360 MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 



a tri-radiate involution into the three horns of the lateral ventricle. The three 

 primitive occipitofrontal and temporal fisssures follow the same arching lines as 

 the mesial arched and are repetitions of the same lines of depression indicating the 

 continuance of the interaction of the same general relation of forces. This plan of 

 evolution is illustrated in the brains of the Carnivora, in Cheiromys and some of the 

 Lemuridge. Finalty, with the evolution of the occipital lobe and the enormous 

 antero-posterior expansion of the parieto-frontal region is introduced a new system 

 of pressure forces and as a result a new type of fissuration, shown by the production 

 of the fissura centralis and repetitions of its line of direction in the fissurae, precen- 

 trales, postcentralis, etc.; whilst the occipital lobe itself is strongly marked off from 

 the rest of the hemisphere by the formation of the primary occipital arch, O 1 O 2 . 

 and its surface furrowed by the production of lines of fissuration respectively parallel 

 to O l and O 2 , the secondary occipital arch O 1 ' O 2 '. 



Thus the type of fissuration of the Primate brain is due to the direct inter-action 

 of the pressure forces resulting from the differential strains produced by the expand- 

 ing brains, compounded with the resisting forces of the more slowly expanding skull. 



These relations of forces produce the fundamental morphological type and this 

 is now modified by local differentiations due to processes of growth taking place in 

 the cerebral cortex itself. 



In the earlier stages of evolution the hemispherical mass develops as a whole, 

 but as differentiation progresses local peripheric swellings commence to make their 

 appearance, and it is by the meeting of these local outgrowths that the fundamental 

 type becomes variously modified in the different families, genera, species and even 

 individuals. In the earlier stages these local outgrowths are not sufficiently differ- 

 entiated to produce any marked results, but as the brain mass finally approaches 

 its limit of surface growth, and the skull becomes a more and more rigid environ- 

 ment these swellings assume a more and more prominent aspect as causative 

 elements in producing the generic,, specific and individual convolutional conforma- 

 tions characterizing particular brains. It is in this field that the application of the 

 principle of the formation of partitions by expanding spherical films can be applied 

 to the explanation of the formation of the complex sulculi, rami and extremities of 

 the various primary and secondary fissures, and also to the markings of the various 

 so-called lobuli and convolution's. We have found that the character of these par- 

 titions can be determined by the relative surface-tensions of the expanding spheres, 



and that these can be determined bv the formula r= -^— that is, the surface-tension 



P— P 

 or pressure produced is inversely as the radii. Hence we have produced, when the 



pressures are equal and two surfaces meet, a plane, Fig. 28, or curved partitions 

 whose radius of curvature depends on the relative relations of the radii of the 

 expanding spheres, Figs. 29 and 30. When three surfaces meet they produce a tri- 

 radiate partition the angles of which are 120°, the length of the individual parti- 

 tions depending on the relative length of the radii of the spheres, Fig. 24. When 

 four surfaces meet we have a quadradiate type produced, Fig. 26. 



