Thompson, Medullation of Cortical Fibers. 369 



association layer toward the surface. This explains how it is 

 possible that the reduction in cortical thickness, although pri- 

 marily due to a spreading of the medullary center, should show 

 itself in the measurements as a thinning of the II and III Mey- 

 nert layer. But the appearance of the Baillarger and Gennari 

 layers fixes a permanent boundary between the outer associa- 

 tion layer and the II and III Meynert layer. ^ From that point on, 

 Kaes tells us the II and III Meynert layer and the outer associa- 

 tion layer of the child measure about the same as, or a little less 

 than, those of the adult. ^ 



The method of measuring the whole unmedullated portion 

 of the cortex between the zonal layer and the partially formed 

 outer association layer as II and III Meynert layer, makes the 

 measurements for the early stages somewhat misleading. But 

 aside from this fact, Kaes' description of the course of cortical 

 development might pass unquestioned, if his measurements 

 fitted his description, but there are several discrepancies between 

 the two which are difficult to reconcile. The most obvious of 

 these is one that comes out in the relation between the thick- 

 ness of the cortex, and that of the medullary center. The 

 child cortex is given as broader than the youth or adult at all 

 three levels, summit, sides and bottom of the gyrus. ^ Meas- 

 ured through from side to side, the total thickness of any gyrus 

 would, of course, be equal to the thickness of the medullary 

 center, plus the thickness of the cortex on either side of the 

 center. Since the thinning of the cortex has been explained 

 as the result of a spreading of the medullary center, if we sup- 

 pose that the gyrus remains the same size from childhood to 

 youth, then the thinning of the cortex in the youth would be 

 exactly equal to the increase in the breadth of the medullary 

 center. If the gyrus increases in size in the youth, the increase 

 in the breadth of the medullary center would have to be enough 

 to account not only for the thinning of the cortex, but also for 



' 5. P- 32- 

 • 5. P- 33- 

 » Tables 5, pp. 32, 34. 



