THE NEURON 39 



adult body are developed from the cells of the embryonic nervous 

 system; but it is now generally accepted that each neuron is 

 developed from a single embryonic cell (known as a neuroblast), 

 and that in the adult body each neuron has a certain measure of 

 anatomical and physiological distinctness from all of the others. 



The very young nerve-cell (neuroblast) is oval in form and is 

 composed of a nucleus and its surrounding protoplasm (cyto- 

 plasm) ; but in further development it rapidly elongates by the 

 outgrowth of one or more fibrous processes from the cell body, 

 so that the mature neuron may be regarded as a protoplasmic 

 fiber with a thickening somewhere in its course which is the cell 

 body of the original neuroblast and contains the cell nucleus 

 and a part only of its cytoplasm (this part being called the 

 perikaryon), the remainder of the cytoplasm composing the 

 fibrous processes, that is, the nerve-fibers. The cell body of 

 the mature neuron is sometimes loosely termed the nerve-cell, 

 though the latter term should strictly include the entire neuron. 

 The importance of the conducting function is reflected in the elon- 

 gated forms of the neurons and in the peculiar protoplasmic struc- 

 ture of the nerve-fibers. The function of the cell body is chiefly 

 nutritive; the entire neuron dies if the cell body is destroyed. 



Each neuron may be regarded as essentially an elongated con- 

 ductor, and these units are arranged in chains in such a way that 

 a nervous impulse is passed from one to another in series. Since 

 the arrangement is such that the nervous impulse usually 

 passes through the series in only one direction (see the typical 

 reflex arc, Fig. 1, p. 25), each neuron has a receptive function 

 at one end and discharges its impulse at the other end. This is 

 what is meant by the polarity of the neuron (see pp. 52 and 97). 



The simpler forms of neurons are bipolar, with one or more 

 processes known as dendrites conducting nervous impulses toward 

 the cell body, and (usually) only one process, the axon or neurite, 

 conducting away from the cell body. The dendrites are usually 

 short, and in this case their structure is similar to that of the cell 

 body. But where the dendrites are long, as in the neurons of the 

 spinal and cranial ganglia (Figs. 1, 10), they may have the 

 same structure as the axon. The axons are the axis-cylinders 

 of the longer nerve-fibers and ar.e structurally very different from 

 the protoplasm of the cell body, being composed chiefly of 



