Thompson, Medullation of Cortical Fibers. 367 



Kaes regards the method of measurement as the most ex- 

 act procedure in determining the growth process of the cortex. 

 It consists in recording the thickness of the cortex as a whole and 

 of each of its constituent layers at successive ages and in the vari- 

 ous regions. The most striking fact revealed by the method of 

 measurement is that the cortex of the l ^ year child is very much 

 broader, or thicker, than that of the adult. The average thick- 

 ness of the child's cortex is 6.5 mm., that of the 18 year adult 

 3.63 mm., and the average adult 4.94 mm.^ Comparing the 

 average thicknesses of the cortex for the successive ages, Kaes 

 discovers this general relationship between cortical thickness 

 and stage of development. The undeveloped child cortex is 

 thickest of any. From childhood to youth — about 18 years, the 

 cortex grows gradually thinner as it increases in complexity. It 

 is thinnest during the period of youth. ^ 



From youth to about 53 years, there is a gradual increase 

 of thickness, due to the addition of new fibers. Up to the 

 period of youth, therefore, thicker cortex means a lower stage 

 of development. Beyond the period of youth, thicker cortex 

 means a higher stage of development,^ The process by which 

 the initial thinning of the cortex takes place is, according to 

 Kaes' account, as follows,* The boundary line between the 

 cortex and the medullary center, is marked by the layer of fibrse 

 propriae. In making the measurements Kaes reckoned the 

 fibrae propriae as part of the medullary center, and not as part 

 of the cortex. The portion which was measured as cortex, 

 was therefore that which lies between the pia on one side and 

 the ectal border of the fibrae propriae on the other. In the pro- 

 cess of development the medullary center spreads outward on 

 either side through the addition of new projection fibers and 



» 5, p. 32 ; 6, Tabella I. 



* It must be remembered that Kaes's statements are based upon a series of 

 brains which contains none between the ages of \% and 18 years, nor between 

 18 and 38 years. 



»6, p, 4; 8, p. 3, 



* 5. PP- 32-33 5 8, p, 3. 



