MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 333 



the nature of which we will discuss under the heading of the plis de passage. In 

 Ateles, fig. 17, PI. XXXVII, the parieto-occipital assumes more the conditions as 

 found in the human brain, following an oblique direction forward and downward 

 instead of being directed perpendicularly. In the Chimpanzee, PL XXXIX, fig. 3, 

 a slight obliquity can also be discerned. 



Primitively, therefore, the occipital lobe, as marked off by the primary occipital 

 arch, lies farther back on this upper mesial surface, exhibiting the relations as found 

 in Chrysothrix, PL XXXVIII, fig. 8, and it attains its greatest size and development in 

 the higher monkeys and Man by the development of swellings within the superior 

 occipital cleft or fissure. 



The gap and the developing convolutions can be seen in the human foetal brain, 

 PL XXXVI, figs. 2 and 4, during the ninth month. 



We have at various times spoken of certain small connecting, bridging or annectant 

 gyri, passing from the occipital to the temporal and parietal lobes, which Gratiolet 

 has distinguished as the plis de passage, and to these we shall now direct attention. 



PLIS DE PASSAGE. 



Gratiolet attaches great importance to these plis de passage as points of diagnosis 

 in different brains, and he distinguished altogether six of these transition gyres : four 

 external and two internal. The four external pass from the lateral portion of the 

 occipital lobe to join the parietal and temporal lobes. He named the uppermost of 

 these the first or superior external pli de passage, and the others the second, third 

 and fourth respectively. The two internal transition gyres he named the superior 

 internal and the inferior internal plis de passage. These, according to him, connect 

 the cuneus with the precuneus. Kolleston in this connection also remarks " in one 

 part of the brain, where two of the five great brain masses, into which its convoluted 

 surfaces may be mapped out, abut upon each other, what are but connecting spurs in 

 the ape's brain, overhung and concealed by the beetling parietal and occipital lobe, 

 rise in Man to the dignity of connecting table-lands, filling up and bridging over 

 at level what is a valley, or rather a chasm, in most simious encephala. " 



The superior and inferior internal plis de passage are shown as developed in 

 many monkeys in PL XXXVII, fig. 14, marked 1 and 2'. 



The four external plis de passage, pass from the lateral portion of the occipital 

 lobe to join the convolutions of the parietal and temporal lobes. They are the first 

 or superior external pli de passage, the deuxihne pli de passage externe and the 

 troisieme et quatrihne pli de passage externe. 



Of these the superior or first is the gyrus, 2, of which we have given the manner 

 of development and connections when describing the separation of the fissura occipi- 

 talis prima superior into its two divisions, the mesial and the lateral. The second 

 is generally concealed under the operculum and joins the angular gyrus or posterior 

 parietal gyrus, only developing completely to the surface in the human brain. The 

 third is the gyrus marked 4, fig. 8, PL XXXVII, and passes forward from the apex 



