NERVOUS SYSTEM. VERTEBRATA. 413 



ventro-lateral margin and then bends upward in front so as 

 either to superficially join the Affenspalto or, as in this case, 

 to end between the lower end of the latter and tho parallel 

 sulcus. 



Two fragmentary sulci, generally regarded as inferior 

 temporal, are found in the triangle formed by the parallel, 

 inferior occipital, and collateral sulci. There is also a 

 shallow horizontal sulcus on the cranial aspect of the occipital 

 pole, corresponding to that called " lateral occipital " in the 

 Cebidae. 



On the mesial surface we find the deep calcarine sulcus 

 placed almost horizontally and passing without any line of 

 demarcation into the stem of the T-shaped retro-calcarine 

 sulcus. The collateral sulcns begins in the inferior angle 

 of the T and, after an extensive course along the inferior 

 margin, ends in close proximity to the rhinal fissure 

 (fig. 241). 



The parieto-occipital, being essentially a compensatory 

 sulcus, exhibits extreme variation. We may find a simple 

 deep incision (a) in the dorso-mesial angle of the hemisphere 

 opposite the apex of the occipital arc of the intraparietal, 

 which it joins. This fundamental sulcus may be surrounded 

 by a U-shaped arc (/3 + 7) on the mesial surface, the posterior 

 limb (7) of which is essentially " parieto-occipital," whereas 

 the anterior limb () really belongs to the intercalary series- 

 being the "compensatory" or so-called u post-limbic" sulcus 

 of Broca. The fundamental sulcus (a) in this case is exactly 

 analogous to that which was named ramus parieto-occipitalis 

 sulci intraparietalis in Cebus (vide supra). The anterior limb 

 (/3) of the U-shaped compensatory sulcus is often separated 

 as a distinct " compensatory " element. While all this 

 variety bears out the fundamentally compensatory nature 

 of the sulcus, it renders a concise definition of the parieto- 

 occipital sulcus impossible *. 



A glance at the illustration (tig. 241) or at the mesial 

 surface of the brain of any Ape, will at once show how 

 the majority of the (posterior) mesial sulci are essentially 



* The human parieto-occipital sulcus consists of the representatives of 

 the sulci a and y, the intervening area being submerged so that the two 

 sulci appear to form one cleft. 



