GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM 157 



tion and its occurrence is taken to mean that chemical processes do 

 go on in nerve-fibers. They must be of very small magnitude, 

 however, for their products are so quickly dissipated as not to 

 hinder the functioning of the nerve at all. 



While indefatigability is thus seen to be a property of axons, 

 we shall learn presently that other nervous structures are highly 

 susceptible to it. Nervous fatigue is a common phenomenon, but 

 it is localized in the region of the synapses and not in the axons. 



Nature of the Nerve Impulse. Although we know these things 

 about nerve impulses, we do not know what the nerve impulse it- 

 self really is. There have been many interesting and ingenious 

 theories of its nature proposed. Some of these attempt to describe 

 it as a purely physical process, the transmission of a physical stress 

 from particle to particle along the nerve; others would consider it 

 as a chemical process, too delicate and transitory to be detected. 

 All theories of its nature agree that the change transmitted along 

 the nerve is not a continuous flow, like an electric current along 

 a wire, but is an exceedingly brief impulse or series of impulses. 

 The name given to the nervous discharge implies this character. 

 During continuous excitation of a nerve, as in prolonged voluntary 

 contraction of a muscle, the individual impulses follow each other 

 in rapid succession. The exact rate is not known, but is believed 

 to be in the general neighborhood of 50-100 a second. Quite re- 

 cently evidence has accumulated which indicates that individual 

 nerve impulses are on the whole of nearly equal intensity. This 

 is important in view of the familiar fact that nervous activities 

 in general may show widely different intensities. The prevailing 

 explanation accounts for this on the ground that nervous activ- 

 ities depend, not on single impulses but on streams of impulses 

 which latter may vary even though the individual component 

 impulses are equal. In accordance with this view we have to 

 suppose that a weak stimulus gives rise to one sort of stream of 

 impulses, a stronger stimulus to a different sort, and so on. 



Conduction Involving More Than One Neuron. Reflexes. In 

 the actual passage of nerve impulses through the Body more 

 neurons than one are always involved. Let us examine a simple 

 case of conduction by which the Body adapts itself to its surround- 

 ings. Accidentally my finger comes in contact with a hot surface. 

 Quite involuntarily I jerk my hand away. The chain of events is 



