GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM 135 



pulses may be started in various ways. A sharp blow on a living 

 nerve starts impulses traveling along it; a good example of this is 

 the effect of striking the " funny" bone. Nerves may be stimu- 

 lated by heat or by cold, by chemical agents or by an electric 

 spark. Whatever the nature of the stimulus the nerve impulse 

 which it arouses is, so far as we can tell, the same in all cases. 



Speed of Nerve Impulses. The nerve impulse travels from 

 the point of stimulation over the neuron at a regular and rather 

 slow rate which probably varies somewhat in different animals 

 and in different nerves of the same animal, but at ordinary tem- 

 peratures approximates 30 meters (97 ft.) per second in most ver- 

 tebrate nerves in which it has been measured. 



Spread of Impulses in Both Directions. Through observations 

 of the action currents of nerves it has been shown that the impulse 

 spreads from the point of stimulation in both directions along 

 the neuron, finally traversing all parts of it. This fact could 

 never have been learned if the existence of the action currents 

 were unknown because our only other method of detecting the 

 presence of nerve impulses depends upon the production of effects 

 in the organs to which the neurons lead; and in the body each 

 neuron has such connection only at one end; a nerve impulse 

 imparted to a motor nerve will cause contraction in its connected 

 muscle but produces no effect whatever at its other end. 



Nerve Impulses Vary in Intensity. It is part of ou? every 

 day knowledge that nerve impulses may vary greatly in intensity; 

 slight contractions of the muscles are produced by feeble impulses; 

 when stronger contractions are desired impulses of greater in- 

 tensity must be sent in. 



Fatigue. Finally, it has been proven beyond question that 

 the passage of impulses over nerve-fibers does not fatigue them 

 to an appreciable degree. In this respect the nerve is comparable 

 to a telephone wire; in each case the message is transmitted with- 

 out impairing the ability of the structure to transmit other mes- 

 sages. 



Nature of the Nerve Impulse. Although we know these 

 things about nerve impulses, we do not know what the nerve 

 impulse itself 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 



