COORDINATION 



411 



mo^or naupon 



receptor 



sensory neuron 



Fig. 16-12. Neurons, motor and sensory. A myelinated nerve fiber is also shown in cross-section. The two neurons 



constitute a reflex arc. 



Another set, the motor nerves, carried the 

 adjusted impulse to the glands and mus- 

 cles where the final response was executed. 

 With such a system it was possible for an 

 animal to reach great size and complexity 

 and still be intricately coordinated. The 

 analogy of the centralized telephone system 

 may be recalled to good advantage at this 

 point. 



It may seem surprising to learn that this 

 very complex system is composed of only 

 one general kind of cell, the neuron. Here 

 the telephone analogy breaks down, be- 

 cause, while the organization may be simi- 

 lar, the nature of the wire itself is vastly 

 different from the neurons which go to 

 make up the entire nervous system of ani- 

 mals. 



The neuron. Although all cells may con- 

 duct an impulse within themselves, the 



neuron has developed this characteristic to 

 an advanced stage. Anatomically the neu- 

 ron is divided into three parts: the cell 

 body, or cyton, containing the nucleus, the 

 dendrites, which consist of numerous proto- 

 plasmic outgrowths from the cell body, and 

 the axon, a single, much longer extension 

 terminating in a brush-like filament (Fig. 

 16-12). The cyton resembles many other 

 cells in appearance and the dendrites 

 merely extend its surface of contact. The 

 axons are usually covered with a fat-con- 

 taining myelin sheath which functions like 

 an electrical insulator. The axons are usu- 

 ally long and an extreme example is illus- 

 trated by the giraffe, where they extend 

 from the tip of the toe to the back, a dis- 

 tance of 6 feet or more. Many axons are 

 bound together to form the nerve trunk, 

 spoken of as the "nerve" by anatomists, 



