CENTRAL NERVOUS SYSTEM. 285 



in this organization. This is maintained for a time, and then in old age it 

 breaks clown, at first gradually, and later rapidly. It becomes important, 

 therefore, to examine the manner in which this organization is accomplished. 



Organization in the Central System. — When first formed the cells com- 

 posing the central system are completely separated from one another. In the 

 mature nervous system the impulses, as has been pointed out, probably travel 

 for the most part from the axones of one unit to the dendrites of another. 



For organization the most important changes, however, are those affecting 

 the cell outgrowths, both dendrites and axones. During growth both of these 

 increase in the length of their main stems and of their respective branches. 

 In picturing the approach of two elements within the central system the pro- 

 cess is usually described as that of the outgrowth of the axone toward the 

 dendrites or bodies of those cells which are destined to receive the impulse, 

 but it must not be forgotten that the dendrites are also growing, and the 

 question of the approximation of the branches of these latter to those of the 

 axone depends in part on their own activities. 



The conditions modifying this process are, however, obscure. It is evi- 

 dent that medullation outside of the central system is not necessary to the 

 functional activity of a fibre, and therefore probably in the central system 

 unmedullated fibres are also in many cases functional. Whatever may be 

 the relation of the establishment of new pathways to the acquisition of medul- 

 lary sheaths by the axone and its branches, it is also found that all fibres 

 which when mature are medullated begin as unmedullated fibres, and that 

 the increase in medullation throughout the central system is an index of the 

 increase in organization. A consideration of the facts of growth in the layers 

 of the cortex, for instance, will show them to be open to this interpretation. 

 Applying these ideas concerning organization to the three classes of cells, 

 afferent, central, and efferent, which compose the nervous system, we find the 

 following: In the central system the afferent cells contribute to organization 

 by the multiplication of the collaterals. At the periphery the division of the 

 branches of the axone increases the number of opportunities for excitation 

 which such an element oilers. These cells are, lor the most part, without 

 dendrites. Among the central cells all possible modes of growth are con- 

 tributory ; that is, the branches of both kinds add directly to the complexity 

 of the central pathways. On the other hand, the efferent group contribute- 

 to this complexity almost solely by the lbrniati< f dendrites, the collaterals 



which come from the axones of these cells forming but an insignificanl con- 

 tribution. Not only, therefore, is organization in large part dependent on 

 changes in the central cells by reason of their numerical preponderance, but 

 also by reason of the fact that to them a multiplication <>t' pathways both by 

 elaboration of the axones and the dendrites is alone possible. 



Defective Development. — In view of these tacts, defective development 

 in the nervous system may depend on failure in one or more of these several 

 processes by which the system is organized, and it should be possible to corre- 

 late defective development involving mainly one set of elements with a dis- 



