MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 291 



pose for these bridging convolutions the name transverse calcarine lobules or rather 

 the transverse lobuli of the calcarine fissure: lobulus calcarina transversus. A com- 

 parison of the degree of development and the position of these fissures and lobules 

 in different races will, I believe, prove interesting and important; for in Man the 

 occipital lobe reaches its highest complexity, as it is also the last to completely 

 develop. Naturally, therefore, in this lobe especially, we may expect to find racial 

 peculiarities. Thus in the negro as shown in fig. 2, Plate XLII, and fig. 5, Plate 

 XXXVI, the calcarine is continuous throughout its whole length, and this I have 

 found is usually the case in this race, just as it is in the Simians. The development 

 of these annectant folds is, therefore, an indication of high cerebral development 

 in this region, corresponding in this respect to the evolution of the pits de passage 

 extending between the occipital and parietal and temporal portions of the brain 

 surface. I have been able to trace the evolution of these transverse calcarine 

 lobules from the stage in which they lie entirely concealed in the depths of the 

 calcarine fissure and not separating the transverse from the horizontal portion, up 

 to a stage of development in which they have not only become widely separated, 

 but, as is shown in fig. 6, Plate XXXVI, the transverse portion has been pushed so 

 far back as to become apparently a fissure and convolution belonging to the lateral 

 surface; and it is in this way that those perpendicular furrows often found on the 

 human occipital lobe are produced. Here we have a condition of affairs which 

 has not been properly recognized by previous observers, and it introduces a 

 new element into the mechanical theory of the evolution of the convolutions. In 

 this case, instead of fissuration giving rise to convolutions, we find through local 

 morphological development, the evolution of convolutions giving rise to the pro- 

 duction of new fissures; and it is for this reason that I have accepted the third view 

 as to the origin of the fissures and convolutions already stated. We shall find that 

 the recognition of the differences between these two processes of cerebral differen- 

 tiation will greatly aid us in solving man} r of the complexities of fissural develop- 

 ment in the human brain. Of the same character with the transverse calcarine 

 lobules are the pits de passage, and they always arise in the depths of fissures pro- 

 duced by the differential action of the pressure forces due to the growth of the 

 cerebral surface within its bony environment. The vertical or transverse direction 

 of these transverse calcarine fissures is evidently due to the fact that the developing 

 backward of the occipital lobe which is characteristic of the higher Primates, is 

 limited by the supra-occipital bone. 



The calcarine fissure is peculiar to the Primate brain and develops onby in con- 

 nection with a posterior lobe and a posterior horn to the lateral ventricle. 



The primary or fundamental fissures, viz.: the Sylvian, and the fissura tri- 

 radiatus with its three branches, form the groundwork or plan upon or around 

 which the remaining fissures and convolutions are arranged. Their relations to the 

 hemisphere are exhibited in the accompanying diagrams, figs. 4 and 5. We have 

 already studied the relations that these fissures bear to the hemisphere in its early 

 stages of development and also the forces acting to produce them. On Plates XXXV 



