I 

 ARCHITECTURAL CHANGES DUE TO GROWTH. 235 



these changes is more striking than that produced by the 

 formation of the gyri of the cerebral hemispheres. Up 

 to the fifth week of fcetal life the hemispheres are 

 smooth. Then appear the transitory fissures, caused by 

 the infolding of the entire thickness of the wall of the 

 hemispheres, a process which has been attributed by 

 Cunningham I to inequality in growth caused by the 

 over-rapid enlargement of the encephalon as compared 

 with that of the cranium. This inequality is soon 

 relieved by proportionate expansion of the cranial 

 bones, and almost all the transitory fissures disappear, 

 one, or perhaps two, remaining. The next formation of 

 gyri, which begins about the twentieth week with the 

 appearance of the central fissure, does not involve the 

 entire thickness of the hemispheral wall, but is due to a 

 retardation of growth along the line of the future 

 fissures. In the growing cerebrum both the bottoms of 

 the fissures, as well as the summits of the gyri which 

 limit them, move away from the geometric centre of the 

 brain, the latter more rapidly, and hence the fissures 

 first formed become gradually deeper so long as the 

 brain continues to enlarge. Variations in the rate at 

 which the radii of the several portions of the cortex 

 elongate become more numerous as the brain grows 

 larger, so the number and extent of the fissures is also 

 increased. The causes of this change are but poorly 

 understood. Both the radial fibres that grow into the 

 cortex, as well as those that grow out from it, must have 

 an influence, and probably an important one, since they 

 are the fibres by which it is most affected, while the various 

 sorts of non-radial fibres are of hardly less significance. 

 One effect of this fissuration is to increase the surface of 

 the cortex, a change which must be accompanied by a 



1 Cunningham, A Contribution to the Surface Anatomy of the 

 Cerebral Hemispheres, Dublin, 1892. 



