ANATOMY, PHYSIOLOGY, OF NERVOUS SYSTEM. 431 



next step is to study in detail the individual units of which 

 these systems are composed. 



The unit by the multiplied arrangements of which the en- 

 tire structure of the nervous system is built up is called the 

 neuron. A neuron is a nervous cell together with the nerves 

 running out from it. By this name, therefore, is included 

 not the nerve cell merely, but the nerve fiber as well. It 

 has the advantage of doing away with that arbitrary dis- 

 tinction between nerve cells and nerve fibers, and gives em- 

 phasis to the fact that a nerve cell with the fibers springing 

 from it is a unit physiologically as well as anatomically. 



But the value of this name is further increased by the 

 fact that all the neurons in the entire nervous system are 

 supposed to be distinct and independent anatomically and 

 possibly to some extent physiologically. A neuron is to the 

 nervous system what a single citizen is to the state. In 

 fact, recent researches show that the different neurons of 

 the body are not in direct anatomical continuity, but that 

 each is an anatomical entity, and that impulses from one 

 neuron to the other can be sent only by having one neuron 

 act as a stimulus in arousing a new impulse in the second. 

 It is not the simple original impulse that can reach the sec- 

 ond neuron, any more than in two persons joining hands is it 

 the pain experienced by one that is transmitted to the other. 

 The first as a stimulus must reproduce the same pain in the 

 second anew. The relative arrangements and the super- 

 position of the various neurons of the central nervous system 

 will be discussed further on in this chapter, it being the 

 point here to treat of their general histology merely. 



Neurons. 



One of the most remarkable things about a neuron is 

 usually its size. While there are neurons lying within the 

 central nervous system only a few centimeters or less in 

 length, there are, on the other hand, neurons having a 

 length of several feet. Such neurons, for instance, as reach 

 from the cortex of the brain to the lumbar cord, or still 



