THE NEURON 43 



The different dendrites of a neuron may be physiologically 

 all alike, or they may spread out in different directions to receive 

 nervous impulses of diverse sorts from different sources. Simi- 

 larly the axon may discharge its nervous impulse into a single 

 nerve center or peripheral end-organ, or it may branch, thus 

 connecting with and stimulating to activity two or more diverse 

 functional mechanisms. In other words, a given neuron may be 

 a link in a chain of some simple nervous circuit (Fig. 1), or it 

 may be adapted to collect nervous impulses from different 

 sources and discharge them into a single final common path, or 

 in the third place it may receive nervous impulses of one or more 

 functional sorts and then discharge its own nervous energy into 

 several remote parts of the nervous system. This, in brief, is the 

 mechanism of correlation, and illustrations of these different 

 types of connection will be found in the following chapters. If 

 animal reactions were simple responses so arranged that a given 

 stimulus could produce only one kind of movement, the only 

 nervous mechanism required would be a single neuron transmit- 

 ting the excitation from the point of stimulation to the organ of 

 response, as a call bell may be rung by pulling a bell cord. But 

 the actual reactions are always more complex than this, so that 

 several neurons must be connected in series with various di- 

 vergent pathways of nervous discharge which reach different 

 correlation centers, all of which must cooperate in the final 

 response. Illustrations of some of these complicated reflex 

 mechanisms will be found in Chapter IV. 



Neurons with short dendrites and a single long axon are the 

 most common form and were termed Type I by Golgi (Fig. 8). 

 In some cases (Fig. 9) the axon also is very short, breaking up 

 in the immediate neighborhood of the cell body; these are the 

 Type II neurons of Golgi and appear to be adapted for the diffu- 

 sion and summation of stimuli within a nerve center. The 

 neurons of the spinal and cranial ganglia form a third type. 

 In embryonic development they begin as bipolar cells with a 

 dendritic process at one end and an axonal process at the 

 opposite end of the cell body; but in the course of further devel- 

 opment (Fig. 10) the two processes approach each other and 

 finally unite for a short distance into a single stem, which then 

 separates into an axon and a highly special form of dendrite 



