136 READINGS IN BIOLOGICAL SCIENCE 



lines, which includes all long, bilaterally symmetrical beings from worms 

 to men, there is an axial nervous system. This means that there is a main 

 trunk of nerves running down the center line of the animal with branches 

 leading off from it to the various organs. From our point of view, these 

 organs may be divided into two classes, the receptors, such as eyes, nose, 

 and ears, which are in touch with the outside world and receive stimuli 

 from it, and the effectors, such as the muscles, which act to bring about 

 changes adapting the individual to his immediate surroundings. The func- 

 tion of the nerves is to carry stimuH from the receptors to the effectors 

 much as a telephone line carries messages from one person to another. 



The link-up of receptor, conductor, and effector is known as the reflex 

 arc and is the mechanical basis of behavior in all organisms advanced 

 enough to have nervous systems. In those which have axial nervous sys- 

 tems, the structure of the conductor part of this circuit is highly compli- 

 cated. The nerves which link receptor and effector are composed of a 

 series of specialized cells, 7ieurons, whose ends approach but do not ac- 

 tually join each other. The gaps between the neurons are called sy?iapses 

 and play a vital part in all the more complicated forms of behavior. 

 Neurons are so organized that they will carry impulses in only one direc- 

 tion. The impulse started by a stimulus impinging on one of the receptors 

 passes along the connecting neuron at the rate of about 400 feet a second 

 until it comes to a synapse, which it jumps, passing on into another neuron, 

 and so on until it reaches the effector. At the synapses there is a resistance 

 of some sort which affects the impulse. It may be slowed down or even 

 blocked at the point. It may also be deflected to any one of several neurons, 

 if their ends lie close enough, or split so that it continues to travel down, 

 several of them simultaneously to different effectors. However, the re- 

 sistance to impulses offered by the synapses diminishes with use. The oftener 

 a synapse has been jumped, the easier it is for the next impulse to jump it. 

 This wearing of paths through the synapses is the neurological basis of 

 learning and habit formation. 



In the more complex organisms, such as our own, there is a constant 

 reception of varied and often conflicting stimuli. The impulses arising 

 from these stimuli have to be sorted out and directed to ensure the sort 

 of reaction which will be most profitable to the whole body. The con- 

 ductors of the various reflex arcs are therefore routed through various 

 reflex centers, which serve somewhat the functions of a telephone central. 

 In these centers the ends of many neurons are brought close together so 

 that the incoming impulses can be sorted out, switched from one line to 

 another or distributed. Just how the reflex centers distinguish between im- 

 pulses, inhibit some, and direct others is still a profound secret, but they 

 do this in frogs and philosophers alike. The mechanics of the reflex arcs 

 and reflex centers are the same in all animals having axial nervous systems. 



The main trunk of an axial nervous system (in vertebrates, the spinal 



