68 THE CEREBRUM. 



occipital lobe extends from the occipito-parietal sulcus to the pre- 

 occipital notch, and is separated from the temporal lobe on the 

 basal surface by an imaginary line drawn from this notch toward 

 the posterior end of the corpus callosum, to the isthmus of the 

 limbic lobe. The occipital lobe has the form of a triangular 

 pyramid whose borders are the supero-medial, the infero-lateral 

 and the medial occipital borders of the cerebral hemisphere. 

 Those borders meet at its apex, the occipital pole. 



The occipital lobe is somewhat rudimentary in man, though 

 present only in apes and men (Cunningham). It makes its ap- 

 pearance at the fourth month in utero, and is distinctly outlined by 

 fissures on all three surfaces at the sixth month, when it resembles 

 the same lobe in the adult ape's brain (Cunningham Memoirs). 

 After the sixth month the fissural boundaries largely disappear 

 from the human brain on the convex and basal surfaces; hence, 

 the artificial boundaries in the adult. Its rudimentary character 

 probably accounts for the great variability in the sulci and gyri 

 of the occipital lobe. On the convex surface they are usually 

 as follows: 



( Ramus occipitalis of interparietal sulcus. 

 Sulci < Transverse occipital (s. occipitalis transversus) 

 [ Lateral occipital (ss. occipitales laterales). 



. | Superior occipital (gg. occipitales superiores) 

 ' | Lateral occipital (gg. occipitales laterales). 



The lateral occipital sulcus is the only one properly belonging 

 to the convex surface of the occipital lobe (sometimes there are 

 two of them). It divides that surface almost equally into a 

 superior and lateral gyrus, both of w r hich may be double. The 

 sulcus begins near the supero-medial border. It follows a merid- 

 ian which is nearly parallel with the infero-lateral border of 

 the lobe and runs forward, often presenting one interruption, 

 to the occipito-temporal boundary line where is bifurcates and 

 forms a short perpendicular sulcus, which represents the affens- 

 palte of the ape (Elliot Smith). When the lateral occipital sulcus 

 is double there are two lateral gyri (Figs. 20, 23 and 24). 



The occipital limb of the interparietal sulcus descends in 



