MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 



•295 



Fig. 7. 



wish to direct attention to is the fact that two branches of the primary occipital 

 arch, O 1 and O' 2 , take the place, direction and relations of the two temporary radiat- 

 ing mesial occipital fissures amongst the permanent fissures of the hemisphere. We 

 have given good reasons to show that the production and lines of direction of these 

 temporary fissures were produced by a differential action of the pressure forces pro- 

 duced by the growing brain and the more slowly expanding skull at that period. 

 We then found that owing to an accelerated development of the bony environment 

 the cerebral surface became once more almost entirely smooth; and finally, when 

 these relations are again reversed we find appearing as permanent fissures the two 

 branches of the primary occipital arch, having the same relative directions and 

 bearing the same relations to the calcarine as the primary furrows did. From these 

 facts, since the mechanical conditions have not materially changed, relatively, we 

 must, I think, conclude that the primary 

 occipital arch in its turn, like the primary 

 radiating occipital furrows, owes its origin 

 to mechanical adjustments of the pressure 

 forces acting upon the cerebral surface. 

 The accompanying diagrams, figs. 7 and 

 8, exhibit the morphological plan of 

 the cerebral hemisphere as regards the 

 division into the three lobes which I have 

 proposed. The occipital lobe, 0, in many 

 of the monkeys, is on its lateral and 

 mesial surface entirely smooth as may be 

 seen in figs. 19 and 20, Plate XXXVII, 

 and comparing these with Plate 

 XXXVII, fig. 1, Plate XXXVIII, fig. 11, 

 etc. In others, it becomes furrowed, and 

 this furrowing gradually increases in 

 complexity as we ascend towards Man, 

 at the same time that the whole lobe be- 

 comes relatively smaller, its place being- 

 taken by the progressively increasing de. 

 velopment in the region between the 



occipital and occipo-frontal lobes of the pits de passage The consideration of the 

 evolution of these fissures and convolutions we will reserve for future discussion. 



BOUNDARIES OF THE LOBES OF THE CEREBRAL HEMISPHERE. 



1. The occipito-frontal lobe is separated posteriorly from the occipitnl lobe by 

 the upper branch of the occipital arch, 1, superior occipital fissure, which in its 

 turn has been separated by previous writers into two separate fissures, the internal 

 perpendicular (parietooccipital), and the external perpendicular (occipital), and pro- 



37 JOURX. A. X. S. PHILA., VOL. X. 



Fig. 8. 



