240 THE GROWTH OF THE BRAIN. 



plete development, that the number of elements which 

 might possibly develop in any given case is far beyond 

 the number that actually does so, and that the charac- 

 teristic appearance of the cortex in the various localities 

 depends in a measure on the expansion of dissimilar 

 layers of the primitive granules. 



Just here can be met a difficulty sometimes felt in the 

 explanation of these changes. It has been urged that 

 the possible number of cells latent and functional in 

 the central system is early fixed. At any age this 

 'number is accordingly represented by the granules as 

 well as by the cells which have already undergone 

 further development. During growth the proportion of 

 developed cells increases, and sometimes, owing to the 

 failure to recognise potential nerve cells in the granules, 

 the impression is carried away that this increase implies 

 the formation of new elements. As has been shown, 

 such is not the case. 



These conclusions require to be tested further, but 

 bearing on them is the fact that the number of granules 

 and partially developed cells was excessive in the defec- 

 tive portions of the cortex of the blind deaf-mute, Laura 

 Bridgman,in whom normal development in these localities 

 ceased at the end of the second year of life (Donaldson), 

 and similar appearances have been found in the cortex of 

 persons congenitally defective. Applying these ideas to 

 the comparative anatomy of the cortex, there is reason to 

 think that in passing down the zoological scale the pro- 

 portion of undeveloped elements would increase, and for 

 this there is positive evidence. It is conceivable, too, 

 that in the case of man wide individual variations in the 

 proportion of developed granules may occur, and with 

 it would come corresponding differences in the organisa- 

 tion of the cerebrum, without much alteration in its 

 absolute weight. If the history of the cortex up to 



