ARCHITECTURAL CHANGES DUE TO GROWTH. 233 



axis in such a manner that the larger axes have the 

 thicker sheaths. A physiological connection between 

 the two is thus suggested, and it has been shown that 

 with the acquisition of the myeline the nerve fibres 

 become functional, a fact which has been effectively 

 used to determine the order in which the several parts 

 of the central system are organised. Medullation begins 

 in the cord and spreads to the encephalon, the local 

 mechanisms in the cord being functional before the 

 connections between them and the higher centres are 

 completed. In general, the elements first developed 

 become largest in size, and since the most necessary 

 functions presumably are those for which provision is 

 earliest made, the relative size of the nerve elements in 

 a given animal points to the degree of their physiological 

 value. 



In the central system the distance to be traversed 

 by the neurons is an important factor. It appears 

 that the long distance connections tend to be estab- 

 lished early when the low specific gravity, the paucity of 

 supporting tissues, and the small absolute size of the 

 entire system, all combine to offer the least obstruction 

 to changes of this nature. The associations later 

 formed become predominantly more local with ad- 

 vancing age, consequently the axis cylinder which is to 

 make a long distance connection early spans the greater 

 parts of the interval between the centres that are to be 

 associated. Later, not only is the absolute length greatly 

 increased, but it approaches nearer the elements which 

 it is destined to control. The first arrangement may be 

 considered as a sketch plan of the adult system, and for 

 the reason just given the further the foetus departs from 

 the first stages of its development the less feasible is any 

 alteration in this framework. Yet the outlines may 

 become heavier, and the few neurons which in the first 



