ; cal., calcarine , col., collateral ; o-t., occipito-temporal fissures. 



The lateral occipital sulcus arches horizontally forward below the lower end 

 of the preceding furrow, not infrequently dividing into an ascending and a descending 

 limb. 



The superior and inferior occipital gyri are the upper and lower areas into 

 which the outer aspect of the occipital lobe is somewhat uncertainly subdivided by 

 the lateral occipital sulcus. Secondary furrows and ridges often obscure the charac- 

 teristic modelling of this surface, whilst annectant convolutions connect its gyri with 

 the parietal and temporal lobes. 



The mesial surf ace of the occipital lobe presents one sulcus, the calcarine fissure, 

 a triangular tract, the cuneus, and part of the gyrus lingualis. 



The calcarine fissure begins by a forked extremity, the longer lower limb of 

 which incises the occipital pole in the impression made on the hemisphere by the 

 lateral sinus. It then continues forward, slightly arched, a short distance above the 

 border of the lobe formed by the junction of the falx cerebri and the tentorium, and 



