156 THE HUMAN BODY 



How Nerve Impulses Are Aroused. We know that nerve im- 

 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. In frogs' nerves at 

 ordinary temperatures the rate approximates 30 meters (97 ft.) per 

 second. In human nerves the rate is probably two or three times as 

 high. 



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 demonstrated if the existence of the action currents (p. 103) 

 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. 



Fatigue. It has been proven beyond question that the passage 

 of impulses over nerve-fibers does not fatigue them to an appre- 

 ciable degree. In this respect the nerve is comparable to a tele- 

 phone wire; in each case the message is transmitted without im- 

 pairing the ability of the structure to transmit other messages. 



We learned in connection with our study of muscular fatigue 

 (p. 101) to look upon fatigue as the result of the accumulation of 

 waste substances. Its absence from active nerve-fibers indicates 

 one of two things. Either the transmission of nerve impulses does 

 not involve the production of fatigue substances or the fiber is 

 able to get rid of such as are produced so quickly that they cannot 

 affect its working. Exceedingly delicate tests which have recently 

 been devised indicate that in nerve trunks there is a small produc- 

 tion of carbon dioxid. This gas is known to be a product of oxida- 



