THE PROBLEM OF NEURON PATTERN 159 



brief survey serves merely to call attention to some of 

 the more common and characteristic aspects of neuron 

 pattern. 



The most characteristic feature of the differentiation 

 of the neuron from the neuroblast is the appearance of 

 outgrowths of the cell body which often develop an 

 extremely complex system of branches, resembling the 

 branching of the multiaxial plants. In most vertebrate 

 neurons the two kinds of outgrowths, axon and dendrites, 

 are clearly distinguishable. The axon is slender and of 

 uniform diameter over all or most of its length and 

 possesses a characteristic structure differing from that 

 of other parts of the cell. The axon may extend for a 

 long distance from the cell body with no, or very few, 

 branches, as in many motor cells (Fig. 45), or it may 

 give rise to lateral branches, the collaterals, in the 

 immediate vicinity of the cell body (Fig. 46). In other 

 neurons it may break up into branches and terminate 

 within a short distance from the cell (Fig. 47). It may 

 terminate in a complex system of branches, the terminal 

 arborization, near or far away from the cell body, or it 

 may end with little or no branching. The relations of 

 axons to dendrites of other neurons also differ widely in 

 different cases. In some the axon or some of its branches 

 form extensive spirals about the dendrite while in others 

 the approach is more or less direct. Usually a neuron 

 gives rise to only one axon, though dioxonic and even 

 polyoxonic neurons occur, and usually also the axon is 



the primary outgrowth of the cell. But some neurons 







remain without axons. 



The dendrites, on the other hand, are structurally 

 very similar to the cytoplasm of the cell body and are 



. 



