CENTRAL NERVOUS SYSTEM. 733 



growth the system becomes capable of new reactions in the sense that its 

 various responses are controlled and directed by a larger number of incoming 

 impulses, and thus the number, complexity, and refinement of the reactions 

 is increased, and in this sense it really attains new powers. 



With the change in the age of the central system there occurs from 

 birth to maturity, if we may judge from general reactions, an increase in 

 this organization which is maintained during the prime of life, and then 

 in old age this breaks down, at first gradually, and later rapidly. It 

 becomes important, therefore, to examine the manner in which this organ- 

 ization 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 neurons of one unit to the dendrons of another. 

 From the original position in which the young cells, the neuroblasts, are 

 produced, they plainly migrate, and often these migrations involve groups of 

 cells, as in the case of those forming the olivary bodies (His). 



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

 the branches, both dendrons and neuron. During growth both of these in- 

 crease in the length of their main stem and of their respective branches. In 

 picturing the approach of two elements within the central system the process 

 is usually described as that of the outgrowth of the neuron toward the den- 

 drons or bodies of those cells which are destined to receive the impulse, but it 

 must by no means be forgotten that the dendrons are also growing, and the 

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

 neurons depends on their own activities as well. 



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

 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 medullary sheaths by 

 the neuron and its branches, it is also clear that all fibres which when mature 

 are medullated begin as unmedullated fibres, 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, composing the nervous system, we find the fol- 

 lowing : 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 neuron increases the number of opportunities for excitation 

 which such an element offers. These cells are without dendrons. Among 

 the central cells all possible modes of growth are contributory; that is, 

 the branches of both kinds add directly to the complexity of the central 

 pathways. On the other hand, the efferent group contributes to this com- 



