338 MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 



It will be seen from this that the identification which some writers have 

 made in comparing the crucial fissure. Cr, of the carnivora to the fissure of 

 Rolando is morphologically incorrect. The crucial fissure is really a vertical and 

 lateral development of a fissure of the mesial surface, whilst the fissure of Ro- 

 lando morphologically is a displaced and modified portion of a lateral superior 

 occi pi to-frontal fissure. 



It is due, therefore, to the development of a distinct occipital lobe, and the 

 enormous expansion of the fronto-parietal region that we have produced the 

 peculiar fissural conformation of the Primate brain, as distinguished from that 

 of the other mammalian orders. Such development and antero-posterior elonga- 

 tion in a confined space must necessarily modify the primitive longitudinal ar- 

 rangement, and as a result we have produced the deep occipital cleft, O 1 , with 

 its overhanging operculum, the central fissure, the precentrals and post-central, 

 the inferior occipital and, finally, the secondary occipital arch and special trans- 

 . verse markings. 



The occipito-temporal lobe retaining its more primitive growth relations, be- 

 comes only slightly modified, but is separated from the developing occipital lobe 

 by the inferior occipital fissure (inferior branch of the primary occipital arch O 2 . 



If these views are correct, then the plis de passage which we have claimed above 

 as being only the developed posterior portions of the occipitofrontal and occipito- 

 temporal convolutions concealed by the largely developed operculum of the Sim- 

 ians, should correspond in number and position with these convolutions, and the 

 facts seem to bear out the theor3'. 



From this standpoint we will now examine the relations of these annectant 

 folds, which have caused so much confusion and introduced so much complexity 

 into the nomenclature of the cerebral convolutions. 



MORPHOLOGY OF THE PLIS DE PASSAGE. 



1. The superior external pli de passage, 2, Fig. 21, connects the gyrus parie- 

 talis with the occipital lobe. 



2. The deuxieme pli de passage externe, 3, Fig. 21, usually lies hidden be- 

 neath the operculum, but occasionally rises to the surface, giving this region an 

 unusual appearance. It connects the conjoined gyrus parietalis inferior and the 

 gyrus occipito-temporalis superior with the occipital lobe. 



3. The troisihne pli de passage externe, 4, Fig. 21. separates the fissures, O 1 

 and O 2 , of the primary occipital arch and passes from the apex of the occipital lobe 

 into the second occipitotemporal convolution. 



4. The quatrihne pli de passage externe, Y, Fig. 21, passes into the third 

 or inferior occipito-temporal convolution. 



5. The pli de passage superiettr interne, 2', Fig. 22, when it exists as a dis- 

 tinct and separate development, passes into the mesial surface of the superior 

 occipitofrontal convolution. 



