THE PROBLEM OF NEURON PATTERN 167 



extend more or less parallel to the surface, while the 

 secondary branches are in general vertical to it (Fig. 48) . 

 Axons in many cases show similar changes in direction, 

 sometimes very abrupt. Again, the dendrites of certain 

 cells may be at first unoriented but later, as the axons of 

 certain other neurons grow into their vicinity, they may 

 become definitely oriented with respect to these and 

 grow toward them. 



Other variations and complications in the form, 

 arrangement, and developmental changes of the out- 

 growths of the neuron might be described almost 

 indefinitely, but those mentioned suffice to call atten- 

 tion to the bewildering complexity of the facts and the 

 difficulties apparently involved in any attempt at physio- 

 logical analysis of neuron development. In addition to 

 this problem of external morphology and development 

 the neuron presents various other physiological problems, 

 e.g., the origin and function of the internal structural 

 features, the Nissl substance and the neurofibrils, the 

 physiological relations of different portions of the 

 neuron, both as regards nutrition and maintenance, as 

 indicated by the data of degeneration and regeneration, 

 and as regards functional conduction of impulses. For 

 the present, however, we are primarily concerned with the 

 question of physiological polarity or axiation and the 

 development of the external morphology. 



NEURON PATTERN AS A CASE OF AXIATE PATTERN 



Neurons are very commonly distinguished as unipolar, 

 bipolar, or multipolar, according to the number of out- 

 growths, though the terms "uniaxial," "biaxial," and 

 "multiaxial' 1 are perhaps preferable. There are of 



