ARCHITECTURAL CHANGES DUE TO GROWTH. 237 



maturity acquires longer and more numerous branches, 

 which may be lost again under pathological conditions. 

 In Fig. 45 are given the outlines of the arbor vitae as 

 observed by Engel. 



The cortical layers both in the cerebrum and cere- 

 bellum grow thicker as the brain increases in size. The 

 studies of Krohn on the cerebellar cortex of the cat 

 show the molecular layer to be thicker in the cat of six 

 months than in the kitten of three months. 



TABLE 54. SHOWING THE THICKNESS OF THE MOLECULAR 

 LAYER OF THE CEREBELLAR CORTEX IN CATS AGED RE- 

 SPECTIVELY THREE MONTHS AND Six MONTHS. MEASURE- 

 MENTS IN MILLIMETERS. 



It will also be noted that this layer is thicker on the left side. 

 This is constant. (Condensed from Krohn.} 



Probably the thickness of the cerebral cortex in- 

 creases so long as any of its layers continue to grow in 

 thickness, and there is reason to think that this may go 

 on up to the fortieth year. The development of the 

 cortex is very instructive, and is illustrated by the 

 accompanying figures, based on those by Vignal. 1 



In the foetus of the twenty-fourth week small granule- 

 like cells, arranged in vertical rows and closely packed 

 together, form the bulk of this layer. In the twenty- 

 eighth week it has increased in thickness, the ceils have 



1 Vignal, Developpment des Elements du Systeme nerveux 

 cerebro-spinal, Paris, 1889. 



