234 THE GROWTH OF THE BRAIN. 



instance form them may be later increased by others 

 following them as guides, in much the same manner 

 that the regenerating peripheral nerve follows by the 

 aid of the old sheaths its previous course down a limb, 

 although quite unable to force a new passage. From 

 the observations of Cajal it appears that the lengthening 

 neuron is enlarged at its tip in a way that suggests the 

 comparison of it with a growing rootlet. 



Often the axis cylinder acquires a great length before 

 medullation occurs, and experiments showing that 

 functional activity accompanies or immediately follows 

 the formation of that sheath, have led to the view that the 

 sheath is necessary for the activity of the neuron. It is 

 not possible to make general statements concerning the 

 significance of the sheath, for the reason that there are 

 fibres which are functional, but in which the sheath is 

 permanently lacking, and also because even in the 

 typical medullated neuron there is one stretch of the 

 axis just after it leaves the cell-body, and others at its 

 final termination, where no sheath is found. It there- 

 fore is more in harmony with the facts to consider that 

 in those neurons usually medullated the establishment 

 of the final relations which makes the transfer of the 

 nerve impulses possible, is the prime event, and that this 

 is accompanied by the physiological change leading to 

 medullation, a change which in these cases is an index 

 of organisation. For the absence of this sheath in 

 certain classes of nerves, and in portions of others, no 

 explanation has been offered.. 



In one way the final approximation of the cell out- 

 growths, accompanied by medullation, is the most 

 important condition modifying brain architecture, but it 

 is by no means so evident as the grosser changes which 

 accompany it. 



Preceding medullation for the most part, none among 



