PHYSIOLOGY OF THE NERVOUS SYSTEM 305 



passed through it into the axon. This movement through 

 the neuron is called the nervous impulse. 



Purpose of the Impulse. : Though the nature of the nerv 

 ous impulse is not understood, 1 its purpose is quite appar- 

 ent. It is the means employed by the nervous system 

 for controlling and coordinating the different parts of the 

 body. The arrangement of the neurons enables impulses 

 to be started in certain parts of the nervous system, and 

 the property of conductivity causes them to be passed as 

 stimuli to other parts. This enables excitation at one place 

 to bring about action at another place. 



Acting as stimuli, the impulses seem able to produce 

 two distinct effects : first, to throw resting organs into 

 action and to increase the activity of organs already at 

 work ; and second, to diminish the rate, or check entirely, 

 the activity of organs. Impulses producing the first effect 

 are called excitant impulses ; those producing the second 

 effect, inhibitory impulses. 



Functions of the Parts of Neurons. The cell-body serves 

 as a nutritive center from which the other parts derive 

 nourishment. Proof of this is found in the fact that when 

 any part of the neuron is separated from the cell-body, it 

 dies, while the cell-body and the parts attached to the cell- 



l At different times the nervous impulse has been regarded as a current of 

 electricity; as a progressive chemical change, likened to that in a burning fuse; 

 as a mechanical vibration, such as may be passed over a stretched rope; and as 

 a molecular disturbance accompanied by an electrical discharge. The velocity of 

 the nervous impulse, which is only about one hundred feet per second, proves that 

 it is not a current of electricity. It takes place with little or no exhaustion of the 

 cell protoplasm and consequently is not due to chemical action. And the loose, 

 relaxed condition of the nerves prevents their transmission of physical vibrations, 

 like those on a stretched rope. The view that the impulse is a progressive molecu- 

 lar disturbance, accompanied by an electrical discharge, has much evidence in its 

 favor, but it has only recently been proposed and is likely to be modified upon 

 fuller investigation. 



