370 Journal of Comparative Neurology. 



the increased breath of the gyrus. The only hypothesis on 

 which the decrease in the thickness of the cortex could be more 

 than the increase in the medullary center, is that the gyrus as 

 a whole should decrease in size from childhood to youth. The 

 measurements which Kaes gives for the breadth of the mednllary 

 center and the thickness of the cortex at the sides of the gyras in 

 the child and i8 year brains are — child cortex 4.9 mm., 18 year 

 cortex, 3. 1 8 mm. ; child medullary center 2. 73 mm. , and 1 8 year, 

 2.91 mm.^ The difference between the thickness of the child cor- 

 tex and that of the youth is 1.72 mm., on each side of the gyrus, 

 or 3.44 for both sides. If the thinning of the cortex is to be 

 explained as a spreading of the medullary center toward the 

 surface of the gyrus, we should expect to find the 3.44 mm. 

 reduction in the cortical thickness at the sides of the gyri, accom- 

 panied by at least an equal increase in thickness of the medul- 

 lary center. But the medullary center of the child brain meas- 

 ures 2.73 mm., while that of the 18 year brain measures but 

 2.91 mm. The difference is only .2 mm., instead of about 3.5 

 mm., as we expected to find it. The only hypothesis on which 

 these measurements could be reconciled is, as we have seen, 

 that the total gyrus grows thinner from childhood to youth — a 

 supposition which seems very improbable. Kaes no where 

 recognizes that such a result follows from his tables of measure- 

 ments, nor does he mention any process by which the total size 

 of the gyrus could be reduced during the period of growth. 



It will be remembered that in the description of the growth 

 process of the fiber systems of the cortex the statement was 

 made that the reason the II and III Meynert layer seemed so 

 disproportionately broad in the child cortex, was that all the 

 space between the zonal layer and the ectal edge of the devel- 

 oping outer association layer was measured as II and III Mey- 

 nert layer.* But with the appearance of the Baillarger layer, 

 the permanent boundary line between the II and III Meynert 

 layer and the outer association layer was fixed, and from this 



1 6, Tabella I. 5, p. 32, table. 

 ' 5. p. 33- 



